Many people assume that mental disorders are hereditary and that they are generally passed on from parent to child down through the ages. But this seems counter-intuitive to evolutionary theory. Scientists have struggled with this question for a long time but may have recently found an answer.
Mentally ill individuals have fewer children
Research from the Institute of Psychiatry at King's College London, lead by Robert Power, has found that individuals with certain cognitive or mental disorders (such as autism, schizophrenia, anorexia nervosa, etc.) tend to have fewer children than those who do not. Given this, which has likely been true for generations, it's unlikely that mental disorders are purely hereditary.
Several factors work into this. First, individuals with disorders are less likely to be physically capable of procreating (either because of their illness or because of medications). They are also less likely to be attractive to potential mates due to the stigmas often attached to mental disorders or the lack of social interaction due to the disorder. Many also spend much of their productive lifespan institutionalized or otherwise unavailable. Abortions are also much more likely amongst the mentally disabled who may get pregnant.
Children of mentally ill parents no more likely to have disorders
The study also looked at children who were born to those with cognitive or mental disorders. Researchers found that the chances of them having a disorder that one or both of their parents had was not significantly higher than average.
Most mental disorders likely caused by random mutations
Some disorders, such as bipolar and depression, were extrapolated separately due to their more complex nature and the likelihood that they are caused by multiple genes. Strong selection against schizophrenia, autism, and anorexia, however, proves their general point.
It's more likely that these disorders are caused by random mutations rather than genes passed on from parent to child. While genetics can certainly play a role in some mental disorders, it's unlikely that it does so for most.
