A mother leaves a toddler in an unfamiliar room with a stranger, then comes back three minutes later. One child wails while she is gone, then settles within seconds of her return. Another barely reacts to the departure, and when she walks back in he carries on stacking bricks as though nobody had left. In a laboratory in Baltimore in the 1960s, Mary Ainsworth watched that scene play out hundreds of times. What interested her was never the crying. It was the reunion. Key Takea