Studies show that older people tend to remember the positive things in life rather than the negative things, while younger people remember the positive and negative equally well. The dominant psychological theory to explain this is that older people are aware of their limited time left, so they prioritize positive emotional experiences. But about a decade ago, I worked with biologist Robert Trivers on his idea that there was an evolutionary basis for older people's increased positive outlook. Our research took us in the fascinating direction of exploring how the body uses its energy.
When our ancestors needed more energy than usual, perhaps while being chased by a tiger, they had to get that energy from somewhere in the body. Could they borrow it from the brain? That organ uses 20 percent of our metabolic (新陈代谢) output, whether we are solving math problems or watching television reruns. Due to this constant energy requirement, borrowing energy from the brain when our need surpasses the available supply is not an option. Perhaps we could borrow energy from our muscles. Because we use far more muscle energy when we are active than when at rest, in principle, we could borrow energy when we are sitting. But the problem is that most of the energy-demanding emergencies of our ancestors required a muscular response. There was no way to borrow energy from our muscles during an emergency because relaxing when a tiger showed up was not an effective response. This brings us to our immune system, which, when strong, protects us from many illnesses and diseases. Like the brain, the immune system works at great metabolic cost, but largely in the service of keeping us healthy in the future. We have an enormous number of immune cells coursing through our body, a momentary break from production is fine. So, when our body needs extra energy, one of the places it goes is our immune function. When you're being chased by a tiger, you don't need to waste energy making immune cells to fight off tomorrow's cold. What you need is to shift all available energy resources to your legs, with the hope that you will live to experience another cough or sneeze.
As a result, our immune system evolved to run in maximum amounts when we're happy, but to slow down dramatically when we're not. With this background in mind, Trivers supposed that older people evolved a strategy of turning this relationship on its head, becoming more focused on the positive things in life in an effort to enhance their immune functioning. This was helped along by their knowing much more about the world than younger adults, so they can deal with some of the unpleasant things in life more easily.
