As Philadelphia police step up patrols to crack down on marauding mobs of teenagers who have assaulted pedestrians and damaged property in recent weeks, one question keeps popping up: Why? According to Laurence Steinberg, a psychology professor at Temple who has researched adolescent behavior extensively, kids do things when they're with their friends that they wouldn't do if they were by themselves. "What we find is that when teenagers are in the presence of their friends they tend to pay more attention to the possible rewards of a risky decision, and therefore less attention to the possible cost."