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
A General, Inherited Risk
Some of these hallmarks of risk are unsurprising. Most people know that addiction runs in families, and they may intuit that certain brain functions could differ in addiction-prone individuals. But how can people's gregariousness or their loathing of dull tasks or the age at which they first had sex show a vulnerability to addiction? The answer seems to be that although addiction risk is strongly heritable, the inheritance is fairly nonspecific. The inherited risk corresponds to a certain temperament or disposition that goes along with so-called externalizing tendencies. Addiction is only one of several ways this disposition may be expressed.
Externalizing behaviors include substance abuse, but also "acting out" and other indicators of behavioral undercontrol or disinhibition. In childhood, externalizing traits include hyperactivity, "oppositionality" (negative and defiant behavior) and antisocial behavior, which breaks institutional and social rules. An antisocial child may lie, get in fights, steal, vandalize or skip school. In adulthood, externalizing tendencies may lead to a personality marked by low constraint, drug or alcohol abuse, and antisocial behaviors, including irresponsibility, dishonesty, impulsivity, lawlessness and aggression. Antisociality, like most traits, falls on a continuum. A moderately antisocial person may never intentionally hurt someone, but he might make impulsive decisions, take physical and financial risks or shirk responsibility.
It's worth reiterating that an externalizing disposition simply increases the risk of demonstrating problematic behavior. An individual with such tendencies could express them in ways that are not harmful to themselves and actually help society: Fire fighters, rescue workers, test pilots, surgeons and entrepreneurs are often gregarious, relatively uninhibited sensation-seekers—that is, moderate externalizers.
So a genetic inclination for externalizing can lead to addiction, hyperactivity, acting-out behavior, criminality, a sensation-seeking personality or all of these things. Although the contents of this list may seem haphazard, psychologists combine them into a single group because they all stem from the same latent factor. Latent factors are hypothesized constructs that help explain the observed correlations between various traits or behaviors.
For example, grades in school generally correlate with one another. People who do well in English tend to get good marks in art history, algebra and geology. Why? Because academic ability affects grades, regardless of the subject matter. In statistical lingo, academic ability is the "general, latent factor" and the course grades are the "observed indicators" of that factor. Academic ability is latent because it is not directly measured; rather, the statistician concludes that it exists and causes the grades to vary systematically between people.
Statistical analyses consistently show that externalizing is a general, latent factor—a common denominator—for a suite of behaviors that includes addiction. Furthermore, the various markers of risk support this conclusion: Childhood characteristics that indicate later problems with alcohol also point to the full spectrum of externalizing behaviors and traits. Thus, drinking alcohol before 15 doesn't just predict future alcohol and drug problems, but also future antisocial behavior. A parent with a history of excessive binge drinking is apt to have children not only with substance-use problems, but with behavioral problems as well. And a reduced-amplitude P300 not only appears in children with a familial risk for alcoholism, but in kids with a familial risk for hyperactivity, antisocial behavior or illicit drug disorders.
The associations between externalizing behaviors aren't surprising to clinicians. Comorbidity—the increased chance of having other disorders if you have one of them—is the norm, not the exception, for individuals and families. A father with a cocaine habit is more likely to find that his daughter is getting into trouble for stealing or breaking school rules. At first glance, the child's behavioral problems look like products of the stress, conflict and dysfunction that go with having an addict in the family. These are certainly aggravating factors. However, the familial and genetically informative MCTFR data have allowed us to piece together a more precise explanation.
Environment has a strong influence on a child's behavior—living with an addict is rife with challenges—but genes also play a substantial role. Estimates of the genetic effect on externalizing behaviors vary by indicator and age, but among older adolescents and adults, well over half of the differences between people's externalizing tendencies result from inheriting different genes.
Our analysis of the MCTFR data indicates that children inherit the general, latent factor of externalizing rather than specific behavioral factors. Thus, an -antisocial mother does not pass on genes that code simply for antisocial behavior, but they do confer vulnerability to a range of adolescent disorders and behaviors. Instead of encounters with the law, her adolescent son may have problems with alcohol or drugs. The outcomes are different, but the same genes—expressed differently under different environmental conditions—predispose them both.
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