What is the argument about?

The inheritance of intelligence is one of the most hotly disputed topics in this field. The argument is not really about whether it is inherited; hardly anyone nowadays disputes that intelligence is, to some extent, handed down from parents to children. The real debate is about whether the contribution of inheritance to a person's intelligence is a major or a minor factor. It is sometimes claimed that as much as 80 per cent of intelligence is attributable to inheritance, while more modest claims put it between 40 and 60 per cent. Yet others, like Leon Kamin, have doubted whether there is any good evidence for the heritable component to be much above zero. It is rare for anyone, it seems, to spend much time considering what these percentages actually measure and how relevant they are to the point at issue. That is the subject of the present chapter.

To begin with, we shall couch the discussion in terms of the generic term ‘intelligence’, since many of the points we wish to make are relevant whether we are talking about a g-score, IQ or any other similar index purporting to measure intelligence. Many writers are far from clear about what it is that is supposed to be inherited. A more precise discussion of heritability requires us to speak in terms of some particular measure of intelligence.