The Genetic and Environmental Bases of Addiction
Posted by dlende on May 6, 2008
As Presented By: Reid, Takashi, Sheeva and Michael
A man with deep set eyes and a tired, drawn face wanders aisle to aisle, seemingly lost amongst the labyrinth of supermarket shelves. His bloodshot eyes, bent forward posture and slight stature are indicative of years of hard living. His pain is readily apparent as he nervously shifts his weight from one foot, to the other and then back again. He rolls up his sleeve to scratch an unseen itch, briefly revealing a patchwork of new and old needle marks along the veins of his forearms; intermeshed with a few cigarette burns and dry, yellowing skin. How did he get this way you might ask? What is it about this particular man that caused him to become an addict?
The Genetic Element
Today many people would say “his genes” predisposed him to become an addict. Addiction has historically been known as a disease that runs in families, and in the past 30 or 40 years, this long-standing belief has been verified using systematic scientific investigation. The bulk of the research suggests that drug dependence functions much like other diseases, with certain people having a genetic makeup that increases their risk.
This was the case for Caroline Knapp, an alcoholic who skillfully describes her battle and eventual victory over addiction in her book Drinking: A Love Story. Knapp struggles with her genetic predisposition saying, “It’s encoded in my DNA, embedded in my history, the product of some wild familial aberration.” This conclusion is not limited to Knapp. One study found that children of alcoholics were four times more likely to become alcoholics themselves.
Modern scientific inquiries tell us that the inheritance of these addictive tendencies cannot be attributed to a single gene, as is the case for some diseases. Its transmittance is much more complicated.
For instance, genes involved in the metabolism of alcohol can be implicated in increased risk of addiction. For instance a major study found that young men who required more alcohol to experience an effect had higher rates of alcohol problems later in life. However, other genes, including those known to affect behavior and mood, are thought to be connected with addiction as well (National Institute on Alcohol Abuse and Addiction). Currently, scientists point to differences in clusters of genes on chromosomes 1, 2, 3, 4, 7, 11, 15, and 16 as important in chemical dependence (Goldman Review).
Some of the most convincing evidence for a genetic susceptibility for addiction can be seen for those people who are predisposed not to become addicted. Some people, most commonly Asians, have a deficient gene for the enzyme ALDH2 which is in involved in the breakdown of alcohol. Upon consuming alcohol, these people experience a buildup of the metabolic intermediate acetaldehyde which causes nausea and is toxic to the body. Acetaldehyde also dilates blood vessels, resulting in the facial flushing that is commonly associated with the condition. Not surprisingly, these people avoid alcohol and are at a very low risk of developing dependence (CHSSC Health and Wellness).
Twins studies are another important approach to understanding genetics and addiction. These studies attempt to analyze the contributions of genetics versus environmental factors to a specific trait.
If genetic makeup influences the risk for a disease, identical twins (who have identical genomes) will tend to display concordant behavior; that is, either both will have the disease or neither will develop the disease. Fraternal twins, whose genomes are not identical, are less likely to be concordant. Therefore, if drug dependence has a genetic component, identical twins will share addictive behavior at a higher rate than fraternal twins (National Institute on Drug Abuse). This is exactly what many twin studies show.
One study analyzing 1,934 twins conducted at the Medical College of Virginia found a strong genetic link for cocaine addiction. The study indicates that concordance for cocaine use was 54 percent for identical twins and 42 percent for fraternal twins. However, the most substantial differences were seen for abuse (defined as recurring use that presents a danger) and, more seriously, dependence (defined as physical tolerance, and increasing, dangerous use). The study found concordance for cocaine abuse was 47 percent for identical twins and 8 percent for fraternal twins; and for dependence, 35 percent for identical twins and zero for fraternal twins (National Institute on Drug Abuse).
Dr Kendler, one of the study’s authors said, “For both cocaine and marijuana, genetic factors are responsible for roughly 60 to 80 percent of the differences in abuse and dependence between fraternal and identical twin pairs.” While placing percentages on the role of genetics in addiction is a difficult and variable process, this study and others strongly implicate genetics in drug abuse and dependence.
The Environmental Element
A careful study of the genetic causes of addiction can provide a wealth of knowledge about the subject; however, one must not forget about environmental triggers and experiences. Many anthropologists and other social scientists have made the case that social conditions matter, that is to say, that addiction “runs along the fault lines of society” (Neuroanthropology Blog). In the experiment described in this link, scientists showed that monkeys who were regularly dominated by other monkeys were much more likely to self-medicate with cocaine than those monkeys at the top of the social ladder. Therefore, they theorized that the “derived stress from being dominated” played a significant role in the likelihood of drug abuse and addiction. Returning to the case of our addict in the introduction, his constant urge to use could be a direct result of his low status within the social structure. Also, the very fact that he is on the fringes of “accepted society” may actually be both a cause as well as a result of his addiction.
Additionally, a great amount of research has been dedicated to environmental triggers of abusive behavior. In the case of alcoholics, “one of the hallmarks…is a difficulty inhibiting responses for alcohol related stimuli” (ATTC). For example, even though a recovering alcoholic may have no problem controlling their addiction in the comfort of their own home, the overwhelming urge to use when they walk by a favorite bar may simply be too much for them to handle. The same is also true of addicts to other drugs. The stimuli from a smell, taste or place commonly attributed an environment of drug use can often set off strong memories of drug abuse for the addict. This process ultimately results in a powerful desire to use, even if they have been away from drugs for a considerable amount of time.
Furthermore, consider the environment in which an addict first experiences the substance(s) they have become addicted to. The use of drugs and alcohol is most certainly a learned behavior, as demonstrated by the cultural emphasis on learning ‘how’ to drink. Therefore, the environment in which a person acquires the knowledge of how to use must be important to the formation of an addiction.
As one site dedicated to the genetics and environmental causes of addiction puts it, “the biggest contributing factor to drug abuse risk is having friends who engage in the problem behavior” (Utah genetics). In the case of the addict introduced earlier, it is quite likely that his first experience with drugs occurred with his peers. In this situation, the mutual support of using as a group became a benefit in itself. Also, having friends that use drugs serves as a powerful cultural force for continued experimentation. Thus, the combination of positive reinforcement and a receptive environment for drug use ultimately results in an increased likelihood for addiction.
Bringing it all Together: A Combined Genetic and Environmental Model
Returning to our addict in the supermarket scenario for one final time, it is not unreasonable to say that some part of his current situation can be attributed to his genetic predisposition to addiction. However, one must not forget that environment plays a major role as well, for even if someone has all the ‘right stuff’ in their genetic code to develop an addiction, if they never come in contact with drugs or alcohol, they cannot possibly become an addict. Therefore, when considering addiction, one must consider both the genetics of a person as well as their environment. This realization will not only help those searching for the causes of addiction, but will also help to provide insight for those suffering from addiction as well.