First, depression induces cognitive changes that focus and enhance capacities for the accurate analysis and solution of key social problems, suggesting a social rumination function. Second, the costs associated with the anhedonia and psychomotor perturbation of depression can persuade reluctant social partners to provide help or make concessions via two possible mechanisms, namely, honest signaling and passive, unintentional fitness extortion. Thus it may also have a social motivation function.
As with many evolutionary psychology hypotheses, there is little empirical support. Observers note that depression is highly prevalent, that it occurs with high frequency even among individuals with high reproductive value, and that it may provide a fitness advantage in certain conditions, such as providing a more accurate assessment of the circumstances (see depressive realism) or providing a convincing signal that the individual needs more support from the group. (Of course, depression is often detrimental to fitness, including reproductive fitness.) The theory that depression is adaptive is interesting; however, it is generous to claim that the evidence for this hypothesis is even weak. It is almost nonexistent.
The more interesting question for my purposes is, if we concluded that depression is an adaptive trait and provided a fitness benefit in our environment of evolutionary adaptation, should this influence our view of the right to suicide? It seems clear that it should not.
If depression is adaptive, that means that the depressed state is a normal, expected part of human life, which conferred a benefit on our ancestors. This hypothesis says nothing about whether people defined as depressed should be forcibly prevented from committing suicide. In fact, if we suppose that depression confers an increase in rationality, we should be even more inclined to respect the depressed person's analysis of his situation, and his decision to commit suicide.
Aside from rationality, the fact that depression may be evolutionarily adaptive would imply that much of our suffering is "by design" - that is to say, part of the evolutionary package. It would be yet another biologically-determined limit on human happiness, and entitle us all the more to refuse to participate in the project of life.
The interesting implication of one hypothesis for the evolutionary adaptiveness of depression has to do with treatment: the idea that people become depressed as a way to credibly signal that they need more support from their group than they are getting. An implication of this would be that depression could be treated by providing depressed people with more "support" (whatever that might mean). It is an interesting hypothesis, but, again, does not provide a compelling reason for prohibiting suicide. First, there is no evidence that any method of treating severe depression is widely effective. Even placebos are much less effective at treating severe depression than at treating moderate or mild depression (meaning severe depression rarely spontaneously improves, and does not respond to sham treatment). Widely prescribed anti-depressants are almost as useless. If some form of "group support" were shown to reliably treat severe depression, that would be wonderful news - but a right to suicide would still be warranted for those who still wish to die despite having submitted to this treatment (or, I would argue, despite refusing this treatment).
I hope to address the question of when, if ever, paternalism is appropriate when dealing with depression and suicide in a later post. I thank reader Mike Kenny for the interesting questions that I'm addressing here.