Nature vs. Nurture: Seasonal Affective Disorder

Leanna M.W. Lui, HBSc


September 16, 2021

With summer coming to an end, and pumpkin spice lattes trending again, we might also expect to say hello to an old friend … seasonal affective disorder (SAD). 

Have you ever woken up one morning during the fall or winter and felt out of it for a prolonged period, not your regular self? I’m not referring to a day here and there, but consistently experiencing this “down mood” around the same time each year? At some point in their life, it is estimated that 2-3% of Canadians will experience SAD. To add to that, 15% of individuals will experience milder (and less impairing) SAD.  

Seasonal affective disorder can be thought of as a type of depression that occurs during a specific time of the year, usually the winter or fall (with remission outside this period). It is typically characterized by symptoms of clinical depression such as low energy, difficulty with concentration, sleep problems, extreme fatigue, and agitation. While the evidence related to the risk factors for SAD are limited, it is suggested that a family history of SAD, female sex, location farther from the equator (ie, fewer days of sunlight), and being between the ages of 18-30 increase your risk for SAD.

The Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) does not provide a separate and distinct categorization for SAD. Rather, SAD is categorized as a subtype of depression. However, it is generally recognized that a diagnosis of SAD is accompanied with two consecutive years of mood episodes within a recurring specified timeframe. 

Nature vs Nurture: An Evolutionary Perspective

The pathophysiology of SAD is not yet well-understood. However, it is hypothesized that SAD is an adaptive response related to physiologic and behavioral patterns of reproduction and childrearing.

Historically, reproduction was closely linked to food and natural resource availability (eg, water, sunlight). Males primarily handled the hunting, while females were primarily responsible for agricultural work, a job closely tied to the seasons. With this in mind, it would logically follow that natural selection favored reproduction during times of food abundance and did not favor reproduction during times of food scarcity (ie, low energy).

Consequently, conception would occur when the growing season began (around the summer), giving females the chance to rest when heavily pregnant in the winter, and give birth in the spring. Accordingly, from an evolutionary perspective, greater seasonal variation in mood and behavior is a function of historic patterns of reproduction and food gathering. 

An alternative hypothesis of SAD is the dual vulnerability hypothesis. This hypothesis posits that SAD is the result of seasonality, and depression (or “vulnerability traits”). Seasonality refers to external environmental factors such as light availability.

It’s quite well-known, and perhaps your personal experience can speak to this topic as well, that shorter days may trigger SAD because reduced light exposure is associated with phase-delayed circadian rhythms. As a result, less dopamine is produced, and relatively higher levels of melatonin are produced, compared to individuals without SAD. “Vulnerability traits” refer to a genetic predisposition, or external effects (eg, stress). 

A Disorder of the Past?

By nature of natural selection, SAD is likely not to be considered an advantageous adaptive trait that would help with survival and reproduction. In fact, it could be considered a maladaptive trait. In that case, will SAD eventually fall to natural selection?

What do you think? Comment below.

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About Leanna Lui
Leanna M.W. Lui, HBSc, completed an HBSc global health specialist degree at the University of Toronto, where she is now an MSc candidate. Her interests include mood disorders, health economics, public health, and applications of artificial intelligence. In her spare time, she is a fencer with the University of Toronto Varsity Fencing team and the Canadian Fencing Federation.


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