Seasonal affective disorder: a vestigial evolutionary advantage?

The typical symptoms of recurrent winter depression include lowered mood, lethargy, hypersomnia, social withdrawal, decreased libido, increased appetite and weight gain. Mild hypomania often occurs in spring and summer. It is argued that this pattern of attenuated hibernation constituted an adaptive evolutionary mechanism which enhanced the likelihood of reproductive success, most notably for females, among populations living at temperate latitudes. Women were more likely to become pregnant in the summer and thus to give birth at a time of year when their babies had a higher chance of survival. Winter depression symptoms also promoted healthier pregnancies and gave rise to enhanced female-male pair-bonding which improved the survival chances of both mothers and babies. Hypomania in spring and summer also served to increase the likelihood of procreation at the optimal time of year. In the modern era, it is probable that recurrent winter depression is becoming a reproductive disadvantage.