Shift in house price estimates during COVID-19 reveals effect of crisis on collective speculation

Abstract

We exploit a city-level panel comprised of individual house price estimates to estimate the impact of COVID-19 on both small and big real-estate markets in California USA. Descriptive analysis of spot house price estimates, including contemporaneous price uncertainty and 30-day price change for individual properties listed on the online real-estate platform Zillow.com, together facilitate quantifying both the excess valuation and valuation confidence attributable to this global socio-economic shock. Our quasi-experimental pre-/post-COVID-19 design spans several years around 2020 and leverages contemporaneous price estimates of rental properties – i.e., off-market real estate entering the habitation market, just not for purchase and hence free of speculation – as an appropriate counterfactual to properties listed for sale, which are subject to on-market speculation. Combining unit-level matching and multivariate difference-in-difference regression approaches, we obtain consistent estimates regarding the sign and magnitude of excess price growth observed after the pandemic onset. Specifically, our results indicate that properties listed for sale appreciated an additional 1% per month above what would be expected in the absence of the pandemic. This corresponds to an excess annual price growth of roughly 12.7 percentage points, which accounts for more than half of the actual annual price growth in 2021 observed across the studied regions. Simultaneously, uncertainty in price estimates decreased, signaling the irrational confidence characteristic of prior asset bubbles. We explore how these two trends are related to market size, local market supply and borrowing costs, which altogether lend support for the counterintuitive roles of uncertainty and interruptions in decision-making.

1 Introduction

One of the most impactful financial life-course events that individuals may encounter is buying a house, and in the United States (US) this fundamental decision is increasingly facilitated by online real-estate platforms such as Zillow.com, Trulia.com and Redfin.com. These marketplace service platforms aggregate available property information into virtual marketplaces, thereby facilitating the rapid and remote comparison of individual candidate houses, estimation of mortgage repayment schedules, and assessment of the overall real-estate market. Their user bases are broad, including professional investors, traditional homeowners and sellers, and casual browsers alike [1]. Consequently, the inflow of high-frequency market information that is aggregated by online real-estate platforms informs potential buyer and seller speculation, defined as near-term expectations of price and price movements [2], which is invariably conditioned by individuals’ sensitive and variable perceptions of uncertainty.

Against this backdrop, one of the many perplexing outcomes of the COVID-19 pandemic was the emergence of exuberant markets in the US after the dust settled from the first shock wave. This was particularly evident in the housing market, as illustrated in Fig. 1(A), which shows the official US government All-transactions House Price Index for several regions in California (CA), where average home sale prices grew by up to 23% in 2021. Similar levels of price appreciation occurred in metropolitan areas across the US.

Figure 1

Schematic of data sampling and before- and after-1/2020 matching design. (A) All-Transactions House Price Index data by region, obtained from the US Federal Reserve Bank of St. Louis (www.fred.stlouisfed.org). Annual percent increase from Oct. 2020-2021 are listed in the legend (2021 data not yet available for Mariposa; for more details see Fig. S1). (B) Longitudinal panel of Zillow Inc. house listings across 10 regions in northern California, USA constructed over 4-year time period 2018-2021 (see Fig. S2 for sample size information). Shown are the locations and names of the 10 principal cities – separated into big market (magenta) and small market (green) groups based upon 2021 population sizes, which are proportional to each circle radius. (C) Spatial distribution of mean house price estimate calculated for properties listed for sale in San Jose before 2020; each grid is color-coded according to its corresponding distribution quintile. (D) Mean 30-day price changes, color scale corresponds to distribution quintiles. (E) Mean price estimate after 1/2020 using values deflated to 1/1/2018 US$. (F) Percent difference between grid values in panels B and D. (G,H) Schematic of house matching design. For each house listed after 1/2020 (denoted by the index h), we identified two sets of similar houses, denoted by $\{N_{h}\}_{\text{Bef}}$ and $\{N_{h}\}_{\text{Aft}}$, based upon three criteria. Matched houses must be listed for sale in the same calendar month phase (e.g. if h is from July then matches must be from May, June or July), in the same price strata (i.e., matches must be within ± 1 price decile of h), and within a 1/2 mile radius of the central house. The set of matches $\{N_{h}\}_{\text{Bef}}$ are used for causal inference by way of a difference-in-difference identification strategy. The set $\{N_{h}\}_{\text{Aft}}$ is only used to estimate the contemporaneous neighborhood housing supply, denoted by the activity $A_{h,m} = \vert \{N_{h}\}_{\text{Aft}}\vert$. (G) Candidate matches before 2020 (10 matches indicated by orange dots); and (H) after 1/2020 (8 matches). Candidate houses within the same period not meeting these criteria are indicated by blue dots

The initial reaction of US financial and housing markets to the COVID-19 outbreak were sharply negative, as this pervasive shock disrupted the health and security of individuals, thereby extending to entire socio-economic systems [3–6]. So why the rapid turnaround in these markets in the second half of 2020? Prior empirical and theoretical work on real-estate markets establishes various factors underlying market volatility, but distinct differences in situational context make it challenging to infer wether or not the prior insights readily extend to the events defining 2020-2021. One particularly relevant feature of the pandemic period was the prompt and unexpected societal shift towards interpersonal interaction and information consumption modes that were entirely mediated by the internet and electronic displays, the impacts of which are only now beginning to be understood [7]. Given the prevalence and impact of online real-estate platforms in the US [1, 8–10], there is need for better understanding their role in facilitating multi-scale correlated phenomena such as collective herding behavior [11–16]. Other factors relevant to understanding the pandemic real-estate market include the rapid deployment of work-from-home accommodations that decreased the demand for metropolitan amenities [17], and also shifted perspectives on work-life balance and associated household expenditures [18, 19].

Understanding the housing market’s response to macro-economic shock is critical to understanding the resilience of this fundamental global market. However, unlike stock markets, where an abundance of high-frequency data provides a clear avenue for analyzing market response to both anticipated and surprise news [20, 21], there are scant high-frequency data sources for operationalizing such research on the real-estate market, even during ‘normal’ market periods. In this regard, our data collection approach exhibits the utility of novel high-resolution and real-time altmetrics for research at the intersection of real estate and urban development [22–29].

In particular, here we contribute to the literature on real-estate market dynamics and speculation by tracking individual property valuations for nearly 2 years before and two years after the onset of the pandemic in January 2020, which we hereafter denote by “1/2020”. A distinguishing feature of our study is the construction of a high-resolution property-level dataset that captures two specific elements necessary for analyzing price speculation: (a) the 30-day change in estimated house price, which measures near-term price movements; and (b) the high-low range in estimated house price, which quantifies uncertainty in price estimation.

As such, our multi-year analysis leverages the sudden emergence of widespread uncertainty as an instrument for analyzing the impact of collective speculation. We leverage this systematic market shift by implementing a difference-in-difference research design that compares the price dynamics of properties listed for sale (on-market) to properties listed for rent (off-market). Those rental properties that were simultaneously available – just not for sale, and thus transparent to speculation deriving from short-term expectations of resale returns – provide a counterfactual baseline for addressing our main research question: to what degree was excess real-estate price growth attributable to COVID-19 pandemic uncertainty?

