Genetic explanations for the Initiation of addictions
Heritability of addictions such as alcoholism and gambling can be studied through family and twin studies. The presumptions of twin and family studies are that if family members share similar behaviours, then the degree to which the behaviours are caused by genetic factors can be examined.
There are two types of twins, identical and non-identical. Identical twins are monozygotic (MZ) as they came from the same egg, and therefore share an identical set of genes. Non-identical twins are dizygotic (DZ) as they come from two different eggs that were fertilised at the same time, and as a result their genes are no more alike than any brother or sister's.
Twins can also differ in the environment that they are brought up in. Some twins may, rarely, be separated at birth and therefore have very different upbringings. If researchers are able to find MZ twins who have been raised apart (i.e. same genes but different environments) then they will be able to distinguish between the influence of genes and environment on a certain behaviour. If MZ twins who have been raised apart booth show the same behaviour, then the behaviour is probably genetic; but if only one of them has that behaviour then it may well be caused by the environment. The degree to which twins and family members share a behaviour is termed a concordance rate, with 1 (or 100%) being entirely genetic, and 0 (or 0%) being entirely environmental.
McGue (1999) found a concordance rate of 50-60% for alcohol addiction, and Agrawal & Lynskey (2006) found a concordance rate of 45-79% for addiction to illicit drugs. This means that just over half of the factors that cause addiction to alcohol are genetic in origin, as are half to three-quarters of the factors causing addiction to illegal drugs. A problem with this conclusion, however, is that the genetic link may be for behaviour disorders rather than addiction, and these behaviour disorders may manifest as many antisocial behaviours including alcohol and drug use.
Are there specific genes implicated in addiction?
The sensitivity of the dopamine system is one biological explanation for the imitation, maintenance and relapse of addictive behaviours (see below), and so it would make sense for researchers to look for genes that are responsible for causing particularly sensitive or insensitive dopamine systems. One such gene is called the DRD2 gene (or dopamine receptor density 2 gene).
The DRD2 gene codes for the number of dopamine receptors available in the mesolimbic dopamine system. A person with fewer dopamine receptors is unable to 'feel' the effect of dopamine as well as someone with a higher number of dopamine receptors, and so needs higher levels of dopamine to produce the same effect as a person with more dopamine receptors. People with the DRD2 A1 variant of the gene have fewer dopamine receptors, and also seem more vulnerable to addictions.