Vol. 7, No. 4, 2001 Page 1&2

Young delinquents: twin study points to role of genes

Millions of teens and young adults, most of them male, commit delinquent acts. However, only a fraction of this group—around 5 percent—exhibit chronic, severe antisocial behavior.

One factor that differentiates "experimenters" from chronic offenders is the age at which they begin exhibiting antisocial behavior. Those who start before puberty tend to be lifelong offenders, while those who commit their first offenses after puberty usually "grow out" of their antisocial behavior.

A recent study by Jeanette Taylor and colleagues offers strong evidence that genetic factors play a strong role in the development of early-onset antisocial behavior, while these factors play less of a role in the transitory delinquency of many young males.

Taylor and colleagues studied sets of male twins, evaluating them every three years beginning at age 11. Participants included 36 individuals with early-onset antisocial behavior, 86 late-starters, and 25 non-delinquent controls. Measures of antisocial behavior included teacher ratings, parental reports, self-reports, and information regarding subjects' contacts with police.

Taylor et al. say that early-starters exhibited lower verbal intelligence and made significantly more errors in measures of delayed memory than did late-starters, who did not differ significantly from controls. These results, they say, "are consistent with other reports of cognitive and executive functioning deficits found among persistently antisocial boys." (Executive function refers to cognitive skills involved in self-control, long-term planning, and related abilities believed to be mediated by the frontal lobes of the brain.)

In addition, early-starters exhibited higher rates of attention deficit hyperactivity disorder and oppositional defiant disorder than either late- starters or controls. They also were significantly more impulsive than either late-starters or controls. The researchers note, however, that late- starters were more impulsive than non-delinquent controls, suggesting some genetic liability among even transient delinquents. Unlike several previous research groups, Taylor et al. did not find consistent differences in skin conductance reactivity (a measure of autonomic nervous system function) between control subjects and the early- or late-onset antisocial groups.

Comparing monozygotic ("identical") and dizygotic ("fraternal") twins, the researchers found that "the risk of being an early-starter was substantially greater for co-twins in monozygotic pairs (55 percent concordant) than for co-twins in dizygotic pairs (29 percent concordant) in which one boy was an early-starter." This is a strong indication of genetic influences, since monozygotic twins share twice as many genes as dizygotic twins. The researchers also found that when both monozygotic twins in a set exhibited antisocial behavior, they nearly always exhibited the same subtype (early-onset or late-onset), "indicating that when two genetically identical individuals are concordant for delinquency they almost always have the same form."

In addition, early-starters had significantly more first-degree relatives with adult antisocial behavior than either late-starters or controls, and significantly more second-degree relatives with either adult antisocial behavior or childhood conduct disorder than controls.

In summary, the researchers say, "[E]arly starters had lower verbal and spatial memory functioning, more problems related to psychological, emotional, and behavioral inhibition, higher negative emotionality, earlier and more persistent association with antisocial peers, and higher familial transmission of antisocial behavior and greater genetic influence on their phenotype."

The researchers say that this genetic influence "represents an underlying biological liability toward disinhibition. That is, early starters possess behavioral and personality problems related to inhibition that increase the likelihood that they exhibit antisocial behavior at an early age (because many of these characteristics develop early) and over the course of the life span (because many of these characteristics are fairly stable)."

Taylor et al.'s findings are consistent with a 1997 large-scale twin study by Wendy Slutske et al., which found that genes play a powerful role in determining a child's risk for conduct disorder (see related article, Crime Times, 1997, Vol. 3, No. 3, Page 1). Similarly, a twin study byThalia Eley et al. found that early, aggressive bullying was strongly influenced by genes (see related article, Crime Times, 1999, Vol. 3, No. 3, Page 1 & 6).


"Evidence for a genetic etiology of early-onset delinquency," Jeanette Taylor, William G. Iacono, and Matt McGue, Journal of Abnormal Psychology, Vol. 109, No. 4, 2000, 634-43. Address: Jeanette Taylor, Department of Psychology, Florida State University, Tallahassee, FL 32306-1270, taylor@psy.fsu.edu.

Related Article: [2001, Vol. 7]

Return to:
[Author Directory] [Front Page] [Issue Index] [Subject Index] [Title Index]