In what follows we address this question by way of the following three research questions. First, what are the characteristics of high-frequency real estate price dynamics at the 1-month resolution, and to what degree did they change after the COVID-19 pandemic? Second, to what degree did the pandemic shock to market uncertainty affect collective speculation – namely, in house price estimates and certainty in those estimates? And third, how did shifts in speculation relate to fundamental market factors, such as market size, supply, and benchmark borrowing rates? While our results are based upon select regions in California, our results may provide insights into other US regions that featured prominent real-estate price growth during the pandemic followed by subsequent market relaxation, in particular given the ubiquity throughout the US of the underlying market factors (low interest rates, high uncertainty, supply constraints, online real-estate platform use) over our period of analysis.

2 Literature review

This work contributes to two distinct research streams. First, the empirical analysis of real-estate price dynamics, price elasticity [30–33] and the overall real-estate market’s response to exogenous shocks [34–37]. And second, the understanding of decision-making under extreme uncertainty following sudden interruptions to normal daily life [38, 39] within the context of the COVID-19 pandemic [3–6, 40].

A common methodology in the real-estate literature are hedonic regression models, applied to identify attributes associated with a given property and neighborhood that are positively and negatively correlated with property valuations. Hedonic factors include property-level features such as building type, materials and floor area, combined with important local amenities [23] such as access to public transportation [41, 42], noise pollution [29], and security of clean tap water [43–45]. Other studies identify externalities that are more pervasive, such as the shifting valuation of tree shade coverage with climate change [46]. While there is a large literature exploring such factors, we do not employ hedonic factor analysis in this work because our data source lacks consistent property-level features. Moreover, we do not model the estimated property valuation nor do we account for the final sale price. Instead, we take estimated property valuations as a given, and then analyze how valuation changes are correlated with micro-economic factors such as market size, local housing supply, and benchmark borrowing rates.

According to established economic theory, lower mortgage rates contribute to increased housing demand [31, 47]. Yet few housing market analyses are performed over periods featuring systematic urban-to-rural migration, such as observed in the US during the pandemic [48], because most studies focus principally on select large metropolitan markets. Hence, there is scant research comparing urban and rural markets within the same mega-region and period. As such, a distinction of the present work is the construction of a balanced panel of multiple neighboring regions, for both large and small market size, over a significant time horizon. We selected 10 proximate regions in CA based upon the accessibility of consistent property-level data from Zillow.com, familiarity with the region, and most importantly, regional context. In particular, California has been affected for decades by an affordable housing crisis that is concentrated in regions with high wealth inequality, the Bay Area mega-region being a case example [49, 50].

The real-estate market literature commonly uses property sales data that are aggregated at both the annual and regional level, which fails to capture market dynamics of individual properties. Instead, much research is based upon house sales transaction data aggregated as mean values over sizable regions such as US ZIP codes or census tracts [17, 19, 31, 33, 35, 51]. One relevant example is the recent work by Mondragon & Wieland [52] who use house transaction data aggregated across US counties over the period 12/2018–11/2021, reporting that a 1% increase in a region’s share of remote-work explains 0.93% increase in average house prices across the US, which accounts for roughly half of the price growth over that period analyzed. The scant availability of high-resolution data at the property level follows from the technical challenges associated with collecting data from online real-estate platforms, with a few recent exceptions [25, 26, 37].

As the unit of analysis in our study is an individual property, this work also contributes to the broader research stream on asset price dynamics [22]. Various asset classes, such as stock prices, firm sizes and human productivity, are amenable to analysis over variable time windows ranging from intraday, to monthly, to intra-annual and decadal scales [12, 53–55]. The most relevant study of real-estate market dynamics is by Landvoigt et al. [31], who analyze capital gains on sold properties over a 5-year horizon for the specific region of San Diego, CA. We are unaware of research analyzing the dynamics of individual real-estate valuations at the 1-month frequency, which is a unique feature of our property-level data source.

A final consideration regarding the extant COVID-19 research is the predominant focus on the short-term market decline in real estate markets immediately following the onset of the pandemic [17, 35, 36, 56]. This focus neglects the overwhelming market reversal that followed the initial negative market reaction. Such a narrow window also tends to disregard the pre-existing trends in market appreciation that preceded the pandemic in California, the US and elsewhere.

3 Data collection methods

3.1 Data source

The primary data used in this study come from two open data sources: the US Federal Reserve Bank of St. Louis and Zillow Inc. For each sampling month m, we collected data from the US Federal Reserve on the average US 30-year fixed rate mortgage, denoted by $M_{m}$, which provides a macro-economic indicator of borrowing costs.

From Zillow Inc. we exploit their internal system of unique property identifiers (ZPID), which facilitate property disambiguation. Consequently, we are able to assemble a city-level panel of property-level data with four notable features. The first feature refers to the unit of analysis, namely property-level data collected at high spatiotemporal resolution. When combined, these data yield a 10-region balanced panel, which distinguishes our study from literature based upon specific temporal and geographic cross-sections. Instead, we are able to compare market dynamics across markets of different size: three regions are associated with big (urban) markets (San Jose, Modesto, Fresno), and the remaining seven are associated with small (rural) markets, as proxied by the principal city population for each region.

The second feature refers to the comprehensive and algorithmically consistent generation of the data, since they derive from a single primary data source – the prominent online real-estate platform, Zillow Inc. As the top real-estate website in the U.S. in 2021 with roughly 36 million visits per month [8], Zillow Inc. is a leading real-estate platform in an increasingly ubiquitous IT service sector [57]. These primary source data are readily available to the public and have fostered data science education and research by way of open competitions [58, 59]. Importantly, Zillow provides real-time house price estimates deriving from a proprietary in-house algorithm that estimates individual house prices based upon a massive and near comprehensive historical database extending back to the mid 2000s, including ask prices elected by the sellers and subsequent sale prices. By maintaining a nearly real-time catalogue of available listings and estimated valuations, Zillow facilitates comprehensive market assessment in addition to mediating buyer-seller interactions. Alternative methods collecting ask and sales prices from regional multiple listing services (MLS) involve data collected from different brokers, realtors and sellers, and do not satisfy this consistency criterion.

The third feature refers to the unique data provided for each property on Zillow.com that facilitate developing property-level metrics for short-term valuation change, valuation uncertainty, and collective speculation. And the final feature refers to the unique conditions for quantifying speculation. Specifically, the Zillow GetSearchResults API provides property estimates for on-market properties listed for sale as well as off-market properties available for rent. In the present study, rental properties entering the habitation market played an important role in accommodating the desire to escape high population density and/or to take advantage of remote work opportunity – two factors associated with the pandemic housing market. Hence, in what follows we juxtapose the price dynamics for these two distinct classes of available real estate to estimate the impact of pandemic uncertainty on the housing market. The key distinction being that buyer-seller interactions implicitly incorporate speculation on future price movements. By contrast, rental property owners instead opt for an incremental revenue strategy based upon cash flowing from future rents, which is less dependent on property and real-estate market speculation. To be clear, data obtained for rental listings are not monthly rent estimates, but are estimated valuations of the rental property, i.e. deriving from same algorithm as those properties that are listed for sale, rendering these distinct property classes directly comparable. See the Supplementary Information (SI) (Additional file 1) Appendix for an elaboration on the data source, collection and analysis.

