Many people have a sense of about how long they’re going to live, which can significantly influence their attitudes and actions.

Those life expectancy hunches are usually incorrect, however.

Our parents’ life span only has a 3 percent effect on ours. Identical twins die an average of 10 years apart, too. And a study of 10,000 pairs of twins by epidemiologist Kaare Christensen has found that the effects of genetics on life span are negligible after age 60.

Even the most strongly family-linked diseases like breast and colon cancer are poor predictors; if one twin develops such a cancer, the other still has less than a 15 percent chance of getting it.

“We’re pretty good at predicting on a group level,” says Christensen, “but we’re really bad on the individual level.”

In view of this, the best bet is to assume you have the average remaining life span for your age group and well-established risk factors like smoking. Don’t let a feeling that you haven’t got much time be an excuse for overindulging and neglecting practical matters; you might regret that when you’re 85.

On the other hand, don’t let expectations of a long life deprive you of enjoyment here and now, either. “Live all you can,” as Henry James said; “it’s a mistake not to.”