determined by specific genes, the Nature Theory takes this further, attributing some traits such as intelligence, personality, aggression, and sexual orientation to an individual 's DNA. The search for "behavioural" genes is the source of constant debate, one of the most controversial of which is the existence of a "gay gene," suggesting that there may be a genetic component to sexual orientation4. Similarly, can aggressive behaviour and criminality be the result of a behavioural gene? The study of twins