Temperament and personality
Infants with temperaments classified as 'difficult' at 3 years of age are more likely to be referred for aggressive problems later on. (3Z38) The dimensions involved are behavioural impulsivity (lack of restraint), short attention span, and motor restlessness. Although not occurring at a clinical level, these are precisely the constituents of the hyperkinetic syndrome; together with the trait of negative emotionality (irritability, anger, and bad moods) they have a clear modest effect in predicting later antisocial behaviour of the early-onset type. Social anxiety on the other hand is protective.
Significant cognitive attributional bias has been shown in aggressive children and youths. They are more likely to perceive neutral acts by others as hostile, and are more likely to believe conflicts can be satisfactorily resolved by aggression. As the individual becomes more disliked and rejected by his peers, the opportunity for seeing things this way increases.(39) By the late teenage years these youths can have a highly suspicious attitude, and be quick to perceive disrespect from others and sometimes react explosively. Social skills are lacking. Emotional processes in antisocial children have been little studied, although self-esteem is often low and coexistent misery common. Youths with conduct disorder have been shown to plan their lives less than controls, so that they are more driven by events than in control of them.
Early-onset lifetime-persistent children have IQs between 8 and 10 points below controls. The relationship with antisocial behaviour weakens after allowing for social class but is still present, although it only accounts for a few per cent of the variance. It is not just the effect of missing schooling, since it is present before children go to school. Much of the association can be accounted for by the association with hyperactivity. (49 It seems likely that low IQ alone does not confer much increased risk of antisocial behaviour, but in the presence of adverse parenting and other risks it has an interactive effect. (41
Physique has little influence on antisocial propensity. The relationship between testosterone (and other androgens) and antisocial behaviour is complex. Levels vary with age, type of antisocial behaviour, and social rejection, so that no straightforward conclusion can be drawn; (42> certainly to date no obvious relationship has been shown between higher levels and more aggression. Serotonin is being investigated since reduced levels have been associated with suicidal, impulsive, and aggressive behaviour in adults. However, as samples have to be obtained by lumbar puncture, studies are usually performed on small numbers of incarcerated individuals, but reduced levels of metabolites have been found in aggressive adults. Moffitt et al.'(43> found reduced blood levels (effect size, 0.5 standard deviations) in an epidemiological study of young adults, but the relationship of blood to brain levels has yet to be clarified. Studies of children are few and show no consistent picture.
However, autonomic arousal has shown a consistent, albeit modest, association with persistent antisocial behaviour in young people. Their pulse rates are lower, their skin conductance is less, as is their rate of adrenaline and cortisol excretion. Some have postulated that this leads to less anxiety when taking risks, (44) while others see it as part of a wider picture of decreased central nervous system inhibition. (45) With all these physiological substrates and mechanisms, the direction of effect is unlikely to be only biology-driven behaviour; equally, confrontational situations, social dominance, peer rejection, and self-esteem do influence physiological parameters.
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