This chapter discusses how theories from biology and psychology can help in understanding crime. It studies individual positivism: that is, those aspects of positivist criminological explanations that look for differences between criminal and non-criminal populations. Biological and psychological positivists believe that by measuring biological and psychological differences between offenders and non-offenders they will discover a clear explanation of criminal behaviour, a truth that explains criminal actions. When researchers discovered physical or biological differences between offenders and non-offenders they tended to assume that those characteristics were causative and explained the behaviour. However, there is a big jump between finding differences and assuming that the difference explains the behaviour. The chapter traces the journey of biological and psychological positivist thinking from its roots in the 19th century through to the approaches in the 21st century where these biological and psychological traits are merely seen as one factor which may increase the likelihood of criminality rather than causing it.