Modern times bring new pressures, in part because society has become increasingly complex. “Each of us are members of many different social groups, and each of them have different expectations of us,” says Randolph Nesse, a physician and evolutionary biologist at Arizona State University. “All of them want more of our time than we can actually provide.” That’s not to say we’re more stressed than early humans. “Nero thought Roman times were stressful,” Nesse says. Ours just involves trying to parse Facebook’s privacy settings.