In studies designed to measure how social support influences the stress response, Coan brings volunteers into an MRI scanner and threatens to zap them with an electric shock. Periodically a symbol flashes before their eyes, indicating there’s a 20 percent chance they’ll receive a shock in the next few seconds. The goal, he says, is to create an “anticipatory anxiety” that mimics the feeling you get from everyday stressors like a looming work deadline.

But the volunteers aren’t in it alone. Some are holding the hand of someone they trust—a romantic partner, parent, or close friend. Others are holding the hand of a stranger. Coan has found that brain activity in the hypothalamus, the region heavily implicated in the body’s stress response, differs between those holding a loved one’s hand and those holding hands with a stranger. Clasping hands with a loved one tamps down threat-related activity.