FEATURE ARTICLE
Predicting Addiction
Behavioral genetics uses twins and time to decipher the origins of addiction and learn who is most vulnerable
Lisa Legrand, William Iacono, Matt McGue
The Role of the Environment
Even traits with a strong genetic component may be influenced by environmental factors. Monozygotic twins exemplify this principle. Despite their matching DNA, their height, need for glasses, disease susceptibility or personality (just to name a few) may differ.
When one member of a monozygotic pair is alcoholic, the likelihood of alcoholism in the other is only about 50 percent. The high heritability of externalizing behaviors suggests that the second twin, if not alcoholic, may be antisocial or dependent on another substance. But sometimes the second twin is problem free. DNA is never destiny.
Behavioral geneticists have worked to quantify the role of the environment in addiction, but as a group we have done much less to specify it. Although we know that 50 percent of the variance in alcohol dependence comes from the environment, we are still in the early stages of determining what those environmental factors are. This ignorance may seem surprising, as scientists have spent decades identifying the environmental precursors to addiction and antisocial behavior. But only a small percentage of that research incorporated genetic controls.
Instead, many studies simply related environmental variation to children's eventual problems or accomplishments. A classic example of this failure to consider genetic influence is the repeated observation that children who grow up with lots of books in their home tend to do better in school. But concluding that books create an academic child assumes (falsely) that children are born randomly into families—that parent-child resemblance is purely social. Of course, parents actually contribute to their children's environment and their genes. Moreover, parents tend to provide environments that complement their children's genotypes: Smart parents often deliver both "smart" genes and an enriched environment. Athletic parents usually provide "athletic" genes and many opportunities to express them. And, unfortunately, parents with addiction problems tend to provide a genetic vulnerability coupled with a home in which alcohol or drugs are available and abusing them is normal.
To understand the true experiential origins of a behavior, one must first disentangle the influence of genes. By using genetically informative samples, we can subtract genetic influences and conclude with greater confidence that a particular environmental factor affects behavior. Using this approach, our data suggest that deviant peers and poor parent-child relationships exert true environmental influences that promote substance use and externalizing behaviors during early adolescence.
When considering the effect of environment on behavior, or any complex trait, it's helpful to imagine a continuum of liability. Inherited vulnerability determines where a person begins on the continuum (high versus low risk). From that point, psychosocial or environmental stressors such as peer pressure or excessive conflict with parents can push an individual along the continuum and over a disease threshold.
However, sometimes the environment actually modifies gene expression. In other words, the relative influence of genes on a behavior can vary by setting. We see this context-dependent gene expression in recent, unpublished work comparing study participants from rural areas (population less than 10,000) with those from more urban settings. Within cities of 10,000 or more, genes substantially influence which adolescents use illicit substances or show other aspects of the externalizing continuum—just as earlier research indicated. But in very rural areas, environmental (rather than genetic) factors overwhelmingly account for differences in externalizing behavior.
One way to interpret this finding is that urban environments, with their wider variety of social niches, allow for a more complete expression of genetically influenced traits. Whether a person's genes nudge her to substance use and rule-breaking, or abstinence and obedience, the city may offer more opportunities to follow those urges. At the same time, finite social prospects in the country may allow more rural parents to monitor and control their adolescents' activities and peer-group selection, thereby minimizing the impact of genes. This rural-urban difference is especially interesting because it represents a gene-by-environment interaction. The genes that are important determinants of behavior in one group of people are just not as important in another.
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