Praying for Rain

This paper studies rainmaking as an instrumental religious belief. The central research question is: why do people believe that prayer can bring rain, even though it does not work? The authors develop a model of cultural evolution in which a religious leader prays for rain at an arbitrary time, and people update their beliefs about whether the leader can cause rainfall based on whether rain follows. The key mechanism is the local rainfall hazard function — the probability of rain conditional on how many days have passed since the last rainfall. In environments where the hazard is increasing (rain becomes more likely the longer a drought continues), a leader who prays during a drought will tend to be followed by rain, creating the illusion of efficacy. In environments with a flat or declining hazard, prayer cannot be systematically followed by rain in a persuasive way. The model yields five predictions: rain ritual traditions will select for prayers correlated with rainfall; the level of average rainfall does not determine persuasiveness; constant-hazard environments cannot support persuasive prayer; increasing-hazard environments are more likely to adopt rainmaking; and higher net benefits of rainfall (e.g., settled agriculture) further increase the likelihood of ritual.

The authors test these predictions with two empirical strategies. First, they use daily data from the Catholic church in Murcia, Spain, covering 1600 to 1836. Church records provide the daily timing of pro pluvia rogations (prayers for rain), while municipal council records — kept independently of the church — record notable rainfall events. Murcia’s rainfall hazard is estimated to be increasing after long dry spells: the hazard rate after a long drought is roughly double the hazard rate two months after the last rainfall. The main finding is that a prayer for rain in the last 30 days predicts a 0.144 percentage-point higher daily probability of notable rainfall (standard error 0.057 pp), relative to a baseline mean daily rainfall probability of 0.203 pp — a 71% increase in the predicted probability. Prayer also Granger-causes rainfall conditional on lags of recent rainfall, and the predictive power holds within a given calendar month, ruling out a purely seasonal coincidence.

Second, the authors construct an original dataset covering rainmaking practice for 1,208 ethnic groups drawn from the Ethnographic Atlas (Murdock, 1967), coded from 370 anthropological sources. They match each ethnic group to its nearest weather station and estimate the rainfall hazard function each group faces in its ancestral location. Of the 1,208 groups, 33% face an increasing rainfall hazard, and 39% of all groups practice rain ritual. The main global finding is that ethnic groups facing an increasing rainfall hazard are 14 percentage points more likely to practice rainmaking (standard error 3.7 pp), relative to a base rate of 30% among groups facing a non-increasing hazard — a 47% increase. This result is robust to continent fixed effects, geographic and climatic controls (longitude, latitude, elevation, distance to coast, ruggedness, mean temperature, mean rainfall, coefficient of variation of rainfall, maximum dry spell length, and the Giuliano-Nunn 2021 climatic variability measure), alternative hazard estimation methods, and linguistic family fixed effects. Crucially, lower average rainfall, longer droughts, and greater climatic variability are not associated with more rain ritual conditional on hazard shape — it is specifically the shape of the hazard function, not aridity or variability per se, that drives adoption.

A second global finding concerns demand: groups dependent on agriculture are 11 pp more likely to practice rainmaking; those dependent on intensive agriculture, 21 pp more likely; and those dependent on intensive irrigated agriculture, 32 pp more likely (on a base of 32%). The scope of the findings is the pre-modern or traditional period captured by the Atlas; the Murcia case covers 1600–1836. The authors conclude that some environments create an illusion of efficacy that sustains instrumental religious belief through cultural selection, without requiring that believers be irrational.

Q: What is the paper’s central theoretical claim about why rainmaking beliefs persist? A: The paper argues that in environments where the rainfall hazard is increasing during a drought, a leader who begins praying during a dry spell will tend to be followed by rain, because the probability of rain rises as the drought lengthens. People who cannot observe the counterfactual hazard (what rainfall would have been without prayer) interpret this coincidence as evidence that prayer works. Cultural selection then favors leaders whose prayer timing is more persuasive, causing the belief to persist across generations even though prayer does not actually cause rain.

