Question

A recent newspaper headline reads "teenagers use more drugs if their friends use drugs." it implies that hanging out with drug-abusing friends can cause subsequent drug use disorder. the major problem with this assumption is that it assumes the direction of _____. alternately, perhaps teenagers who are using drugs may select friends who also use drugs.

Answer 1

I believe the answer is: causality.

The initial assumption above  is that a teenager who use drugs would most likely influence other non-users to use drug. There is nothing from the finding that disprove otherwise. So, when user hang out with non-users, there is a pretty good chance that the non-users could influence the users to stop consuming the drug.