Travis and Marcus, close friends, grew up in the same neighborhood. Both were exposed to chronic poverty, gang violence, and family dysfunction. By the time they were in high school, Travis was flunking most of his classes, had a child, and had been arrested numerous times for drug-related crimes and theft. Marcus, on the other hand, was a good student, played football, and had aspirations to attend college. These different developmental trajectories, despite similar environmental influences, illustrate the concept of _____