New research shows that even severe stress can have an upside
For centuries physicists have used the word stress to describe force applied to materials. It was not until the 1930s that Hans Selye, a Hungarian-born endocrinologist, began using it of live beings. Selye injected rats with cow hormones, exposed them to extreme temperatures and partially severed their spinal cords to prove that all these sort of maltreatment affected the rodents in the same ways：they lost muscle tone, developed stomach ulcers and suffered immune-system failure. He used the word for both the abuse of the rats and the health effects. Later on, it started to be used for psychological suffering as well.
Today, the Oxford English Dictionary defines stress as “a state of mental or emotional strain or tension resulting from adverse or demanding circumstances”. The causes vary enormously：one person may be stressed by exams but happily swim with Great White sharks. Another may have to take sedatives before flying, but adore speaking to a crowd. This makes stress hard to measure. Proxies, such as the Negative Experience Index produced by Gallup, a pollster, suggest the world is growing more pessimistic, which may indicate increasing stress levels. Other surveys confirm what is perhaps obvious：stress is universal.
The American Psychological Association (APA) suggests that, at least in America, the most common cause are to do with money, work and family. Women report being more stressed than men and are twice as likely to be diagnosed with anxiety disorders. Studies on rats indicate that sex hormones such as oestrogen and progesterone may play a part；so may the double burden of family and work. Men may also be more likely to conceal their distress. Black and Hispanic Americans, as well as poor people and parents, also report higher levels of stress. In 2015 half of Americans starting university reported being stressed most or all of the time.
Young people have long reported more stress than old people, says Mary McNaughton-Cassil of the University of Texas at San Antonio. But she believes today's youth are more overwhelmed than ever before. Globalisation means rapid change in the workplace, and firms increasingly expect employees to be constantly connected. The mass media flood us with bad news while creating unattainable aspirations, she adds：“You have to look like a movie star, stay informed about politics, take care of the kids and hold down a job.” Social media, which may lower stress when used to strengthen connections with friends, have been associated with higher stress when they deliver news of friends' travails, such as divorces and accidents.
Many studies have shown that stress has similar effects on humans as on Selye's rats. It has been linked to high blood pressure, headaches, stomach upset and insomnia. According to the APA, chronic stress can “ravage” the immune system and increase unhealthy behaviours, such as drinking and smoking, that raise the risks of diabetes and cardiovascular diseases. A recent study by Bruce McEwen, a neuroscientist at Rockefeller University, showed that exposing rats to stress for just three weeks changed their brain architecture. Forcing his rats to swim, among other unpleasant tasks, shrank the dendrites in their amygdalae, the parts of the brain that control emotional responses, decision-making and memory. Though reversible, such changes in humans increase the risk of anxiety disorders and depression.
Late in his career Selye came to distinguish between “eustress”, or the good stress caused by positive experiences, such as falling love, and distress, the bad sort. Other scientists extended the original physics metaphor：just as many materials can withstand stress until a certain point, it was thought that humans could cope with stress if it did not become too severe. Indeed, the idea took hold that moderate stress might be a good thing. In 1979 Peter Nixon, a consultant at Charing Cross Hospital in London, described a “human function curve”：a moderate amount of stress, such as a deadline or race, was now understood as not just harmless, but beneficial. But above a certain threshold humans, like metal bars, would break.
