New mutations, or as-yet unknown mechanisms, are likely also at work in autism and anorexia, the researchers wrote in their article, published Nov. 12 in the journal Archives of General Psychiatry.
For the study, the researchers gathered data on people born in Sweden between 1950 and 1970. They looked at how many children people had, and diagnoses of mental health conditions.
The researchers also looked at the number of children born to siblings of people with mental health conditions.
"We might be able to work out why some people, seemingly genetically predisposed to psychiatric illness, actually do better than expected," which could point toward new treatments, Power said.
A genetic balancing act
The finding that, among those with mental health conditions, men saw greater reductions in their number of children than women did was unsurprising, the researchers said. Women tend to be "choosier" than men in selecting mates, and so women would be less likely to have children by men who have these conditions.
In the study, people with autism and schizophrenia tended to have the fewest children. This suggests that these disorders, perhaps more so than other disorders the researchers considered, are maintained by new mutations.
With autism, the results suggest genes involved in the disorder are mostly rare mutations that have occurred in the recent generations and are not shared across affected individuals or families, Power said, adding that the results of this study should be interpreted cautiously.
Bipolar disorder, on the other hand, seemed to have less of an effect on how many children people had. It may be that lithium treatments have allowed people with this disorder to function more normally, so the number of children they have is less affected, the researchers said.
The puzzle of depression
Among people in the study with depression, there was only a slight change from the average in terms of the number of children they had, and the unaffected siblings had more children than average, Power said.
Depression is likely caused by many different genes, and it could be that these siblings have an intermediate number of such genes. This intermediate number may be better than having too many or too few, Power said. Being depressed may be a detriment to survival and reproduction, but so is being overly optimistic, for example, when it comes to judging risks.
"Perhaps those individuals who fall somewhere along the middle of a 'depression spectrum' have the best chance at a healthy life," he said.
The researchers noted that some people with mental health disorders may take medicine that affects their fertility, or have been hospitalized for a time during their reproductive years, and these factors may have affected the results.
Pass it on: Certain mental health disorders may persist because of new mutations rather than by heredity.