David Hirshleifer and Tyler Shumway, were curious about what effect the weather has on stock trades. So they gathered weather data for twenty-six international cities from 1982 to 1997. Then they compared stock returns for each city on each day. What they found is remarkable: sunshine strongly correlated with daily stock returns—in ways that couldn’t easily be explained by any other factors. If it was sunny in the morning, stocks were more likely to go up.
