However, a surprising study suggests an ancient taboo regarding the number four may explain why some Asian-Americans are more likely to die of a heart attack on the fourth day of a month than any other day.
In some Asian cultures here and abroad, four is considered an unlucky symbol of death. Whites, who don't have a similar superstition, die at equal rates throughout the days of the month, say researchers at the University of California at San Diego.
"When you see there's a 40 percent increase in deaths from heart attacks on a frightening occasion, it's a very large effect," says study co-author David Phillips, a professor of sociology. "It means you probably should worry about worry."
Phillips has long been fascinated by possible links between state of mind and time of death. He remembers growing up as a child in South Africa, and being intrigued by a horrifying scene in the Sherlock Holmes mystery, "The Hound of the Baskervilles."
In the story, a character kills a man who has a heart condition by scaring him to death. The murder weapon is a dog disguised to look like a ghostly hound from hell.
Phillips has spent years looking for a good way to study whether fear can kill people. Previous studies only looked at deaths after disasters like earthquakes, when a variety of other factors -- such as limited access to health care -- might explain why more people died of heart attacks, he says.
Then one day, Phillips went out to lunch with some Chinese students and drove past an Asian supermarket named "99 Ranch Market."
"Being incurably curious, I asked them why it's called 99 Ranch. They said nine sounds like prosperity in Chinese. I asked if there were any unlucky numbers in Chinese, and they said four sounds like death. It's true in Cantonese, Mandarin and Japanese," Phillips says.
Indeed, the word for four in those languages sounds the same as the word for death, he says.
An intrigued Phillips examined U.S. mortality records from 1973 to 1998, including 210,000 deaths of Chinese and Japanese people and 47.3 million deaths of whites. His findings appear in a recent issue of the British Medical Journal.
He discovered that Chinese and Japanese deaths peaked on the fourth of the month; they were 13 percent more likely to die on those days. No other day showed such an abnormal spike.
In California, the rate of death on the fourth was 27 percent higher than chance would predict among Chinese and Japanese people.
Since many of those who died were in hospitals, it seems unlikely they had changed their behavior significantly on those days, Phillips says. A hospital patient "can't exert himself inappropriately. He can't start drinking alcohol or taking street drugs," he adds.
Deaths not related to heart attacks didn't show a similar pattern.
To make sure that four is indeed an unlucky number, Phillips and his students investigated its role in Chinese and Japanese culture. They found some Chinese hospitals don't have a fourth floor. Also, the Chinese air force doesn't use the number on any of its aircraft.
His students also checked California phone books, and found Japanese and Chinese restaurants were less likely than other eateries to include the number four in their phone numbers.
Sean O'Connell, academic director of the East Asian Language and Cultures Department at the University of Southern California, says, "You don't want to use the number four or be associated with it because you'll pass away."
O'Connell says his wife, who is of Japanese descent, sent back a license plate because its numbers began with a four: "There's just this type of pervasive belief, an underlying current."
The San Diego researchers plan to further investigate how Asian-Americans feel about the number four and the fourth of the month, Phillips says.
"We just studied death certificates. We don't know exactly what the psychological processes are," he says.
What To Do
Researchers other than Phillips are studying whether people can actually die from fright. To learn more, read this BBC News story.
Want to catch up on your Sherlock Holmes by reading the story that inspired Phillips' study? Read a free copy of "The Hound of the Baskervilles", courtesy of Bibliomania.com.