3.2 Data collection

We collected monthly snapshots from March 2018 to September 2021 for 10 proximal CA cities and their surrounding regions belonging to the Bay Area mega-region shown in Fig. 1(B); see Fig. S2(A,B) for monthly sample sizes. The largest principal city by population is San Jose (∼1 million inhabitants in 2021); and by area is Fresno (116 square miles); the smallest city by population is Mariposa (∼1500 inhabitants) and by area is Livingston (3.7 square miles). For spatiotemporal context, the distance separating San Jose and Fresno is roughly 150 driving miles (240 km) corresponding to 2.5 driving hours. Despite a wide variation in size, location and socio-economic backdrop, these 10 regions all feature shortages in affordable housing, a longstanding problem plaguing California and various other metropolitan areas in the United States [49, 50]. Seven of the principal cities are located along a major industrial and commuter transportation highway (CA 99), and are within the 3-hour super-commuter travel-time from the greater Bay Area, thereby qualifying as bedroom communities. Conversely, two regions (Mariposa and Oakhurst) are oriented around recreational tourism in and around Yosemite National Park. All together, these municipalities span a wide range of house prices, market size and turnover to support within and across-city analysis at high geo-temporal resolution.

In the remainder of the analysis, for data sampled between March 2018 and May 2019, we denote this sample as “before 2020”; and we denote data sampled between May 2020 and September 2021 as “after 1/2020”. See Fig. S2(C,D) for sample sizes grouped by 6-month non-overlapping periods that facilitate a visual comparison of average-property trends before and after 1/2020. In total, we analyze a dataset comprised of 57,414 individual properties listings spanning a nearly 4-year time period (2018-2021) [60].

3.3 Property-level metrics

For each unique property h, we obtained the following data from the Zillow GetSearchResults API:

• (1) the official address (including zip code and city name);

• (2) the longitude and latitude (centroid of the property);

• (3) the Zillow price estimate, termed the Zestimate®, which we denote by $P_{h,m}$;

• (4) the high and low range for $P_{h,m}$, denoted by $P^{+}_{h,m}$ and $P^{-}_{h,m}$, respectively;

• (5) the 30-day change in the $P_{h,m}$, denoted by $\delta P_{h,m}$.

Fig. 2 shows a sample Zillow webpage for a property in San Jose CA, illustrating the prominence of contemporaneous $P_{h}$, $\delta P_{h}$, $P^{+}_{h}$ and $P^{-}_{h}$ data as well as 10 years of historical data that confronts both casual and purposeful platform users.

Figure 2

Schematic of quasi-experimental design for estimating the magnitude of price shifts attributable to COVID-19 market speculation. (A) Shown is a Zillow webpage for an actual on-market property listed for sale. Red highlights indicate the primary source data obtained from the open-access Zillow Inc. GetSearchResults API; yellow highlights indicate additional standardized data that feed into the proprietary Zillow Inc. algorithm that yields real-time estimates for $P_{h}$, $\delta P_{h}$, $P^{+}_{h}$ and $P^{-}_{h}$. In addition to contemporaneous valuation estimates, users are also confronted with longitudinal $P_{h}(t)$ histories extending up to a decade, which includes actual sales events indicated in the “Price History” section of each listing page. (B) Our quasi-experimental design leverages the algorithmically consistent data ($P_{h}$, $\delta P_{h}$, $P^{+}_{h}$ and $P^{-}_{h}$) available for on-market properties listed for sale (which are sensitive to market speculation) as well off-market properties listed for rent. Rental properties represent appropriate counterfactuals in that while they are available for habitation, they are off-market, meaning that they are neutral to short-term market speculation (since the time horizon for entering the market is well beyond the horizon for contemporaneous speculation). Consequently, whereas price changes for on-market properties depend on shifts in the valuation of fundamentals in addition to market speculation, price changes for rental properties primarily reflect shifts in the valuation of fundamentals (e.g., the incremental value of an additional bedroom). Hence, this study applies a difference-in-difference (DiD) design to net out shifts in the valuation of fundamentals in order to isolate shifts attributable to speculation – see Eq. (4). Moreover, by comparing shifts after versus before 1/2020, we estimate the effect of market speculation deriving from COVID-19 uncertainty on the real-estate market

The price estimates ($P_{h,m}$ and $\delta P_{h,m}$) are calculated by Zillow Inc. based upon their proprietary in-house algorithm that incorporates a battery of hedonic factors. For example, inputs used to estimate $P_{h,m}$ include macro-economic market data (such as mortgage rates, regional and neighborhood data such as schools and similar houses), house-specific data provided by the seller and from external sources (habitation area, number of floors, construction materials and date, pool and yard dimensions, garage capacity, school district, neighborhood amenities, and other web-metrics such as house-views which are an indicator of housing demand [24]), and other properties in the neighborhood of h that are either contemporaneously for sale or were listed in the recent past.

Note that $P_{h,m}$ is not the asking price set by the listing agent, but rather an estimate of the property’s market value. As illustrated in Fig. 2(A), it is common for Zillow.com property profiles to feature up to 10 years of historical price estimates as a time series, also annotated by point events corresponding to prior ask and sales prices, which together inform buyer and seller speculation. Manual inspection of 10-year Zestimate® time series indicates that new listings and updated ask prices are rapidly incorporated into the Zestimate® algorithm [26]. This rapid information collection is a critical feature that facilitates collective co-production of market speculation deriving from individual seller and online platform service user activity. In this regard, $P_{h,m}$ represents a dynamically updated estimate of the fair market value based upon market information that is comprehensive, real-time and localized.

Notably, the Zestimate® error rate, measured as the percent difference between $P_{h,m}$ and the property’s actual sale price, has decreased over time as their proprietary algorithm becomes more accurate. According to Zillow Inc., the median error rate (such that 50% of property valuation errors are less than this value) for on-market homes was 3.2% during our sampling period, and has since decreased to 2.4% [61].

These unique features of Zillow property data – namely, the comprehensiveness, consistency, dynamics and accuracy – facilitate analyzing the evolution of the housing market in specific regions at high geographic and temporal resolution. Without this rich data source, the next best alternative would be to pool records of seller ask prices. However, such data would not be consistent and would not include dynamics, as the ask price occurs at a fixed date and does not tend to change over a 30-day time window. Instead, the Zestimate® is updated in real time. Also, seller ask prices do not include a price range, and so they do not permit analysis of valuation uncertainty.

We constructed our panel of Zillow property estimates by sampling Zillow.com monthly for over 4 years. As such, price values were obtained in nominal US$ at the sampling month m. Hence, in what follows, we deflated all price values to 1/1/2018 US$. We control for the data sampling (calendar) month in our statistical analysis to account for well-known intra-annual housing market activity cycles [62].

Based upon the primary data from Zillow.com, we also computed three additional metrics. First, we calculated the price change as a percent of the initial price,

$$\begin{aligned} \Delta P_{h,m} =100 \times \frac{\delta P_{h,m}}{P_{h,m}-\delta P_{h,m}} \ . \end{aligned}$$

(1)

See Fig. S2(E,F) for the mean and standard deviation of $\Delta P_{h,m}$, grouped by period and property type. Second, we calculated the spot price uncertainty,

$$\begin{aligned} U_{h,m}=100 \times \frac{P^{+}_{h,m}-P^{-}_{h,m}}{P_{h,m}} \ . \end{aligned}$$

(2)

See Fig. S2(G,H) for the mean and std. dev. of $U_{h,m}$, grouped by period and property type. And third, we estimated the neighborhood housing market activity $A_{h,m}$ of a particular listing h by counting the total number of properties within a 0.5 mile (0.8 km) radius, and within the contemporanous three-month period $\{m_{h}-2, m_{h}-1,m_{h}\}$ including the listing month $m_{h}$.