Q: What is the rainfall hazard function, and why does its shape determine whether prayer can be persuasive? A: The hazard function h(t) gives the instantaneous probability of rain at time t days after the last rainfall. If the hazard is flat, the probability of rain is the same regardless of whether prayer was offered or not, so there is no systematic correlation between prayer and rainfall to exploit. If the hazard is declining, prayer during a drought will be followed by lower-than-average rainfall probability, undermining the leader. Only if the hazard is increasing does prayer during a long dry spell systematically coincide with a higher probability of rain, creating a persuasive correlation.

Q: What do Propositions 2 and 3 of the model establish? A: Proposition 2 establishes that if the hazard rate is constant and a person’s prior belief that prayer works is below 0.5, then no prayer start time can persuade them to support the leader. Proposition 3 establishes the converse: if the hazard rate is increasing and the prior is below 0.5, there exists a meaningful belief for which a person will support the leader for any prayer start time. Together these propositions identify the increasing hazard as the necessary and sufficient structural condition for persuasive prayer.

Q: What is the main quantitative finding from Murcia, and what identification strategy supports it? A: A prayer for rain in the last 30 days predicts a 0.144 percentage-point higher daily probability of notable rainfall (standard error 0.057 pp) relative to a baseline mean of 0.203 pp, a 71% increase. The authors additionally demonstrate that prayer Granger-causes rainfall conditional on lags of recent rainfall, and that the effect holds within a given calendar month, ruling out the explanation that prayer simply tracks the rainy season. The prayer and rainfall records are kept by independent institutions (church and municipal council), reducing the risk of strategic recording.

Q: How does the hazard rate in Murcia behave, and does it satisfy the model’s key condition? A: The hazard of rainfall in Murcia is initially high just after rain, declines to a minimum roughly two months after the last rainfall, and then increases significantly thereafter, reaching or exceeding its initial level after a long drought. The fluctuations are large: the hazard after a long dry spell is roughly double the hazard two months after rainfall. This U-shaped pattern means the hazard is increasing during a prolonged drought, satisfying the model’s key condition for persuasive prayer.

Q: How was the global rainmaking dataset constructed, and what is its coverage? A: The authors used the Ethnographic Atlas (Murdock, 1967) as a template, covering 1,290 ethnic groups, and combed 370 anthropological sources — primarily group-specific ethnographic monographs — to code rainmaking practice for 1,208 groups. A group is coded as practicing rain ritual only if there is clear evidence of a practice specifically intended to bring rain through supernatural means. The authors treat their measure as a lower bound. They find that 39% of the 1,208 groups practice rainmaking, across every settled continent.

Q: What is the main global regression result and how robust is it? A: Ethnic groups facing an increasing rainfall hazard are 14 percentage points more likely to practice rain ritual (standard error 3.7 pp) relative to a base rate of 30%, a 47% proportional increase. This coefficient is positive and statistically significant across all specifications, including those adding continent fixed effects, a full battery of geographic and climatic controls (longitude, latitude, elevation, distance to coast, ruggedness, mean temperature, mean rainfall, coefficient of variation of rainfall, maximum dry spell length, and the Giuliano-Nunn 2021 climatic variability measure), alternative hazard estimation methods, linguistic family fixed effects, and restrictions to groups with high-quality rainfall data.

Q: Does aridity or climatic variability explain rainmaking adoption? A: No. Lower average rainfall, longer droughts, and greater climatic variability (measured using the Giuliano-Nunn 2021 index) are not associated with more rain ritual practice, conditional on the shape of the hazard function. This rules out the naive hypothesis that people pray for rain simply because they do not get enough, or because their rainfall is unreliable. It is specifically the shape of the hazard — whether it is increasing during a drought — that drives adoption, not the level or volatility of rainfall.

