Clinical depression is a serious ailment, but almost everyone gets mildly depressed from time to time. Randolph Nesse, a psychologist and researcher in evolutionary medicine at the University of Michigan, likens the relationship between mild and clinical depression to the one between normal and chronic pain. He sees both pain and low mood as warning mechanisms and thinks that, just as understanding chronic pain means first understanding normal pain, so understanding clinical depression means understanding mild depression.

Dr Nesse’s hypothesis is that, as pain stops you doing damaging physical things, so low mood stops you doing damaging mental ones — in particular, pursuing unreachable goals. Pursuing such goals is a waste of energy and resources. Therefore, he argues, there is likely to be an evolved mechanism that identifies certain goals as unattainable and inhibits their pursuit — and he believes that low mood is at least part of that mechanism.

It is a neat hypothesis, but is it true? A study published in this month’s issue of the Journal of Personality and Social Psychology suggests it might be. Carsten Wrosch from Concordia University in Montreal and Gregory Miller of the University of British Columbia studied depression in teenage girls. They measured the “goal adjustment capacities” of 97 girls aged 15-19 over the course of 19 months. They asked the participants questions about their ability to disengage from unattainable goals and to re-engage with new goals. They also asked about a range of symptoms associated with depression, and tracked how these changed over the course of the study.

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