Marijuana: The Genetic Component

Scientists have found that whether an individual has positive or negative sensations after smoking marijuana can be influenced by heredity. A 1997 study demonstrated that identical male twins were more likely than nonidentical male twins to report similar responses to marijuana abuse, indicating a genetic basis for their response to the drug. (Identical twins share all of their genes.)

It also was discovered that the twins' shared or family environment before age 18 had no detectable influence on their response to marijuana. Certain environmental factors, however, such as the availability of marijuana, expectations about how the drug would affect them, the influence of friends and social contacts, and other factors that differentiate experiences of identical twins were found to have an important effect.

http://www.drugabuse.gov/infofacts/marijuana.html