The theories which I have so far discussed have one feature in common: they look for causes in the individual. Some find these in traits which are attributed to his physiological constitution; some in characteristics which result from his upbringing. Both of these types of explanation, however, are 'individualistic' in the sense that, whether the blame is ultimately laid on nature or nurture, they envisage the delinquent as an individual who is more prone than the non-delinquent to anti-social behaviour even when both live in the same society, with the same temptations and opportunities. In contrast, the group of theories which I am about to describe can be called 'environmental', because they emphasise the part played by the economic, ecological or social conditions in which the delinquent develops. 1