Addiction and Twin Studies

Learn about research on addiction involving twins including environmental & genetic factors. Find out how drug addiction treatment can transform your life.

Twin studies play an important role in research to learn the reasons why people become addicted. As in many other debates, such as why some are smarter, healthier or happier than others, one of the main questions is whether nature (genes) or nurture (environment) is more important.

With regard to addiction, environmental effects are considered easier to deal with. Rehab centers and counselors seek to help people overcome addiction by altering their environment. When people avoid parties where addictive substances will be used, for example, they are changing their environment in an attempt to limit their substance abuse.

On the other hand, genetic effects were previously viewed as being beyond help. However, advances in medical research may one day make it possible to isolate the specific genes involved in addiction and create more effective medicines and treatments against addiction.

Twins in Research on Addiction

Identical twins are prized among medical researchers, psychologists, and other scientists who seek to study the effects of genetics and environment on human development. Identical twins, also known as monozygotic twins, share 100 percent of their genes. This effectively eliminates the genetic variable. Any differences that come up between the two twins, then, must be due to environmental factors.

Identical twins are further differentiated into two broad groups. Most identical twins are raised together in the same household. This means that the twins grow up in an environment in which many factors, such as nutrition, access to education and health care, are nearly the same. At least until these twins leave the home, many of the environmental variables are also eliminated. This means that while they are young, any differences that arise between identical twins raised together are due to the unique experiences of the individuals.

The second group, which is rarer still, includes identical twins raised separately. These twins are sometimes separated at birth and given up for adoption to different families. Sometimes the parents divorce and the children are brought up separately. Twins raised in this way can be compared against each other to gain a better understanding of how broader environmental factors shape outcomes in human development.

Non-identical twins, also known as dizygotic twins, are also commonly used in scientific studies. They share 50 percent of their genes. They can also be split into two groups; those who are raised together and those who are raised separately. They can be measured throughout their lives in terms of many different qualities to test the interplay of genetics and environment.

Genetic and Environmental Influences on Addiction

One study comparing alcohol and addiction in identical (monozygotic) and non-identical (dizygotic) twin pairs found that genetics definitely play an important role in addiction. The study found that when one person in a pair of identical twins is addicted to alcohol, there is a high probability that the other will also be addicted to alcohol. For non-identical twins there is also a correlation in alcoholism, but it is not as strong as between identical twins.

Another study found that children of alcoholic parents are far more likely to become addicted to alcohol than children of non-alcoholic parents. However, the children of alcoholic parents are also more likely to grow up in an environment where alcohol is always present and where alcohol use is modeled for them by their loved ones. Therefore, this study cannot show how much of the influence is genetic and how much is environmental.

A broad range of studies indicates that genes and environment play roughly equal roles in addiction. That is, genes are responsible for about 50 percent of the probability that someone will become addicted to substances. However, environment is responsible for the other 50 percent of the probability. This means that even if addiction runs in the family, most people can still avoid addiction by managing their environment. On the other hand, it means that people who may have little genetic predisposition to addiction can fall into it if they find themselves in the wrong environment.

Furthermore, the statement that genes are half responsible for addiction is a bit misleading. No matter what someone’s genetic makeup may be, they will not become addicted to drugs if they never try them. In this sense, a person who can control their behavior and keep from trying drugs can ensure they will not become addicted.

Researchers believe that at least some of the genetic effect may be related to people having a strong desire to try new things or being predisposed to become dependent on things that give them pleasure. These behavioral tendencies make them more likely to come into contact with drugs and increase the possibility that they could become addicted.

However, genetic factors could also have a more direct physiological effect, causing some people to experience a stronger effect on their nervous system from some substances. That is, some people may feel more of a high from some substances than others do. Furthermore, some people may also be predisposed to seek out experiences that give them that feeling, and less able to resist it. Studies on animals have found that the body’s ability to process alcohol and it’s sensitivity to exposure to alcohol are both influenced by genetics.

At least for now, people have no control over their genes. The environment is the factor that people can influence. Researchers therefore focus on what specific factors in the environment can have an impact on a person’s susceptibility to addiction.


A study by researchers at Harvard University, using a database of thousands of male twin pairs, found that there is evidence that people can inherit a tendency to become addicted to multiple substances. Researchers investigated whether people with a genetic predisposition to become addicted to alcohol were also more likely to become addicted to nicotine. They found that a significant portion of the correlation between being addicted to alcohol and cigarettes can be explained by genetic factors. This likelihood to become addicted to multiple substances is called cross-addiction.

This indicates that some people may be generally more at-risk for getting addicted to substances of any kind. This could be because their bodies react especially strongly to substances and experiences that make them feel good. They may get a comparatively large dose of dopamine, a hormone associated with pleasurable experiences, from such substances or experiences. This would make them comparatively less able to resist drugs despite knowing the dangers of high levels of usage. This could also bring about an addiction to certain kinds of behaviors, such as gambling of sexual activity.

Cross-addiction is especially dangerous when someone is trying to recover from addiction to one substance. They may turn to other substances to make themselves feel good, as their former source of pleasure has disappeared. It is important in these cases for people to seek pleasure in activities that are non-addictive and non-harmful. For example, they may get involved in sports, spend more time with friends, or even receive massage in order to fill the void left by the drug.

Genetics and Stages of Addiction

The aforementioned Harvard study also found that genetics can have different effects at different stages in the process of drug experimentation, use and abuse. For example, genes that predispose people to seek to try out new things could make them more likely to try out a drug in the first place. However, different sets of genes, related to the amount of pleasure they get from the drug and their ability to resist the drug, would play a bigger part at later stages and make the person more likely to become addicted.]]>