R. B. J. Casey is a professor of psychology at Weill Cornell Medical College. Her focus on adolescent brains includes those of humans and mice. “There is evidence,” she says, “that even adolescent rodents tend to hang out with same-aged peers and tend to have more fights with their parents.” Sound familiar? These behaviors—sensitivity to influence from peers, taking risks, and pulling away from parents—are hallmarks of human adolescence. They have their roots in the hunter-gatherer world of our early ancestors, where success meant surviving and reproducing. What cause-effect relationship does the author identify in the section?