494 Substance-Related and Addictive Disorders
Among adolescents, conduct disorder and repeated antisocial behavior often co-occur
with alcohol- and with other substance-relate d disorders. While most  individuals with al-
cohol use disorder develop the condition be fore age 40 years, perhaps 10% have later
onset. Age-related physical changes in older individuals result in increased brain suscep-
tibility to the depressant effect s of alcohol; decreased rates of  liver metabolism of a variety
of substances, including alcohol; and decrease d percentages of body water. These changes
can cause older people to develop more seve re intoxication and su bsequent problems at
lower levels of consumption. Al cohol-related problems in older people are also especially
likely to be associated with  other medical complications.
Risk and Prognostic Factors
Environmental. Environmental risk and prognostic factors may include cultural atti-
tudes toward drinking and intoxication, the availability of alcohol (including price),
acquired personal experiences with alcohol, and stress levels. Additional potential medi-
ators of how alcohol problems develop in predisposed individuals include heavier peer
substance use, exaggerated positive expectations  of the effects of alcohol, and suboptimal
ways of coping with stress. 
Genetic and physiological. Alcohol use disorder runs in families, with 40%–60% of the
variance of risk explained by genetic influenc es. The rate of this condition is three to four
times higher in close relatives of individuals with alcohol use disorder, with values highest
for individuals with a greater nu mber of affected relatives, closer genetic relationships to
the affected person, and higher severity of the alcohol-related problems in those relatives.
A significantly higher rate of alcohol use disorders exists in the monozygotic twin than in
the dizygotic twin of an individual with the condition. A three- to fourfold increase in risk