Field Guide to the Loner: The Real Insiders

John Cacioppo, a psychologist at the University of Chicago, has highlighted social isolation as a health-risk factor on par with obesity and smoking. "Loneliness is like hunger and thirst—a signal to help your genes survive," Cacioppo says. "When you're lonely, there's a stress response in your body, and it's not healthy to sustain that for a long time."