Infection, Genetics, and Mental Illness

A comment I had occasion to make made me want to post about the causes of mental illness. I hasten to add that I do this as a layman, not an expert.

There is a lot of data on the prevalence of mental illness in the population. Some of it is methodologically suspect, even a marketing ploy for the "helping professions." But there's no doubt that bipolar (formerly manic-depressive) illnesses and schizophrenias are quite widespread, the former in many people who are high achievers.

Although these diseases are increasingly seen to have biological and not just behavioral effects, their cause is still a mystery.

There's a suggestion that heredity plays a role, but it doesn't seem to be a simple Mendelian heredity. Now, if bipolar disorders are genetic, one would think they would lower the reproductive rate of those who suffer for them (greater difficulty in getting and staying married, tendency to suicide), and thus gradually vanish from the population. Yet these conditions are common, even flourishing.

One possibility is that there is an adaptive polymorphism at work. The classic exampleis sickle-cell anemia. In those with one gene for the condition, some resistance of falciparum malaria is conferred. The lower reproductive rate of those who have two genes and full-blown sickle-cell anemia balances the higher reproductive rate of those with one gene and no disease. In a region with no malaria, like the US, we would expect the gene to die out gradually because the one-gene carriers no longer have a reproductive advantage.

Bipolar disorder could be similar. People without the full-blown conditions, but some of the genetic inheritance might have a tendency to caution and deliberation on the one hand, or courage and innovation on the other (or even an alternation between the two but not past destructive limits). Or the condition might confer greater intelligence or creativity. The reproductive advantage of the only partially-expressed condition might balance the reproductive deficit of those with severe bipolar conditions.

An alternative explanation is that the disease is caused by an infectious agent of some kind, or an autoimmune reaction to such an agent. Just as in my childhood ulcers were thought to be a result of a certain kind of diet, and thus were treated with dietary restrictions, but were then found to be a product of heliobacter, a bacteria, it is likely that other conditions will be found to have infectious origins.

John Derbyshire Cochran-Ewald infection hypotheses of the origins of homosexuality, whose reasoning is similar to the above. You can find More on C-E here.

BTW, it's hard to see how any of this thinking could emerge without employing the principle of natural selection.

Update: Here's Paul Ewald's book.