Genes for depression part of adaptation?

Among 10 to 19-year-olds globally, depression is the main cause of disabling illness. This is according to Health for the World's Adolescents, a report that the World Health Organisation (WHO) released last week.

More accurately called major depressive disorder (MDD), it also tops the number of cases seen at the Institute of Mental Health (IMH).

In Singapore, one in 16 people will have MDD at some point in life, according to a 2011 IMH survey. Locally, the median age at which symptoms first appear is 25 years but all ages can be affected.

Naturally, treatment is needed, especially because suicide is an ever present danger in MDD. Yet experts in an esoteric subdiscipline called evolutionary psychiatry believe that such patients should not be taking antidepressants.

A patient's wife called me to discuss The Depths: The Evolutionary Origins Of The Depression Epidemic (2014). This book argues that, although evolution has made humans the fittest species, we have nevertheless evolved to be subject to depressions. This suggests that depression promotes species survival in some way.

If so, it argues, depression is a good thing, not a defect, which is why patients should not take antidepressants to correct it.

The author, Dr Jonathan Rottenberg, a University of South Florida psychologist, reviews the latest research on the evolutionary function of mood to support this hypothesis. This odd idea is actually a major school of thought within evolutionary psychology.

It was first mooted in 2009 by two other experts who jointly published a paper in Psychological Review arguing that depression was an adaptive trait built into our genomes through natural selection processes.

In evolutionary psychology, human behaviour is seen as the end result of the processes of natural selection working on genes responsible for our ancestors' psychological traits.

If a trait looks unhelpful to us but occurs widely in the population, it is argued that it is pointing to genes that are actually "adaptive" in an evolutionary sense. So depression, for example, looks unhelpful as a trait to have but it may have an adaptive function in the species.

As depression is prevalent across populations, the genes for it could have been naturally selected into the genome to confer advantage. But what good can negative moods do?

Well, the depressed ruminate at length. And patients say they do so to try to solve their problems. Thus negative moods may cause one to take time out from others, as it were, to deliberate about an issue, process information about it carefully and analyse it in its granular details.

So did natural selection choose the behavioural trait of withdrawal from social interaction when a person faces certain types of problems and then cause him to ignore food, drink and sex? Disentangled from these pleasurable activities, he would be able to best focus on those problems and eventually solve them, whereupon he would re-enter his social world.

If depression has indeed been naturally selected as a way to solve complex social problems in this manner, then it should not be treated with drugs.

In this argument, drugs may alleviate symptoms but won't cure the cause, namely, life problems. Conversely, talk therapy that helps to identify and solve important life problems is preferable.

However, no evidence is offered in the book or other published studies to show that MDD is caused by social problems. Neither is evidence offered to disprove the reverse, that MDD causes social problems instead.

Also, no evidence is given to show that MDD actually helps people to solve social problems.

In evolutionary biology, an adaptive trait is behaviour that increases the individual's fitness which specifically refers to the number of offspring he produces in a lifetime. This is because evolution is all about survival and reproduction.

A specific trait that confers a genetic edge must lead to more offspring. Thus, if depression were really an adaptation passed down through natural selection, then patients should have more children than people without depression. But the authors offer no figures to show this is so.

In fact, since MDD patients are more prone to suicide, it seems less likely that they will, on average, have more offspring than normal folk. And depressed people generally have reduced libidos, so it seems unlikely that depression confers a reproductive edge.

Instead, it seems more reasonable to see depression as an illness.

These patients are usually people without an obvious reason to be depressed, like bereavement, job loss and so on. Instead, they may even have a great job, a loving family, supportive friends and secure finances. Yet they are still depressed.

That is why MDD looks more like an illness than an effective response to life problems.

The experimental evidence offered in the subdiscipline is unrealistic such as listening to sad music (as a proxy for depression) while doing mock currency trading (as a proxy for problem solving).

MDD does not lead to intense, focused analysis. Instead, there is a deep loss of function: one's mood, sleep, appetite, goals and thoughts are all adversely affected. One is unmotivated to think carefully through problems or devise workable solutions to them.

Thus the dictum that a maladaptive, prevalent trait may point to genes that are an evolutionary adaptation may need some reworking. After all, foolishness is quite widespread too.

Perhaps the genes for depression are merely hitching a ride in the human genome without conferring any adaptive advantage.

Perhaps its genes are just inherited along with other genes that do confer a good trait, like creativity, wholly unrelated to depression.

Overall, this hypothesis must be viewed sceptically and patients should continue taking their antidepressant pills regardless.

This article was first published on May 23, 2014.
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