At one time, links between adverse experiences and childhood disorder were assumed to run in just one direction. More recently, it has become clear that the situation is more complex. Children are not simply passive recipients of experience; they influence, as well as being influenced by, those around them, and they play an active role in constructing and interpreting their social worlds. ^ Even very young infants influence the nature of their interactions with caregivers, and children's capacities for shaping and selecting their experiences increase as they mature. The temperamentally difficult child is likely to evoke more negative responses from parents; when parents themselves are under stress, or find it hard to maintain consistency, troublesome child behaviours can play a key role in fuelling harsh or punitive responses. Delinquent adolescents may seek out delinquent peers, who further encourage their antisocial activities. Associations between environmental factors and disorder often involve complex reciprocal patterns of effects.
Some of the evocative effects of children's behaviour will reflect heritable traits. (2) The advent of behaviour-genetic studies in child psychiatry has provided important insights into environmental as well as genetic risks. Genetic analyses have shown, for example, that many ostensibly 'environmental' factors include some element of genetic mediation.(3) Parents provide children with their environments and also with their genes, so that in biologically related families, nature and nurture are inevitably interwoven. Musical parents will encourage their children to enjoy music, buy them a violin, and may also pass on musical talents. In a similar way, antisocial parents may rear children in hostile and punitive environments, provide models of antisocial behaviour, and also pass on genes that predispose to disruptive behaviours. Genes and environments will often be correlated in this way; research is only in the earliest stages of disentangling their effects.
In addition, genetic studies suggest that for most child psychiatric disorders, non-shared environmental influences—experiences that make children in the same family different from one another, rather than more alike—are generally more significant than shared effects. (4) Conduct disorders may be an exception here; in relation to other disorders, however, the most salient environmental influences may lie in child-specific risks such as scapegoating or differential treatment within the family, in siblings' differing perceptions of 'family-wide' events, or in experiences outside the family, at school, or with peers.
Finally, genetic factors may contribute to a differential sensitivity to environmental risks. Research has consistently shown marked individual differences in children's responses to all but the most severe forms of psychosocial adversity. As yet, reasons for these differences are not well understood. They may reflect variations in the severity of exposure; individual differences in resilience or coping strategies, or in environmental sources of protection; or variations in vulnerability. In practice, this may result in only subgroups of children being at risk. Genetic predispositions constitute one source of such vulnerability, with genes and environments interacting to increase some children's sensitivity to environmental stress.
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