People with certain mental disorders, such as schizophrenia and autism, tend to have fewer children than the average person, suggesting that these disorders persist not because of heredity, but because of new genetic mutations, according to a new study.
Based on the data of 2.3 million Swedes, researchers found that people average 1.76 children. However, women with schizophrenia or autism average half this amount, and men with these disorders only one-fourth.
Men and women with bipolar disorder, anorexia nervosa or a substance abuse disorder have fewer children than average, and men with depression show a slight decrease in the number of children. However, women with depression have the same number of children as those in the general population.
“The main message of our study is that it appears that suffering from a psychiatric illness severely reduces the number of children an individual has, particularly for men,” said study researcher Robert Power, of the Institute of Psychiatry at King’s College London.
The findings shed light on a longstanding puzzle in psychiatry: How do the genes linked with some mental health disorders persist in the human population, if people with those disorders tend to have fewer children?
For example, schizophrenia is extremely heritable, so it would make sense that it becomes more rare over time. But the disorder seems to persist in 1 percent of the population, which suggests that new mutations are occurring quickly enough for it to remain consistent, said Power.
New mutations are also likely responsible for the persistence of autism and anorexia.
For the study, researchers analyzed data on people born in Sweden between 1950 and 1970. They looked at how many children people had, and diagnoses of mental health conditions. They 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 lead to new treatments, Power said.
The fact that, among individuals with mental disorders, men had fewer children than women was expected, 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 disorders.
In the study, individuals with autism and schizophrenia tend to have the fewest children. This suggests that these disorders, perhaps more than the other disorders studied, are kept going by new mutations.
With autism, the results suggest genes are mostly rare mutations that have occurred in recent generations and are not shared across affected individuals or families, said Power.
In contrast, bipolar disorder seemed to have less of an effect on how many children people had. It may be that drug treatments help people with this disorder function more normally, so the number of children they have is less affected, the researchers said.
Among individuals with depression, there was only a slight change from the average in terms of the number of children they had, and the healthy siblings had more children than average, said Power.
Depression is likely caused by a variety of genes, and it could be that these siblings have an intermediate number of such genes. This midway number may be better than having too many or too few, Power said. Being depressed may be a disadvantage to survival and reproduction, but so is being overly optimistic, for example, when it comes to calculating 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 note that some people with mental disorders may take medication that affects fertility, or they may have been hospitalized at some point during their reproductive years, and these factors may have influenced the results.
Source: Archives of General Psychiatry