Behaviour runs in families

We tend to resemble our parents, siblings, and other close relatives not just in the way we look but in the way we behave. However, the types, patterns, and causes of familial behavioural traits are extremely varied. They range from rare single gene disorders, such as Huntington's disease, that present with dramatic changes in behaviour in adult life, to commoner but more genetically complex disorders, such as schizophrenia and manic depression, through to normal variation in traits that are usually measured quantitatively, such as personality or intelligence.1 They can also include characteristics such as political or religious persuasion and career choice.

One of the basic pitfalls in studying the genetics of behaviour (or indeed any other type of trait) is to assume that familiality necessarily implies genetic transmission or that strong evidence of familial clustering implies single gene effects. Geneticists have long been aware that mendelian inheritance can be simulated by other mechanisms.2 To illustrate this, McGuffin and Huckle studied the educational histories of the families of a cohort of medical students: they …