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.

Their conclusion was that those who experienced mild depressive symptoms could, indeed, disengage more easily from unreachable goals. That supports Dr Nesse's hypothesis. But the new study also found a remarkable corollary: those women who could disengage from the unattainable proved less likely to suffer more serious depression in the long run.

Mild depressive symptoms can therefore be seen as a natural part of dealing with failure in young adulthood. They set in when a goal is identified as unreachable and lead to a decline in motivation. In this period of low motivation, energy is saved and new goals can be found. If this mechanism does not function properly, though, severe depression can be the consequence.

The importance of giving up inappropriate goals has already been demonstrated by Dr Wrosch. Two years ago he and his colleagues published a study in which they showed that those teenagers who were better at doing so had a lower concentration of C-reactive protein, a substance made in response to inflammation and associated with an elevated risk of diabetes and cardiovascular disease. Dr Wrosch thus concludes that it is healthy to give up overly ambitious goals. Persistence, though necessary for success and considered a virtue by many, can also have a negative impact on health.

Dr Nesse believes that persistence is a reason for the exceptional level of clinical depression in America—the country that has the highest depression rate in the world. “Persistence is part of the American way of life,” he says. “People here are often driven to pursue overly ambitious goals, which then can lead to depression.” He admits that this is still an unproven hypothesis, but it is one worth considering. Depression may turn out to be an inevitable price of living in a dynamic society.