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Teenagers who spent long hours in day-care as toddlers are more likely to take risks and act impulsively than peers who spent those years at home, finds the largest and longest study of child-care in America. The findings—sure to stir controversy among parents and policy makers—found that teens rated themselves 16% more impulsive for every extra 10 hours they spent per week in day-care as toddlers.

"These small effects end up being spread and bounce off each other," said one researcher. "In classrooms, the dynamic becomes 'I dare you to take a risk, you dare me to take a risk.'" At least one expert, from New York's Families and Work Institute, thinks the behavioral impact might be good, reports the Los Angeles Times. "All these aspects of impulsiveness and risk-taking can be a positive thing," she noted. (Read more daycare stories.)