Identical twins share all of their genes, and fraternal twins share about half.
And it further emphasizes that drug use and addiction are not simply social problems, but are health issues affected by an individual's biological state. The identical twins answered similarly while the fraternal twins did not. The results indicate that genes have a significant influence on individual pleasant or unpleasant responses to the effects of marijuana. Taking the environmental and genetic influences together, these results suggest that while exposure to marijuana by factors such as social contacts are important, there are individual differences, perhaps in the brain's reward system, associated with genetic factors that influence whether one will continue using marijuana. Environmental factors may lead to an individual's experimenting with a drug, according to Dr. Tsuang, but heredity appears to hold the key to whether or not an individual will continue to use or abuse the drug.