4 Data analysis methods

4.1 Cauchy-Lorentz distribution of 30-day price change, ΔP

Both the positive and negative tails of $P(\Delta P_{h})$ are heavy, extending well beyond the values of $\pm 40\%$. Hence, to avoid parameter estimates in our regression model being biased by extreme outliers, we exclude properties with $\Delta P_{h} > 40$%; see the SI Appendix for additional details. We estimated a best model for the $P(\Delta P_{h})$ distribution using the maximum likelihood method. The best-fit probability density function (PDF) is the Cauchy-Lorentz distribution,

$$\begin{aligned} P(\Delta P) = \frac{1}{\pi \gamma \big(1+(\frac{\Delta P-x_{0}}{\gamma})^{2} \big) } \ , \end{aligned}$$

(3)

which has asymptotic power-law tail behavior $P(\Delta P) \sim \Delta P^{-2}$ for $\vert \Delta P - x_{0}\vert \gg \gamma$. The two Cauchy-Lorentz PDF parameters estimated using both big and small market data pooled together are $x_{0} = 0.2$ (location) and $\gamma = 2.0$ (scale). As illustrated in Fig. 3(A), the vast majority of observations are located around $x_{0}$, with 89% of properties feature $\vert \Delta P_{h} \vert \leq 10$%.

Figure 3

Systematic increase in property valuation and confidence in the after-1/2020 housing market. Kernel density estimate of the probability density function (PDF) calculated for (A) 30-day price change, $\Delta P_{h,m}$, including the best-fit Cauchy PDF calculated using both the big and small market data combined; and (B) PDF calculated for price uncertainty, $U_{h,m}$. Data shown are calculated using properties listed “For Sale”; see Fig. S4 for PDF conditioned on market size, period and property type. (C-F) Mean ($\langle \cdot \rangle$) and standard deviation (STD, $\sigma [\cdot ]$) calculated for $\Delta P_{h,m}$ and $U_{h,m}$ conditional on spot price $P_{h,m}$. Together, these two variables show how the after-1/2020 CA housing market features excess valuation growth and increasing valuation confidence (i.e., decreased uncertainty), patterns that are common to both the big and small markets, and appear to be even stronger for the small market. These effects manifest as systematic shifts in the first and second moments – i.e., the characteristic location (C,D) and characteristic fluctuation scale (E,F) – of the underlying data distributions, and are robust across the entire range of house listing price estimates

4.2 Quantifying the effect of COVID-19 on speculative valuation in a CA real-estate market

We use the rapid onset of the pandemic as an exogenous shock to uncertainty, which thereby facilitates estimating the degree to which shifts in property valuation and valuation confidence during the pandemic were attributable to collective speculation. Our approach contributes to a growing body of quasi-experimental COVID-19 research in the social sciences [40].

As a consistency check, we implemented two complementary quasi-experimental methods: (a) unit-level matching and (b) multivariate regression. Unit-level matching of individual properties leverages the granularity of our data sample to estimate treatment effects manifesting at high spatiotemporal resolution. Instead, multivariate regression yields inferences based upon differences in group-level averages, with the notable advantage that additional regressors can be included in order to control for micro-level (e.g., number of neighboring properties listed for sale, $A_{h,m}$) and macro-level covariates (e.g., contemporaneous mortgage rates, $M_{m}$).

Fundamental to both methods is identifying a counterfactual baseline to net out differences pre-existing the pandemic. To this end, both approaches utilize the rental market – comprised of properties that satisfy the same demand for housing, but were just not available for sale and thus were neutral to contemporaneous speculation – as a counterfactual baseline for comparison. Accordingly, both approaches rely on the parallel trend assumption between on-market (denoted by “For Sale”, FS) and off-market (“Rent”, R) property types, which we demonstrate in Fig. S7.

The logic underpinning this counterfactual approach is as follows. Whereas shifts in the valuation of on-market properties depend on shifts in the valuation of fundamentals in addition to market speculation, shifts in the valuation of off-market properties primarily reflect shifts in the valuation of fundamentals. Hence, we can estimate the impact of speculation on a given quantity Y by way of a difference-in-difference (DiD) strategy denoted by

$$\begin{aligned} \Delta \overline{\Delta}_{Y} := \overline{\Delta}_{\text{Y,FS}}- \overline{\Delta}_{\text{Y,R}} = \Delta (\text{Speculation})\ , \end{aligned}$$

(4)

as illustrated in Fig. 2(B). More specifically, we apply this strategy to estimate the effect of the COVID-19 pandemic on two quantities that are sensitive to uncertainty: $Y=\Delta P_{h}$ and $Y=U_{h}$. Note that Eq. (4), which we further specify in the following section, inherently incorporates a temporal difference between the before and after 1/2020 periods. This second difference implies that the DiD $\Delta \overline{\Delta}_{Y}$ is net of the baseline level of the market before 2020, meaning that this estimator quantifies the magnitude of price shifts specifically attributable to the speculation in the CA real-estate market deriving from COVID-19 uncertainty.

4.2.1 Method 1: unit-level matching

The quasi-experimental matching design leverages notable advantages. Foremost, this approach accounts for unobserved covariates that are nonetheless correlated with the available matching variables. In the present case, while we do not explicitly incorporate house-specific features – such as vicinity to shopping and schools, backyard size and other physical amenities such as a pool and garage – these and many other variables are fundamentally incorporated into each $P_{h,m}$ produced by the Zillow algorithm, and used in the counterfactual matching stage. Moreover, by virtue of its design as a leading e-platform [63] that derives value by aggregating comprehensive and contemporaneous local and national house listings, $P_{h,m}$ values are believed to be consistent and thus well-suited for the purpose of unit-level matching.

Our matching design also exploits the high geo-temporal resolution of the listing data to match properties listed after 1/2020 with similar properties listed before 2020, thereby optimizing measurement precision in the evaluation of market shifts due to pandemic uncertainty. An advantage of this approach is addressing the high degree of price and price change variation that exists even within a single region, as illustrated in Fig. 1(C). To be specific, we account for unobserved unit-level features [64] by strictly matching houses according to three listing features: (a) price strata associated with $P_{h,m}$; (b) calendar month m; and (c) geographic longitude and latitutde of h.

We match on price strata by first calculating an intensive variable $Q_{c}(P_{h,m}) \in 1, 2 \ldots 10$. The quantile $Q_{c}=1$ (respectively, $Q_{c}=10$) represents the lowest (highest) price decile that is a specific to a particular city c and before/after period. Assuming that potential buyers would be open to a range of house prices in excess of a single decile, we then allow for matches within ±1 decile group from $Q_{c}(P_{h,m})$. We constrained matches temporally by requiring matched houses from the same calendar month or 1 calendar month prior of the central house, which accounts for intra-year housing market cycles. For example, if a property was listed in June, then we only accept properties listed in May or June as candidate matches. And we constrained matches geographically by requiring matched houses to be within a 0.5 mile (0.8 km) radius of the central house.

By way of example, Fig. 1(G,H) illustrates the matching procedure using a property from San Jose listed after 1/2020, which also exhibits the reduction in market supply after 1/2020 relative to before. Note that not all houses within the specified radius are candidate matches because the price variations in a single neighborhood can span several $Q_{c}(P_{h,m})$ strata. In Fig. 1(G) we denote the set of matched houses in the same neighborhood of a given central house h by $\{N_{h}\}_{\text{Bef}}$.

