 |
|
|
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.
|
|
