Most violence is committed by men - but while boys may always be boys, only a tiny minority of antisocial teenagers become hardened criminals. Researchers can point to telltale sings - genetic inheritance, abnormal brain structures and chemical imbalances - which can put individuals at risk. Violence linked to PMS, epilepsy or excess testosterone can often be successfully treated with drugs. Such biological influences on criminality do not exclude social and economic factors, but they do call into question many of our moral and legal assumptions. In this text, the authors of "Brainsex" assess the evidence and offer suggestions for policy-makers.