Introduction

In the last decade, there has been an increasing appreciation of the role of genetic influences on many child behavioural traits and disorders (Rutter et al., 1999a,b). Although there has been a growing acceptance of the importance of genetic factors for many behaviours and psychiatric problems, there has been a tendency to view conduct disorder as the exception. The reasons for this point of view are varied. They include the fact that rates of conduct disorder and juvenile delinquency vary considerably from one population to another (Rutter et al., 1998) and that much of this variation can be explained by environmental adversity (Rutter et al., 1975). Rates of both juvenile and adult crime have also changed enormously over time (Rutter et al., 1998), a phenomenon that cannot be explained by genetic effects (because the genetic makeup of a population takes many generations to change). Family-based influences have included a number of risk factors for which a direct effect on disruptive behaviour has seemed likely such as inconsistent discipline and poor supervision (Rutter, 1978). For all these reasons, it has only been in very recent years that the possibility of significant genetic influences has been entertained seriously.

Risk factors for conduct disorder include behavioural problems such as hyperactivity (Stewart et al., 1979; Taylor et al., 1986a, b), a variety of family factors (Frick et al., 1992; Hamdan-Allen et al., 1989) and cognitive deficits that include misinterpretation of social interactions (Dodge et al., 1994) and discrepancies in verbal and performance intellectual abilities (Moffitt, 1993), as well as reading problems (Maughan & Hagell, 1996).