More specifically, for each property h listed after 1/2020, we identify the match set $\{N_{h}\}_{\text{Bef}}$ from the pool of similar properties listed before 2020. We then construct a hypothetical property listed before 2020 that is very similar to h. Ideally, the counterfactual property would be the same property h using data sampled from before 2020. Unfortunately, the Zillow API only returns data contemporaneous to the data download date, and so we are unable to back-sample prior valuation data for any given property h. In order to overcome this challenge, a more sophisticated research design would need to identify a repeated sampling procedure to obtain a balanced Zillow estimates for the same set of properties over time, which was beyond the scope of our data collection capability, and is a limitation shared by most real-estate analyses using on-market property data.

The characteristics of the counterfactual property are given by the average value $\langle Y \rangle _{\{N_{h}\} \text{Bef}}$ calculated across the match set $\{N_{h}\}_{\text{Bef}}$, where Y represents either $P_{h,m}$, $\Delta P_{h,m}$ or $U_{h,m}$. As such, we then compute the counterfactual difference

$$\begin{aligned} \Delta _{Y,h} = Y_{h, \text{Aft}} - \langle Y \rangle _{\{N_{h}\} \text{Bef}} \ , \end{aligned}$$

(5)

which estimates the shift in Y associated with the two time periods for each h. In a companion study, we perform a similar analysis by instead matching first across property types within each time period, and then computing a temporal difference. This approach is more constrained by smaller R sample sizes for the period after 1/2020, yet we obtain largely consistent results [37].

From the set of $\Delta _{Y,h}$ values collected for each region and property type, we then calculate the average difference

$$\begin{aligned} \overline{\Delta}_{Y} = \langle \Delta _{Y,h} \rangle \ , \end{aligned}$$

(6)

where we denote the property type in subscript, e.g. $\overline{\Delta}_{Y,FS}$ and $\overline{\Delta}_{Y,R}$. The impact of the COVID-19 pandemic on the variable Y is then estimated according to the magnitude and statistical significance of $\overline{\Delta}_{Y}$. We evaluate the latter using a one-sample Student T-test to estimate the likelihood of the null hypothesis $\overline{\Delta}_{Y}=0$ representing no pandemic effect. Fig. 4(A-C) show the sign, magnitude and statistical difference of $\overline{\Delta}_{Y}$ calculated for the three property-level variables $P_{h,m}$, $\Delta P_{h,m}$ or $U_{h,m}$. See Fig. S5 for the distribution of individual $\Delta _{Y,h}$ values from which $\overline{\Delta}_{Y}$ are calculated; and see Fig. S6 for $\overline{\Delta}_{Y,c}$ calculated at the city level as a demonstration of robustness over down-scaled regions.

Figure 4

Estimation of housing market valuation shifts attributable to COVID-19. (A-C) $\overline{\Delta}_{Y}$ is the distribution average of the unit-level difference $\Delta _{Y,h} = Y_{h, \text{Aft}} - \langle Y \rangle _{\{N_{h}\} \text{Bef}}$ calculated for the variable Y across properties listed after 1/2020. The counterfactual baseline $\langle Y\rangle _{\{N_{h}\} \text{Bef}}$ is calculated using the set of matched properties that were listed before 2020 (denoted by $\{N_{h}\}_{\text{Bef}}$). In this way, matching facilitates a more precise estimation of the impact of COVID-19 on individual properties. Error bars indicate the standard error of the mean and stars indicate the significance level of a T-Test for the likelihood of the null hypothesis $\overline{\Delta}_{Y}=0$. Each gray bar represents the difference-in-difference $\Delta \overline{\Delta}_{Y} = \overline{\Delta}_{Y,FS} - \overline{\Delta}_{Y,R}$, which is an estimator for the effect of COVID-19 speculation on Y. Note that each market-level $\Delta \overline{\Delta}_{Y}$ is directly comparable and consistent with the corresponding city-level treatment effect $\delta _{TE,Y}$ shown in panel (D), where San Jose and Fresno are big markets, and Merced is a small market. (A) The difference in the price estimate ($Y = P_{h,m}$; all values deflated to 1/2018 US$) shows the average price change for listings after 1/2020. (B) The difference in price change ($Y = \Delta P_{h,m}$) measures shifts in price valuations at high temporal resolution (30-day), and shows that properties listed for sale had excess price valuation relative to those listed for rent. (C) The difference in price uncertainty ($Y =U_{h,m}$) is inversely related to valuation confidence. In the case of properties listed for sale, we observe a 1-percentage point reduction in price-uncertainty, i.e. higher valuation confidence; conversely, we observe drastic price uncertainty increases for rental properties. (D,E) Summary of the COVID-19 treatment effect $\delta _{TE,Y}$ on properties listed for sale, based upon results from a two-period difference-in-difference multivariate regression model. To summarize, average percent price change values increased between 0.85 and 1.21 percentage points, and price uncertainties declined between 3 and 9 percentage points, relative to the baseline levels they plausibly would have maintained in the absence of the pandemic. Note that in both cases, this treatment effect corresponds to properties listed for sale. Error bars represent the 95% confidence interval in each point estimate; full table of parameter estimates are reported in Tables S1-S2. Significance levels indicated by the asterisks: * $p < 0.05$, ** $p < 0.01$, *** $p < 0.001$

Hence, the difference in difference $\Delta \overline{\Delta}_{Y}$ defined in Eq. (4) nets out the overall market shifts that may bias interpretation of $\overline{\Delta}_{Y,FS}$ when considered alone. What remains after subtracting our speculation-neutral baseline for comparison $\overline{\Delta}_{Y,R}$ is the excess impact attributable to speculation implicit in property sales. We evaluate the statistical significance of the null hypotheses $\Delta \overline{\Delta}_{Y}=0$ using the two-sample Student T-test with Welch correction that accounts for varying sample-size and variance between the FS and R samples.

4.2.2 Method 2: multivariate regression

We complement the matching method with multiple regression, which affords estimating marginal relationships with temporal and spatial covariates. In what follows we implement a two-period difference-in-difference (DiD) model for three regions (San Jose, Fresno, Merced) for which sufficient rental property data are available to serve as the before- and after-1/2020 control group. In short, we apply ordinary least squares (OLS) regression using STATA 13.0 software to estimate the following model for a specific region,

$$\begin{aligned} Y_{h,m} = \delta _{TE} (I_{h,\text{ForSale}}\times T_{m}) + \vec{\beta} \cdot \vec{X} + \vec{\gamma} \cdot \vec{I} + \epsilon \ , \end{aligned}$$

(7)

where X⃗ (respectively, I⃗) represents a battery of continuous (respectively, factor) controls, and the DiD interaction term $\delta _{TE} (I_{h,\text{ForSale}}\times T_{m})$ captures the difference between the two property types (specified by the binary indicator variable $I_{h,\text{ForSale}}$) across the two periods (specified by the binary indicator $T_{m}$). Figure S7 shows that the conditions of the DiD parallel trend assumption in the period before 2020 are sufficiently satisfied for both $\Delta P_{h,m}$ and $U_{h,m}$. And for additional cross-validation, see the study by [17] analyzing repeated-transaction home price data within and across the 25 largest metropolitan statistical areas during the before-2020 period. And regarding the exclusion restriction on the treatment, one can verify this assumption by using Zillow.com to manually inspect properties listed for rent, and compare them to those that are listed for sale to see that there are no systematic a priori differences between the two property types.

