Today is Friday the 13th of the year. Even though the date is known throughout Western culture as one associated with negative things, it’s really just another Friday on the calendar. There is no scientific evidence to suggest this day has an increased chance of bad occurrences compared with other days. But that doesn’t mean scientists haven’t tried to find any.

In Thomas W. Lawsons 1907 novel, Friday, the thirteenth, a businessman takes advantage of the superstition (迷信) and creates a Wall Street panic on the day. The book was the likely inspiration for a number of scientific examinations of stock market returns on Fridays that fell on the 13th day of the month compared with all other Fridays. In 2001, Brian Lucey, a business professor at Trinity College Dublin in Ireland, published his analysis of a few of those studies and found several flaws. The studies had narrow assessments overall, focusing on just a few markets or a single stock exchange. But when he analyzed the data as a whole, Lucey found that internationally, with few exceptions, returns on Friday the 13th were typically just a little higher than returns on other Fridays.

The scientific evidence for cause and effect may not be there, but people may still alter their behavior on Friday the 13th in a way that causes certain things to happen. The way people drive might be different from their usual. But the few studies that have examined traffic accidents haven’t found statistically significant trends to suggest Friday the 13th is more dangerous than other Fridays on the road.

The science is clear: Friday the 13th is a normal day. Yet, people continue to think this is meant to be a bad day. “Psychologically, superstitions arise from the desire to influence external events, decrease anxiety and reduce uncertainty,” said Neil Dagnall, a psychologist at Manchester Metropolitan University. “Besides, I don’t think people know often why they do things.”

Growing old does not increase your immediate danger of dying — at least, if you are a fruit fly. The chances of a Mediterranean fruit fly dying out on a particular day reaches a peak (顶峰) and then declines, according to James Carey of the University of California and Davis and James Vaupel of Duke University. Their results disagree with the general idea that the death rate rises with age in all species.

The research will cause a long-running debate over whether or not there is a genetically defined limit (遗传上的极限) to an individual’s lifespan (寿命). If there is not and the fruit fly results extend to humans, then medical advances might eventually allow the elderly to live forever.

In the early 19^th century, the British scientist Benjamin Gompertz formed a law stating that death rates increase with age. For an adult human the immediate chance of death seems to double every eight years. The Gompertz law was believed to hold for all species and to put an age limit on each species.

To test this theory, Carey and Vaupel studied more than a million fruit flies. They found that the death rate reached a maximum of about fifteen per cent when the flies were between 40 and 60 days old, and then fell. Flies that survived to 100 days had only a 4 to 6 per cent chance of dying on a given day. In other words, the chances of dying seem not to increase sharply in advanced age, as predicted by the Gompertz law; rather, they seem to level off.

If Gompertz law does not hold for human beings either, then there may be no genetically defined limit on a person’s lifespan.