Although medical experts are unable to conclusively determine what causes people to abuse drugs, there have been a number of explanations offered. People use drugs for a variety of different reasons: 1. Some may begin using drugs as teenager because of peer pressure; 2. Some believe that it makes them seem more sophisticated or grown up; 3. Some think that using drugs is an expression of rebellion against their parents or society in general. Recently, there has been a great deal of research into the likelihood of a biological influence. Advocates of this theory approach drug addiction as a disease, and point out evidence that suggests a genetic predisposition to drug abuse. Other theorists believe the behavior is learned solely from observing others, or that the use of drugs is somehow reinforced or encouraged by social relationships. In addition, a person's belief systems or expectations about drugs may play a role in drug abuse.