Research on teen smoking and other deviant behaviors has found that the most important factor in statistically predicting if a teen will take up a particular deviant behavior is the presence or absence of peers who also engage in that behavior. This is probably because the other teens are acting:

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Peers can be powerful agents of socialization, influencing each other's beliefs, attitudes, and behaviors. In the case of deviant behaviors like smoking, research has found that having peers who engage in that behavior increases the likelihood that a teen will do the same. It's not necessarily that peers are the most important long-term force, but rather that they can have a significant impact on a teen's immediate decisions and behaviors.

Research on Teen Smoking and Other Deviant Behaviors Has Found