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5. GENETIC BASIS OF SUBSTANCE DEPENDENCE

compensating for stress factors when entering the professional adult world,
and facing retirement between the ages of 55 and 65 years (see Vogt, 2000a,b).
There is some empirical evidence that social inequality and class
differences are related to risky use of both licit and illicit psychoactive
substances. For example, the decline in smoking in some countries has been
more rapid in men and women from higher socioeconomic classes, and
drinking shows an inverse relationship with occupational status (Marmot,
1997). As regards illicit substances, ecological studies have shown that the
poverty status of communities is a powerful predictor of fatal drug overdoses
of cocaine and opioids in developed countries. For example, in a study in an
urban community in New York, 69 % of the variance in fatal drug overdoses
was explained by poverty status (Marzuk et al., 1997).
Poverty also is associated with problems of nutrition and a wide array of
negative contextual conditions: malnourished individuals are especially
vulnerable to adverse effects of consumption of licit and illicit substances
(Charness, 1999). General health status and nutritional state play an
important role: for example diabetes, hypertension and hepatitis C virus
increase vulnerability to alcohol (Regev & Jeffers, 1999; Weathermon & Crabb,
1999. More specifically, health-related knowledge about alcohol use and anti-
drinking attitudes lowered the odds of drinking (Epstein et al., 1995).
Personal resources include coping skills, self-efficacy, risk perception,
optimism, health-related behaviour, ability to resist social pressure and
general health behaviour. These resources will interact with and possibly be
enhanced by community programmes that can fulfil the needs of people to
resort less frequently to substance use, and that provide a healthy
environment in which the individual feels less pressured to use licit and illicit
substances.

Summary
Family, twin and adoption studies provide strong evidence for a significant,
but not exclusive, genetic contribution to the development of substance use
and dependence. Environmental factors, and individual specific experiences,
are also of major importance. Family and adoption studies that have focused
on general risk for substance use show that substance dependence is a familial
trait, which can be attributed to either shared environment or shared genes.
Twin studies consistently show higher monozygotic than dizygotic
concordance for substance dependence, indicating a genetic effect.
The significant and complex genetic contributions to substance
dependence continue to motivate efforts to identify allelic variants that
contribute to dependence vulnerability, even if each allelic variant contributes
only a modest fraction to the whole problem. Genotypes at loci containing
vulnerability alleles could provide improved approaches to treating
vulnerable individuals and thus maximize the use of resources for prevention
and treatment. Individual and societal suffering could be relieved by better

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