We apply this canonical two-period DiD specification to model two different dependent variables: $Y = \Delta P_{h,m}$ and $Y = U_{h,m}$. For each model we implement fixed-effects to account for time-independent factors associated with the calendar month m of the listing ($C_{m}$), and region-specific price strata $Q_{c}(P_{h,m})$, where both quantities are encoded as categorial variables. Hence, the treatment effect $\delta _{TE}$ is the direct analog to $\Delta \overline{\Delta}_{Y}$, and estimates the excess shift in Y attributable to collective speculation deriving from COVID-19 uncertainty.

In the first scenario where the dependent variable is the 30-day percent price change, the model specification is

$$\begin{aligned} \Delta P_{h,m} =& \delta _{TE,\Delta P} (I_{h,\text{ForSale}}\times T_{m}) + \beta _{U} (U_{h,m} \times I_{h,\text{ForSale}}) \\ +& \beta _{U^{2}}(U^{2}_{h,m} \times I_{h,\text{ForSale}}) + \vec{\beta} \cdot \vec{X} + \vec{\gamma} \cdot \vec{I} + \text{const.} + \epsilon \ , \end{aligned}$$

(8)

where the covariates are $\vec{\beta} \cdot \vec{X} = \beta _{M} M_{m} + \beta _{A}(A_{h,m} \times I_{h,\text{ForSale}}) + \beta _{A^{2}}(A^{2}_{h,m} \times I_{h, \text{ForSale}}) + \beta _{P} \ln P_{h,m} + \beta _{P^{2}} \ln ^{2} P_{h,m}$ and the factor variables are $\vec{\gamma} \cdot \vec{I} = \gamma _{I} I_{h,\text{ForSale}} + \gamma _{T} T_{m} + \gamma _{Q}Q_{h} + \gamma _{C}C_{m}$. The interaction between $I_{h,\text{ForSale}}$ and several control variables differentiate responses conditional on property type. Full model estimates are elaborated in Table S1.

Similarly, in the second scenario where the dependent variable is the percent price uncertainty, the model specification is

$$\begin{aligned} U_{h,m} =& \delta _{TE,U}(I_{h,\text{ForSale}}\times T_{m}) + \beta _{ \Delta P} (\Delta P_{h,m} \times I_{h,\text{ForSale}}) \\ +& \beta _{\Delta ^{2} P} (\Delta ^{2} P_{h,m} \times I_{h, \text{ForSale}}) + \vec{\beta} \cdot \vec{X} + \vec{\gamma} \cdot \vec{I} + \text{const.} + \epsilon \ . \end{aligned}$$

(9)

Full model estimates are elaborated in Table S2.

5 Results

5.1 Descriptive statistics grouped by region and period

The quantity $P_{h,m}$ is an extensive variable, and its distribution is approximately log-normal – see Fig. S3. This result is consistent with the Gibrat proportional growth model developed in the context of financial assets and firm growth [12, 55]. Instead, our focal variables $\Delta P_{h}$ and $U_{h}$ are intensive quantities measured as percentages. In the case of $\Delta P_{h}$, the frequency distribution $P(\Delta P_{h})$ shown in Fig. 3(A) features high levels of variance around the roughly 1-2% average price growth levels observed during the sample period. In terms of its shape, $P(\Delta P_{h})$ is asymmetric and leptokurtic, being wider in the bulk than the Laplace (double-exponential) tent-shaped growth distributions observed in other empirical studies of economic growth [53–55, 65].

The tails of $P(\Delta P_{h})$ extend well beyond 10%, indicating that fluctuations in this real estate asset class are more similar to the heavy-tailed price fluctuation distributions observed for the equity asset class [66]. One explanation for the heavy tails is the large scale of real estate depreciation that can occur over the lifetime of ownership, balanced on the other side by relatively sudden appreciation attributable to renovations. Put another way, when a property enters the real-estate market, there is a rapid update in asset valuation that incorporates information that had accrued over wide-ranging time scales. This is of course not dissimilar from stock markets, where the periodic release of earnings and other news are rapidly absorbed into stock prices [21].

In the case of $U_{h}$, this quantity also shows considerable variation, and is narrowly centered around the 10% level, but with significant right-skew – see Fig. 3(B). By way of comparison, consider the distribution $P(\Delta P_{h})$ calculated for properties listed for sale, for which we observe a systematic shift towards an excess frequency of $\Delta P_{h}>0$ values after 1/2020 relative to before. Conversely, in the case of $P(U_{h})$ we observe the opposite trend, signaling increased valuation confidence after 1/2020 relative to before. Interestingly, in the case of rental properties, we observe no shift in $P(\Delta P_{h})$ comparing before and after, whereas the frequency of larger $U_{h}$ values post-1/2020 increases dramatically, possibly reflecting COVID-19 eviction moratorium policy rapidly implemented in the US [67–69]. See Fig. S4 for complementary distributions conditioned on market size, period and property type.

5.2 Prominent shifts in real-estate valuation during COVID-19

Using the CA real estate market before 1/2020 as a comparative baseline, Fig. 3 shows that the post-1/2020 market feature hallmarks of a speculative bubble – namely (a) accelerated valuation growth net of change in fundamentals and (b) increased confidence in excess valuation. Somewhat ironically, these characteristics may have emerged by way of contagious spreading of ‘irrational exuberance’ among market agents [13, 15, 16] who increasingly interact, explicitly and implicitly, in collective information communication platforms [1, 10, 14, 57, 63].

One explanation for the enhanced real-estate speculation derives from the global COVID-19 uncertainty shock, which muddled global expectations for investment returns. This global shock resulted in a confounding and non-uniform impact on the public, as indicated by a diverging “K-shaped” recovery in the US population [70]. The shock was also followed by profound policy interventions, such as the sudden reduction of the US federal funds target rate taking the form of a long-lasting financial-quake [20, 21], which among other immediate effects, promoted aggressive household borrowing that boosted home-purchasing power and home-improvement activity [71]. This also triggered a sudden housing supply-demand imbalance exacerbated by the rapid expansion of remote work-from-home policy [17, 52], in particular in the IT sector that is concentrated in the Bay Area mega-region. While these factors primarily affect the house purchase market, they also affected the rental market, given the coincident increased demand for rent combined with sudden rent protection policy that together shifted risk-levels for both tenants and rental property owners [68].

Combined, these factors are reflected by significant systematic shifts in the characteristic levels of speculation ($\Delta P_{h,m}$) and uncertainty ($U_{h,m}$) across the entire range of $P_{h,m}$ – for both small and big markets. Notably, we observe higher average $\Delta P_{h,m}$ in small markets than in big markets, consistent with nationwide analysis of the impact of state-level shutdowns on price changes in the months before and after their implementation, which were found to be mediated by differences in population and structural density between urban and rural markets [35].

Compared with recent work analyzing the real estate market in southern CA that finds a negative relation between price growth and price [31], a relation that is consistent with other asset classes such as firms and stocks [55], we instead observe an increasing trend in $\langle \Delta P_{h,m} \rangle$ with $P_{h,m}$ after 1/2020, which is indicative of accelerated speculation – see Fig. 3(A). This shift is also readily apparent in the higher levels of price-growth variation ($\sigma [\Delta P_{h,m}]$) observed after 1/2020 – see Fig. 3(B). Again, this pattern deviates from the well-established decreasing size-variance relationship found for other asset classes [12, 53, 55, 65, 66, 72]. Contrariwise, Fig. 3(C,D) indicate a reduction in mean and standard deviation of price uncertainty after 1/2020, also consistent with the conditions of a speculative bubble.

