Although we know that the risk for alcoholism runs in some families, it is not clear exactly what genes cause the increased risk. (There is good evidence that several genes are involved.)

Most of the genetic studies that have been done on addiction have dealt with alcoholism, but there is growing evidence that many of the basic principles apply to other drugs as well.

In fact, statistical analysis shows that if you have relatives with drug dependence, you are at substantially increased risk of becoming addicted to any drug, including alcohol.

But it seems as though the risk is higher pertaining to the specific drug that your relatives are addicted to. If they are alcoholic, your risk of becoming alcoholic increases; if they are addicted to opiates, you are at increased risk for that addiction. The same seems true for marijuana, cocaine, and nicotine.

Whenever we wonder if a condition is inherited, we have to ask ourselves whether the trait that we see in both parent and child is caused by transmission through the genes or by some common factor in the environment. For example, how do we know whether a child who loves music and is talented in that field inherited the characteristic or is just benefiting from growing up in surroundings where music is appreciated?

Researchers approach this question by looking for cases of children who have been adopted, or, even better for purposes of investigation, identical twins who were separated at birth and have been raised in different families. Some countries, such as Denmark, have maintained registries of twins and of adoptions for many years, so that the records are available for research. When these are analyzed regarding the subject of addiction, some patterns begin to emerge.

Studies of twins are especially useful. Geneticists classify identical twins as monozygotic, meaning that they developed from one egg, and fraternal twins as dizygotic, meaning that they came from two different eggs (a zygote is a fertilized egg). Identical twins have 100 percent of the same genes and fraternal twins about 50 percent of the same genes. When twins exhibit the same trait, it is called concordance.

The risk of addiction regardless of family history is about one in ten. If an identical twin is alcoholic, there is more than a 50 percent chance that the other twin will be alcoholic, even if they are reared apart. On the other hand, fraternal twins reared apart have about the same risk of developing alcoholism as other siblings of alcoholics - about a one-in-four risk. The concordance rates, therefore, are about 50 percent for monozygotes and about 25 percent for dizygotes, indicating that genes play an important role in addiction. That identical twins don’t have a 100 percent concordance rate as they would for physical traits such as eye color shows that nongenetic factors are also important.

Adoption studies help us to sort out genetic factors from environmental ones. Children whose biological parents were alcoholic are far more likely than others to become alcoholic even if their adoptive parents are not alcoholic. In fact, the biological child of an alcoholic parent is three to five times more likely to be alcoholic than the biological child of nonalcoholic parents, even in nonalcoholic families that are considered well-adjusted and functional.

It can be shown by statistical means that inheritance accounts for about 50 percent of the risk of an individual becoming alcoholic. Some other traits, such as impulsiveness, aggressiveness, and antisocial tendencies, are also likely to be passed down from the biological parents - regardless of the traits of the adoptive parents. These traits contribute to the risk for addiction but appear to function independently of the basic genetic flaw that puts relatives of alcoholics and addicts at risk for addiction.

One clever way to study the genetics of addiction is to use animal models. A group of researchers in the 1970s began selectively breeding lab rats who appeared to enjoy water spiked with alcohol. Gradually, two groups of rats were bred: “P” rats (alcohol-preferring) and “NP” rats (alcohol nonpreferring). The “P” rats not only bred true but also developed many behaviors reminiscent of a human alcoholic.

For example, they would consistently opt for spiked water in place of some of their rat chow, and they would go on binges, developing blood alcohol levels of up to four times what the legal driving limit would be for humans. This study was important because it helped to prove that genetic factors alone - regardless of the environment - could lead to alcoholism.