Which of the following suggests a genetic influence on addiction?
a. identical twins are less likely than fraternal twins to have similarities in regard to alcoholism.
b. fraternal twins are less likely than identical twins to have similarities in regard to alcoholism.
c. the risk for alcoholism is higher for women versus men.
d. the risk for alcoholism is higher for men versus women.

A - identical twins are less likely than fraternal twins to have similarities in regard to alcoholism. The reason why this tells us about the genetics underlying addiction is because two identical twins share the completely same genetic information while fraternal twins do not. The fact that the first two share the same and higher probability for addiction tells us that it's probably a genetic influence.