Nicholas Christakis

If your belly is hanging over your waistband more than it used to, you probably tend to point the finger at all those macho nachos you’ve been eating. And you should – the macho nachos are definitely to blame. But: if you have any extra large friends, you ought to hold them accountable too. Research suggests obesity is contagious – it can literally spread from person to person, like a fashion trend. And it’s not the only social contagion out there – depression and even goodness appear to be communicable too. Harvard medical sociologist Nicholas Christakis studies social contagion, and he speaks with Faith Salie on Fair Game.