Evidence abounds that intelligence is heritable, which of course implies that a range of other mental states are also heritable. Mental illness was known to run in families centuries ago, and both manic-depression and schizophrenia were recognized as familial diseases by Sigmund Freud. But progress in defining the genetic basis for mental illness has been slow, in part because a definitive diagnosis of mental illness is often so hard to make. In fact, research into the behavioral genetics of mental illness has been filled with false starts, misleading data, and dashed hopes.
However, there is more reason now than ever before to be optimistic that the genetic basis of mental illness can be deciphered. Mental illness is understood in a way fundamentally different now than before; we know that mental illness is a medical problem, not demonic possession or divine visitation. Psychoanalytic tools have become sophisticated enough that psychiatrists can achieve consensus on most diagnoses. While this may sound like a small achievement, it represents a fairly profound advance over the situation of just a generation ago.
Mild retardation is probably only the lower end of a broad spectrum of intelligence, whereas severe retardation may be a genetically more distinct entity. The genes involved in determining high or low intelligence appear to be quite distinct from