5.3 Property-level matching

We first consider results for $Y = P_{h,m}$, which we report primarily for the purpose of demonstrating that the magnitude of price shifts we encountered are not incremental. However, because the same quantity is also incorporated into the matching variable $Q_{c}(P_{h,m})$, we do not explore these results in depth. Fig. 4(A) shows $\Delta \overline{\Delta}_{P} = \overline{\Delta}_{P,FS}- \overline{\Delta}_{P,R}$ of roughly 8000 US$ for both market sizes. This result indicates that the same property h listed for sale is valued 8000 US$ more than if it was listed as available for rent, a result which is significant at the $p<0.001$ level.

The quantities $\Delta P_{h}$ and $U_{h}$ are intensive quantities. Hence, each quantity is more directly comparable across time periods and property types, while also being less correlated with the matching variable $Q_{c}(P_{h,m})$. In the case of percent price changes, Fig. 4(B) indicates excess valuation growth over a 30-day period of $\Delta \overline{\Delta}_{\Delta P} = \overline{\Delta}_{\Delta P,FS}- \overline{\Delta}_{\Delta P,R} = 1.36-0.26 =$ 1.1 percentage points for the average property in the big market, and $1.47-(-0.53) = 2.0$ percentage points for the small market. Both DiD values are significant at the $p<0.001$ level. In the latter case, this result suggests that the valuation of the same property h would appreciate an additional 2% percentage points more if it were listed for sale, as opposed to if it were instead listed as available for rent. In terms of the magnitude of this effect on properties listed for sale, the increase in $\Delta P_{h,m}$ is more than double the characteristic levels observed prior to the pandemic – see Fig. 3(A).

In the case of percent price uncertainty, Fig. 4(C) shows a $\Delta \overline{\Delta}_{U} = \overline{\Delta}_{U,FS}- \overline{\Delta}_{U,R} = -2.4$ percentage point decrease for the big market, and $\Delta \overline{\Delta}_{U}= -7.2$ percentage point decrease for the small market. Both DiD values are significant at the $p<0.001$ level. This result indicates that the certainty in the valuation of a property is higher if it were listed for sale than if it were listed as available for rent.

5.4 Multivariate regression

Fig. 4(D,E) shows the 2-period DiD “treatment effect” $\delta _{TE}$ estimated for the models specified in Eqs. (8) and (9), respectively. Results indicate an excess 30-day percent price change of $\delta _{TE, \Delta P}=0.85$ (Fresno), 1.13 (Merced) and 1.21 (San Jose) percentage points. These values are consistent in sign, magnitude and statistical significance with the corresponding market-level DiD values $\Delta \overline{\Delta}_{\Delta P}$ estimated using the matching method. Both methods indicate excess valuation, or higher valuations than there would have been in the absence of COVID-19 market shock, which is consistent with prior theory of housing-market speculation [2, 15].

Results instead indicate declines in price uncertainties (i.e., increases in valuation confidence) attributable to the pandemic: $\delta _{TE,U} = -3.1$ (San Jose), −3.6 (Fresno) and −8.9 (Merced) percentage points. As a robustness check, we confirm that each point estimate $\delta _{TE,U}$ is consistent in sign, magnitude and statistical significance when compared with the corresponding market-level $\Delta \overline{\Delta}_{U}$ values estimated using the matching method.

5.5 Marginal effects of market supply and mortgage rates

To further explore the relative impact on price change and uncertainty, Fig. 5 shows the margins associated with (a) neighborhood market activity $A_{h,m}$, a micro-level indicator of housing supply measured as the number of potentially competing listings in the immediate vicinity of h; and (b) the average 30-year fixed-rate mortgage $M_{m}$ reported by Freddie Mac®, which is an inverse measure of homeowner borrowing power.

Figure 5

Marginal effects of local market supply and mortgage rate on price change and uncertainty. (A,B) Predictions of the relationship between the supply of alternative houses (defined as the number of matched houses within the same period as the central house listing, $A_{h,m}$) and price change $\Delta {P}_{h}$. Positive shift in $\Delta {P}_{h}$ of roughly 0.5 percent after 1/2020 relative to before, which diminishes at higher levels of market supply for both small and big markets. (C,D) Predictions of the relationship between the average 30-year US Mortgage rate (Fixed rate, shown as percentage) and $\Delta {P}_{h}$. Positive shift on the order of 0.4 percent for both small and big markets. (E-H) Similar to panels (A-D) but showing the OLS model predictions for price uncertainty. As expected, the uncertainty associated with COVID-19 is more clearly manifest in the market valuation uncertainty than the price dynamics. Counterintuitively, the increased levels of uncertainty associated with the pandemic appear to have reduced uncertainty in price estimations, which points to the amplification of market speculation during this period of global stress. Shaded areas indicate 95% confidence interval around the predicted margins of response indicated by the dashed line. All marginal effects are calculated using covariates maintained at their mean values

The specification used to estimate these marginal effects is nearly identical to the DiD models described above. The main difference is we do not include the DiD term ($I_{h,\text{ForSale}}\times T_{m}$). Instead, this model includes an interaction $S_{h} \times A_{h,m} \times T_{m}$ in order to quantify the marginal effect of neighborhood market activity $A_{h,m}$ associated with $Y=\{ \Delta P_{h,m} \text{ or } U_{h,m} \}$, while accounting for differences in period and market size. Full model estimates are elaborated in Table S3.

Figs. 5(A-D) provide an estimate of the semi-elasticity of price with supply, and are consistent in magnitude with prior empirical work by [30] on the full elasticity of housing supply conditioned by land development constraints. For example, an additional 10 local listings (i.e. $A_{h,m}$ shifting from 10 to 20) corresponds to a reduction in price change of roughly 0.6 (resp. 0.7) percentage points for the small (resp. big) market before 2020; however, after 1/2020 this reduction increased in magnitude by roughly 0.1 percentage points for both markets as indicated by the increasingly steep slope after 1/2020.

Another factor explaining price gains during this period are the lower interest rates that directly affect buyer purchasing power and builder construction costs [47]. The slope of the lines shown in Fig. 5(C,D) provide an estimate of the mortgage rate semi-elasticity, indicating a roughly 0.7 percent price increase for a 1 point reduction in $M_{m}$, which is on the lower side but consistent with estimation based upon a wide range of approaches [32]. The discrepancy may be attributable to the relatively low range of $M_{m}$ and relatively high monthly price changes encountered during our sample period. Note that the estimation for smaller (larger) interest rates for before (after) 1/2020 are extrapolations into out-of-sample $M_{m}$ regimes, as indicated by the larger standard errors indicated in the regression fit.

Another relevant analysis for comparison is one based upon the San Diego housing market from 1997-2008, which attributes higher price gains for houses at the lower end of the price distribution to cheaper credit [31]. While we do not explicitly explore the interaction between $M_{m}$ and ΔP conditional on P, we do not see evidence of the differential price gains by price segment over this period for big vis-a-vis small markets, as also indicated by Fig. 3(A).

