The predictability of our mortality rates is something that has long puzzled social scientists. After all, there is no natural reason why 2,500 people should accidentally shoot themselves each year or why 7,000 should drown or 55,000 die in their cars. No one establishes a quota for each type of death. It just happens that they follow a consistent pattern year after year.A few years ago a Canadian psychologist named Gerald Wilde became interested in this phenomenon. He noticed that mortality rates for violent and accidental deaths throughout the Western world have remained oddly static throughout the whole of the century, despite all the technological advances and increases in safety standards that have happened in that time. Wilde developed an intriguing theory called “risk homeostasis”. According to this theory, people instinctively live with a certain level of risk. When something is made safer, people will get around the measure in some way to reassert the original level of danger. If, for instance, they are required to wear seat belts, they will feel safer and thus will drive a little faster and a little more recklessly, thereby statistically canceling out the benefits that the seat belt confers. Other studies have shown that where an intersection is made safer, the accident rate invariably falls there but rises to a compensating level elsewhere along the same stretch of road. It appears, then, that we have an innate need for danger.In all events, it is becoming clearer and clearer to scientists that the factors influencing our lifespan are far more subtle and complex than had been previously thought. It now appears that if you wish to live a long life, it isn’t simply a matter of adhering to certain precautions: eating the right foods, not smoking, driving with care. You must also have the right attitude. Scientists at the Duke University Medical Center made a 15-year study of 500 persons’ personalities and found, somewhat to their surprise, that people with a suspicious or mistrustful nature die prematurely far more often than people with a sunny disposition. Looking on the bright side, it seems, can add years to your life span.1. What social scientists have long felt puzzled about is why ______.

2. In his research, Gerald Wilde finds that technological advances and increases in safety standards ______.

3. According to the theory of “risk homeostasis”, some traffic accidents result from _______.

4. By saying “...statistically canceling out the benefits that the seat belt confers” (Para. 2), the author means ______.

5. Which of the following may contribute to a longer life span?

A.the mortality rate can not be predicted B.the death toll remained stable year after year C.a quota for each type of death has not come into being D.people lost their lives every year for this or that reason问题2： A.have helped solve the problem of so high death rate B.have oddly accounted for mortality rates in the past century C.have reduced mortality rates for violent and accidental deaths D.have achieved no effect in bringing down the number of deaths问题3： A.our innate desire for risk B.our fast and reckless driving C.our ignorance of seat belt benefits D.our instinctive interest in speeding问题4： A.wearing seat belts does not have any benefits from the statistic point of view B.deaths from wearing seat belts are the same as those from not wearing them C.deaths from other reasons counterbalance the benefits of wearing seat belts D.wearing seat belts does not necessarily reduce deaths from traffic accidents问题5： A.Showing adequate trust instead of suspicion of others. B.Eating the food low in fat and driving with great care. C.Cultivating an optimistic personality and never losing heart. D.Looking on the bright side and developing a balanced level of risk.