Consider Stockholm, London, Frankfurt, and Toronto. Each of them has stock exchanges, each of them have dark winters. Weirdly enough, you can see the weather in the stocks.

As University of Toronto professor Lisa Kramer and her colleagues have found, seasonal affective disorder doesn’t only show up in your mood–it’s also present in the stock markets. For as the days grow shorter, we feel less and less like taking risks. As Kramer’s research has found, the further you–or a stock exchange–is from the equator, the more likely your behavior’s going to show risk aversion.

Used with permission of the American Economic Association

As Kramer explained to us, the stock markets show behavioral patterns that have a long, long inheritance:

Our brains are these interesting combinations of stuff that affect the way we make decisions, so as the seasons change, our moods can also change, and some of us experience very extreme changes in mood with changes of the seasons. As the exposure to daylight diminishes, we respond in a biological way to the change in exposure to daylight. We’re not that different than ancestors who would retreat in the winter time. When the seasons changed they had to become a lot more conservative, they had to adopt a lot more conservative practices. We’re still those same people.

What this shows is that our psychology and behavior is super influenced by our environments. Studies have shown that we’re more creative in the dark and less productive in the cold. As anyone who has felt drained by overexposure to fluorescent lights can tell you, light is a major driver of the way we feel. The most profound of these may be what was causing those Swedish markets to get risk averse: Seasonal Affective Disorder.

As the days grow shorter, we feel less and less like taking risks.

First put into psychological language in 1984, seasonal affective disorder (with the unfortunately fitting acronym of SAD) is a way of capturing the way that 10% of Americans are intensely affected by their exposure to sunlight.