Adolescents (12-18 years old) and young adults (18-25 years old), are more likely than older adults to drive-or agree to be driven-recklessly or while intoxicated, to use illicit or dangerous substances and to engage in both minor and more serious antisocial behaviour. Numerous factors during adolescence may lead to or favour initiation of drug use, such as sensation-seeking, gregariousness and social conformity. These aspects, however, cannot be dissociated from the increased sex drive and quest for an integrated self. In the separation-individuation process, relationships with peers play many different roles: a field for experimentation, emotional support, a place for "projection" and "identification", and the possibility of finding a partner. Unsurprisingly, therefore, drug use generally takes place in a group setting. Despite evidence of heightened real-world risk-taking, laboratory studies have yet to yield consistent evidence that adolescents, when on their own, are more inclined towards risky behaviour than their elders. Moreover, their comprehension and reasoning abilities in risky decision-making situations are roughly equivalent to those of adults. Structural and functional neuroimaging studies have shown that neural circuitry undergoes major reorganization during adolescence, particularly in those regions of the brain relating to executive functions, the self and social cognition, and that the "emotional brain" may play a role in that reorganization. Age-related decreases in gray matter volume mainly reflect a reduction in the number of synapses and the complexity of axonal ramifications. By 18-20 years old, most of the subcortical white matter and association pathways have reached a plateau. Risk-taking behavior and novelty-seeking may provide, with an appropriate feed back, a mechanism to optimize brain development in adolescence.
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