INTRODUCTION

Since W. D. Hamilton's seminal work on kin-selection (Hamilton, 1964a, 1964b) advances in the field of behavioral evolution have changed our view of animal and human social behavior. A wide range of social behaviors – altruistic, co-operative, aggressive, parental, and sexual – are now seen as products of past selective forces, many of which were generated by social interactions. Fifty years ago, it was possible for Behaviorists to argue that humans are blank slates whose nature – if such exists – is wholly determined by cultural/environment forces. The most basic emotional attitudes – for example, sexual preferences and parental concern for children – were said to result from training and cultural expectation. That position is no longer tenable, in large part due to concepts from behavioral evolution.

At the same time, important advances have been made in psychiatry. Over the last 40 years, psychiatric researchers have identified mental disorders that can be reliably diagnosed, may affect Darwinian fitness, and are partly caused by genes. A large proportion of people in most or all societies suffer from one or another of these mental disorders. The research on diagnosis culminated in the 1994 publication of the Diagnostic and Statistical Manual of the American Psychiatric Association, Fourth Edition, known as the DSM-IV, which lists objective criteria for diagnosing disorders (American Psychiatric Association, 1994). The DSM-IV criteria for specific disorders are described later in this chapter, along with pertinent prevalence, genetic, and fitness data.