It's a question that's baffled evolutionary theorists for decades: if survival of the fittest is the rule, how have the genes that contribute to serious, debilitating mental disorders survived?
It's been shown that people who suffer from schizophrenia, autism, anorexia and other disorders are less likely to have children. And yet, the genes that cause these disorders aren't going away. In fact, some of the disorders appear to be becoming more common. Evolutionary theory wouldn't predict that.
Scientists have a few theories that attempt to explain this paradox.
One is that the genetic mutations that cause these disorders occurred relatively recently, so not enough generations have passed to allow the evolutionary process to weed them out.
Another theory is that the genetic mutations that cause a disorder in one person somehow make that person's sibling more likely to have children. In a situation like that, the mutation offers a net benefit to a person's family.
A team of Swedish and British scientists recently tested these theories by comparing the rates at which people suffering from mental illness have kids to those of their siblings. The data came from a medical database of more than 2 million Swedes.
The researchers found that the siblings of people who suffer from schizophrenia, autism and anorexia had on average the same or fewer children than the general public, which would seem to confirm the first theory. But they also found that the siblings of people who suffer from depression or substance abuse had significantly more children than the general public, an outcome more in line with the second theory.
We talked with Dr. Peter McGuffin, a professor at King's College London who worked on the study, which was published in the journal Archives of General Psychiatry. Here are highlights from the interview, edited for length and clarity.
Q: You say at the beginning of your paper that "psychiatric disorders have long puzzled researchers by defying the expectations of natural selection." Why?
A: It's particularly the case with schizophrenia, which in this paper and in many other papers has been shown to be a disorder that drastically reduces your fecundity — the number of kids you have. It's often referred to as reduced fertility but, strictly speaking, people with schizophrenia aren't infertile. It's just that they're less often likely to find a partner and have kids.