Fig. 5(E-H) show analog response margins associated with price uncertainty $U_{h}$. For both big and small markets, uncertainty levels tempered after 1/2020 relative to before, corresponding to higher levels of valuation confidence for the same levels of neighborhood supply. Counterintuitively, this result indicates more efficient price discovery [73], despite greatly heightened socio-economic uncertainty. Interestingly, the informational signal captured by $A_{h,m}$ diminished during the pandemic in the small market, as indicated by the relatively flat profile in Fig. 5(F).

6 Discussion

Quasi-experimental contribution to the COVID-19 pandemic literature: The rapid emergence of the global pandemic, followed by pervasive mitigation policy, had broad yet uneven impacts across society [3, 4, 6, 67–70]. Against this backdrop, here we contribute to the rich literature emerging from this global crisis [40] by utilizing this sudden uncertainty shock to analyze the collective dynamics of real-estate price formation.

The pandemic perturbed the housing market, a correlated multi-scale complex systems [11–13], in several critical ways. First, the pandemic shifted social interactions towards virtual modes, which increased the importance of online real-estate platforms as decision-making tools. The subsequent interruption to everyday life had immediate effects, as documented in research showing that US counties featuring stay-at-home orders also had higher property sale prices [35]. Other perturbations include global supply chain disruptions [74] that negatively impacted building costs and exacerbated supply inelasticity [33], two features that are central to the theory of emergent housing bubbles [2, 15]. These supply factors were complemented by the expansion of remote-work options, which effectively increased the search radius of buyers, and decreased the overall demand for amenity density [17]. Another pertinent contextual factor in California are the pervasive regulations regarding real-estate development and new home construction [50].

Viewed from a longer perspective, the US real-estate market has been steadily transforming since the housing boom leading into the bust of 2007-2008. In particular, the growth of the IT service economy [57, 63] has brought online real-estate platforms to ubiquity [8], with roughly 110 million distinct properties tracked by Zillow Inc. [9], corresponding to roughly 3 out of every 4 of the 142 million housing units tracked by the US Census Bureau in 2021. In addition to updating on-market and off-market property data, Zillow also calculates algorithmically consistent property valuations that are increasingly relevant to price formation in the US real-estate market.

The utility of such comprehensive and rapidly-updated market data extends far beyond active buyers and sellers. According to a recent industry survey [1], 75% of the respondents classify their time casually browsing real-estate platforms as an imagination outlet, with only 17% claiming to search listings with serious home-purchase motivations. This statistic suggests that, in addition to fundamental shifts in supply and demand, the extreme levels of price growth during the pandemic may be attributable to behavioral phenomena, heightened levels of life-course uncertainty, and an increased prevalence of naive speculators that are important contributors to bubble formation [10, 14]. Hence, inasmuch as real-estate platform service providers facilitate crowd-sourcing, browsing, and market-making, they also facilitate analyzing the dynamics of speculation at high resolution and vast scale.

Methodological and empirical contributions to the real-estate market literature: In order to address our three research questions, we first constructed a high-resolution multi-region balanced panel comprised of individual property valuation estimates, which thereby facilitates inferential econometric analysis. Our main result is estimating the excess price growth attributable to the COVID-19 pandemic by way of two complementary econometric DiD approaches: unit-level matching and multivariate regression.

Our property-level dataset combined with a pre-post model design leverages the systematic comparison of price estimates for on-market properties listed for sale versus off-market listings for rent, the difference corresponding to the effect of pandemic uncertainty on price speculation. Another unique feature of our panel is its regional composition, including both big (urban) and small (rural) real-estate markets. In our first DiD approach, we matched house listings based upon the set of available characteristics (listing month, price strata, longitude-latitude of the property) to optimize around precision in the calculation of the effect size [64]. In the second DiD approach, we implemented a canonical 2-period and 2-group model that incorporates additional covariates while also exploiting the different valuation and socio-economic features of renting versus buying that were exacerbated during the pandemic. Both approaches yield consistent results summarized in Fig. 4. Also, as the 10 regions analyzed capture a relatively wide variation in size, location and socio-economic backdrop, there is reason to believe our results apply to other US regions with housing markets similar to the Bay Area mega-region that also featured heightened price growth over the same period.

Limitations: Our data and methods are characterized by various limitations. One limitation of our data sample is the lack of additional property-level feature data. As such, unobserved factors may bias the $\delta _{TE,\Delta P}$ and $\delta _{TE,U}$ estimates produced by the multivariate regression method. Relevant omitted variables include construction supply constraints [50, 74], the regulatory environment for affordable housing construction [35], shifts in demand for amenity density [17], and remote-work and associated migration [33, 52]. These estimates may be further biased by spatial autocorrelation, which may call for more advanced econometric methods employing spatial lag variables. However, we do note that our matching method accounts for time independent spatial autocorrelations, which are neutralized in the first difference applied in Eq. (5).

For this reason, we complemented the regression method by a matching method, which constructs a hypothetical counterfactual property according to three matching factors: price, location and calendar listing month. In particular, we assume that the estimated price $P_{h,m}$ incorporates omitted variables in a consistent way. Hence, in matching properties according to price and location, we are able to factor out the missing idiosyncratic property details that contributed to each property’s valuation.

Another notable limitation of our study is the inability to account for two complementary demand-side factors, namely the shift towards remote work and the coincident emergence of online market intermediaries, or iBuyers. Regarding the former, recent work shows that an increasing prevalence of remote work, and subsequent housing demand shifts associated with migration, explains roughly half of the aggregate price changes over 2019-2021 [52]. Meanwhile, recent analysis on the emerging paradigm of instant-offer iBuyer platforms finds that the profitability of this emerging industry is highly impacted by valuation uncertainty [75]. Consequently, despite our analysis subsuming these factors, we are unable to cross-validate or contribute additional insights regarding their role in market speculation.

7 Conclusion

We analyzed the impact of the COVID-19 pandemic shock using a property-level dataset including unique measures of uncertainty and speculation. Despite the drastically increased levels of uncertainty surrounding the scope and duration of the global pandemic, our results indicate a counterintuitive decrease in property-level price uncertainty ($U_{h,m}$). At the same time, we employ two complementary methods to estimate $\Delta \overline{\Delta}_{\Delta P}$ and $\delta _{TE,\Delta P}$, respectively, which quantify the excess price growth attributable to heightened levels of pandemic speculation. Both methods yield consistent estimates, on the order of 1% per month excess price growth, i.e. above the levels of growth that would be expected in the absence of the pandemic, corresponding to roughly +12.7 percentage points when integrated across an entire year. For context, this effect size accounts for more than half of the actual annual growth observed across these same regions in 2021. The coincidence of accelerating price growth and valuation confidence is a hallmark of a speculative bubble, which we found to be stronger in the smaller housing markets, and likely reflects their greater susceptibility to sudden supply contraction.

Considered together, these results are harbingers of ‘irrational exuberance’ [16] in response to the sudden shock to long-term certainty that augmented the dynamics and scale of collective speculation. These findings, when contextualized against the backdrop of major life-course decision-making, are reconciled by behavioral theory regarding the persuasive power of uncertainty [38] and sudden unexpected interruptions [39]. Considered in this light, while also accounting for the magnitude of severity and surprise of this global shock, we speculate that the response to COVID-19 uncertainty and subsequent daily life interruptions combined with the real-time inflow of market information collected by online real-estate platforms may have contributed to collective herding behavior that is central to speculative bubble formation in complex socio-economic systems [11–16].