Natural selection and the manifestations of low mood and depression.

Abstract

Low mood is the normal human response (sadness, fatigue, pessimism, and so forth) to negative life situations, such as deaths of loved ones or failures at important goals. Many aspects of low mood suggest that it was shaped by natural selection, perhaps as a defensive response to fitness-threatening situations that recurred in ancestral environments. If this is correct, the different situations that commonly lead to low mood may evoke more strongly those symptoms that are well suited to solving the fitness problems prominent in each situation. Consistent with this hypothesis, 770 participants who identified the likely cause of a recent episode of low mood retrospectively reported more fatigue, pessimism, and anhedonia---all responses that should reduce goal pursuit---following failed efforts. On the other hand, crying and desire for social support---reactions that should help form or strengthen social bonds---were prominent following social losses. It was difficult to know whether situations actually caused the symptom patterns or if a third variable was responsible for the associations. To control for non-causal explanations, 123 students were randomly assigned to vividly imagine either the death of a loved one or a major failure. Following this mood induction, participants reported their present levels of low mood symptoms. These results imply that failures and social losses cause the different low mood symptom patterns observed in the previous finding. These findings support the hypothesis that low mood symptoms solve problems salient in each situation and provide additional weight to the more general thesis that the capacity for normal low mood is an adaptation. While evidence suggests that normal low mood is adaptive, interpersonal variation in low mood reactions (such as severe depression in response to relatively mild situations) may not be adaptive. Genetic differences between people play the largest etiological role in this variation, and several recent advances in evolutionary genetics might help explain why this genetic variation exists. Specifically, to the degree that numerous genes influence low mood reactions in some way, recurrent mutations could play a large role in disrupting the performance of mechanisms that underlie normal low mood.