Being depressed somehow increases your fitness (your long-term rate of reproduction)?
In an article in Scientific American, Depression's Evolutionary Roots, it is argued that depression is adaptive because it makes us think harder. Think about that for a moment.
Depression seems to pose an evolutionary paradox. Research in the US and other countries estimates that between 30 to 50 percent of people have met current psychiatric diagnostic criteria for major depressive disorder sometime in their lives. But the brain plays crucial roles in promoting survival and reproduction, so the pressures of evolution should have left our brains resistant to such high rates of malfunction.Perhaps I think the current psychiatric criteria for major depressive disorders are rather liberal, but that's a different story. I'll accept that many people experience being really, really sad at some point in their lives.
Here's the link between depression and thinking:
One reason to suspect that depression is an adaptation, not a malfunction, comes from research into a molecule in the brain known as the 5HT1A receptor. The 5HT1A receptor binds to serotonin, another brain molecule that is highly implicated in depression and is the target of most current antidepressant medications. Rodents lacking this receptor show fewer depressive symptoms in response to stress, which suggests that it is somehow involved in promoting depression. When scientists have compared the composition of the functional part rat 5HT1A receptor to that of humans, it is 99 percent similar, which suggests that it is so important that natural selection has preserved it. The ability to “turn on” depression would seem to be important, then, not an accident.Okay, so 5HT1A promotes depression by binding to serotonin, but it also helps fueling neurons so we can think well. They therefore argue that depression is good for you, because you think better when you are depressed.
Studies of depression in rats show that the 5HT1A receptor is involved in supplying neurons with the fuel they need to fire, as well as preventing them from breaking down. These important processes allow depressive rumination to continue uninterrupted with minimal neuronal damage, which may explain why the 5HT1A receptor is so evolutionarily important.
How fast can you say cum hoc ergo propter hoc?
Of course, correlation alone is not enough to imply causation, and the authors do attempt to suggest a mechanism that would make sense of depression causing better thinking:
Many other symptoms of depression make sense in light of the idea that analysis must be uninterrupted. The desire for social isolation, for instance, helps the depressed person avoid situations that would require thinking about other things. Similarly, the inability to derive pleasure from sex or other activities prevents the depressed person from engaging in activities that could distract him or her from the problem. Even the loss of appetite often seen in depression could be viewed as promoting analysis because chewing and other oral activity interferes with the brain’s ability to process information.Except that the two very things that usually make traits adaptive are precisely what are offered as damaging to thinking: sex and food. We are meant to believe that thinking about one's problems is so much more important than having sex and eating that having a depression is adaptive. That is one hell of a big mouthful that requires a few exclamation points.
Laboratory experiments indicate that depressed people are better at solving social dilemmas by better analysis of the costs and benefits of the different options that they might take.In the meantime the rest of us will be over here having dinner and sex.
Here's how it's gonna go down: The brain is developmentally complex, with multiple proteins required to build each component, and proteins involved in building multiple components. Epistasis and pleiotropy, respectively, if you think of proteins as genes. The brain is not optimally designed, but has evolved using available proteins for new functions. It has had to make do with what was already present, at least in many cases, and that has resulted in some constraints that we now have to deal with. Such as being good at thinking analytically unfortunately comes at the cost of higher risk of being depressed, for example.
Related post: Head injury as a cause of ADHD.