The way you make choices about your alcohol consumption and your decisions whether to use marijuana may only be your choices to a degree.

Research has shown that heritable factors are a big part of dependency on alcohol and cannabis, which can influence many other areas of life. However, there has been a lack of research into how heritable influences overlap across the two substances.

A new study seeks to look at the overlap between heritable factors across the two substances and how use might be attributed to environmental or common genetic influences. Sartor, et al., of the Washington University School of Medicine, the Queensland Institute of Medical Research, and the University of the Sunshine Coast studied the relationships between these factors.

The researchers quantified cross-substance overlap in sources of variance and estimated the extent of associations between use and dependence measures and how genetic and environmental factors might be responsible.

To gather their data, the researchers used the Australian Twin Registry. The sample consisted of 2,761 sets of twins and 735 singletons, totaling 6,257 individuals. Telephone interviews were used to gather information about alcohol and cannabis use and history.

Using the data collected via telephone survey, the study employed standard genetic analysis to establish a quadrivariate model that showed estimates of overlap in genetic and environmental factors across the four phenotypes.

The results of the study showed an over 60 percent of variance in alcohol consumption, cannabis use, and cannabis dependence symptoms, and close to a 50 percent of variance in alcohol dependence symptoms were found to be from genetic sources.

The study found that common environmental influences did not contribute to the four phenotypes. The overlap in heritable influences was almost complete for within-substance measures of use and dependence.

The findings of this study indicate that common genetic factors are influential for alcohol and cannabis use and for dependence, but substance-specific influences are largely responsible for the genetic variance in the cannabis use and dependency phenotypes.

In addition, the study has suggested that measures of the frequency and heaviness of use may indicate genetic factors are also responsible for alcohol and cannabis problems, as they are for dependency measures.

In comparing twins and singletons, this study has examined the genetic influences on alcohol and cannabis dependency. The results show that genetics play a big part in determining behaviors related to substance abuse.