You'd think sometimes that girls and boys aren't even in the same elementary schools. They seem to drift in their own gender-specific packs -- the boys oblivious to the girls and the girls disinterested in the boys. This is an over-simplification, of course. And eventually, the boys and girls do start to clue in to one another as they enter adolescence.

But an interesting Canadian study in the Journal of Research on Adolescence found this transition can be harmful, particularly for girls, if they develop opposite-sex friendships too quickly.

Following 400 students of a French Canadian school from ages 12 to 18, researchers found that boys and girls who already had numerous opposite-sex friendships when they entered adolescence were more likely to abuse alcohol over time. Existing opposite-sex friendships also predicted drug use over time by girls. The study also found girls at greater risk of drug and alcohol abuse if they developed opposite-sex friendships more rapidly than others during early adolescence.

Boys were no more likely to abuse drugs and alcohols if they developed rapid networks of opposite-sex friendships during adolescence. The working theory is that boys gain emotional support from friendships in early adolescence with girls, whereas the girls do not receive the same support from these friendships.

Girls who engaged in early opposite-sex friendships were more likely to seek out older boys and engage in anti-social behavior, the researchers found. (Remember, this is Canada, where the drinking age is 18. One dynamic here might be troubled girls seeking out older boys to buy them alcohol.) So it might not be that boys somehow drive girls to drink, but rather that girls with problems are more likely to seek out these relationships earlier in life.

The authors encouraged parents to closely monitor their daughters' friendships, particularly with older boys, during adolescence. They also offered three theories for their findings: (1) both drug abuse and opposite-sex friendships might simply reflect the desire among immature girls to strive for adult status. (2) girls might abuse drugs or smoke cigarettes because they think it will attract boys. (3) girls might form relationships with older boys to gain access to drugs and alcohol.