Why do humans and their primate cousins get more stress-related diseases than any other member of the animal kingdom?

The answer, says Stanford University neuroscientist Robert Sapolsky, is that people, apes and monkeys are highly intelligent, social creatures with far too much spare time on their hands.

All vertebrates respond to stressful situations by releasing hormones, such as adrenalin and glucocorticoids, which instantaneously increase the animal's heart rate and energy level. In the short term, he explained, stress hormones are "brilliantly adapted" to help you survive an unexpected threat. "You mobilize energy in your thigh muscles, you increase your blood pressure and you turn off everything that's not essential to surviving, such as digestion, growth and reproduction," he said. "You think more clearly, and certain aspects of learning and memory are enhanced. All of that is spectacularly adapted if you're dealing with an acute physical stressor--a real one."

But non-life-threatening stressors, such as constantly worrying about money or pleasing your boss, also trigger the release of adrenalin and other stress hormones, which, over time, can have devastating consequences to your health, he said: "If you turn on the stress response chronically for purely psychological reasons, you increase your risk of adult onset diabetes and high blood pressure. If you're chronically shutting down the digestive system, there's a bunch of gastrointestinal disorders you're more at risk for as well."

Studies show that long-term stress also suppresses the immune system, making you more susceptible to infectious diseases, and can even shut down reproduction by causing erectile dysfunction and disrupting menstrual cycles. The bottom line, according to Sapolsky: "If you get chronically, psychosocially stressed, like a Westernized human, then you are more at risk for heart disease and some of the other leading causes of death in Westernized life."

The Founding Fathers probably weren't thinking about health when they declared the pursuit of happiness to be an inalienable right, but when it comes to understanding the importance of a stress-free life, they may have been ahead of their time.

"When you get to Westernized humans, it's only in the last century or two that our health problems have become ones of chronic lifestyle issues," Sapolsky said. "It's only 10,000 years or so that most humans have been living in high-density settlements--a world of strangers jostling and psychologically stressing each other. But being able to live long enough to get heart disease, that's a fairly new world."

"We're now about 70 years into thinking that sustained stress can do bad things to your health. The biggest challenge for the next 70 years is figuring out why some of us are so much more vulnerable than others."

In the meantime, Sapolsky suggested that people do whatever they can to reduce stress in their daily lives. "Try stress management, change your priorities or go into therapy," he said. "It takes work. Some people clearly never can overcome it. But the same things that make us smart enough to generate the kind of psychological stress that's unheard of in other primates can be the same things that can protect us. We are malleable."

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Professor Robert Sapolsky delivered the topical lecture, "Stress, Health and Coping," at the AAAS annual meeting.