Q: How does demand for rainfall, proxied by agricultural subsistence, affect rainmaking adoption? A: Groups dependent on agriculture are 11 percentage points more likely to practice rainmaking relative to other subsistence modes. Groups dependent on intensive agriculture are 21 percentage points more likely, and groups dependent on intensive irrigated agriculture are 32 percentage points more likely, all on a base of 32%. This gradient is consistent with Proposition 5 and 6 of the model: settled, location-specific agricultural investment raises the net benefit of rainfall control, increasing support for rain ritual independently of the persuasion channel.

Q: What does the model’s cultural evolution mechanism (Proposition 4) predict about how prayer timing changes over generations? A: Proposition 4 states that rituals with high support are more likely to persist. In increasing-hazard environments, random variation in prayer timing means some leaders gain more support than others; those with more persuasive timing are more likely to persist. Each generation then adopts a policy at least as persuasive as the prior generation, so support rises over time and prayers gradually converge toward the timing that maximizes persuasiveness. This mechanism does not require deliberate optimization by any individual leader.

Q: How does the paper’s finding relate to the long-standing anthropological debate between the traditional and revisionist schools on rainmaking? A: The traditional school (following Frazer 1890) holds that belief is instrumental — people engage in rainmaking to make rain, and belief responds to empirical evidence. The revisionist school (Wittgenstein, Durkheim) argues that religious belief and rationality are fundamentally separate, and religious practice is performative rather than evidence-responsive. The paper’s finding that rainmaking is more prevalent precisely where it is more persuasive — i.e., where the environment makes prayer appear to work — supports the traditional, instrumental interpretation that belief responds to evidence of efficacy.

Q: What are the scope conditions for the paper’s conclusions? A: The Murcia case study covers the period 1600–1836, ending when the abolition of tithes reduced the church’s funding and influence; it applies to a sophisticated Catholic institutional context. The global analysis covers traditional practices of pre-modern ethnic groups as recorded in the Ethnographic Atlas and anthropological literature; it does not speak to modern religious practice or to religions after substantial modernization. The persuasion mechanism requires that people cannot directly observe what rainfall would have been without prayer, a condition satisfied in pre-scientific contexts.

Rainfall hazard function: In this paper’s usage, the function h(t) = f(t)/(1-F(t)) giving the instantaneous probability of rainfall at time t days since the last rainfall. Its shape — whether flat, declining, or increasing during a drought — determines whether prayer can be persuasive, not the overall level of rainfall.

Increasing hazard: A hazard rate that rises as the length of a dry spell increases, so that rain becomes more likely the longer the drought has continued. The paper defines this specifically as the derivative of the hazard function evaluated at the 99th percentile of spell length. This is the necessary structural condition for prayer to seem efficacious.

Instrumental religious belief: Belief directed at achieving a worldly outcome (here, rainfall), as opposed to purely expressive or social belief. The paper treats belief as instrumental if it responds to perceived evidence of efficacy and is adopted where it appears to work.

Persuasion (in the model): The process by which a leader’s prayer timing causes people to update their belief that prayer works, by generating a correlation between prayer and subsequent rainfall that exceeds what people expect from the background hazard rate. Persuasion is possible only when the hazard is increasing.

Pro pluvia rogations: The Catholic church’s formal prayers for rain, practiced in Murcia since at least the 14th century. In the paper’s data, these prayers follow a pattern of escalation — increasing in number and intensity — during prolonged droughts, consistent with the model’s prediction about prayer timing.

Cultural evolution: The paper’s framework (drawing on Henrich 2015) in which religious leaders act as cultural entrepreneurs; leaders whose prayer timing happens to be more persuasive gain greater support and are more likely to survive across generations, so prayer traditions drift toward more persuasive timing without deliberate design.

Rain ritual (global measure): A binary indicator coded as one for an ethnic group if the anthropological literature contains clear evidence of a practice specifically intended to bring rain through supernatural means, including dances, sacrifices, prayers, and petitioning of rain deities. Treated by the authors as a lower bound on actual prevalence.