The scientific view of what determines a life span or how a person ages has swung back and forth. First, a couple of decades ago, the emphasis was on environment, eating right, exercising, getting good medical care. Then the view switched to genes, the idea that you either inherit the right combination of genes that will let you eat fatty steaks and smoke cigars and live to be 100 or you do not. And the notion has stuck, so that these days, many people point to an ancestor or two who lived a long life and assume they have a genetic gift for longevity.
But recent studies find that genes may not be so important in determining how long someone will live and whether a person will get some diseases — except, perhaps, in some exceptionally long-lived families. That means it is generally impossible to predict how long a person will live based on how long the person's relatives lived.
Life spans, says James W. Vaupel, who directs the Laboratory of Survival and Longevity at the Max Planck Institute for Demographic Research in Rostock, Germany, are nothing like a trait like height, which is strongly inherited.
"How tall your parents are compared to the average height explains 80 to 90 percent of how tall you are compared to the average person," Vaupel said. But "only 3 percent of how long you live compared to the average person can be explained by how long your parents lived."
"You really learn very little about your own life span from your parents' life spans," Vaupel said. "That's what the evidence shows. Even twins, identical twins, die at different times." On average, he said, more than 10 years apart.
The likely reason is that life span is determined by such a complex mix of events that there is no accurate predicting for individuals. The factors include genetic predispositions, disease, nutrition, a woman's health during pregnancy, subtle injuries and accidents and simply chance events, like a randomly occurring mutation in a gene of a cell that ultimately leads to cancer.
The result is that old people can appear to be struck down for many reasons, or for what looks like almost no reason at all, just chance. Some may be more vulnerable than others, and overall, it is clear that the most fragile are likely to die first. But there are still those among the fragile who somehow live on and on. And there are seemingly healthy people who die suddenly.