By some estimates, we humans have about 100,000 genes. About 7,000, experts suggest, affect the length of our lives. But only a fraction of those are of real significance. It is hard to imagine that so few genes can be responsible for such a complex phenomenon as longevity. In looking at personality, psychologists ask how much is nature, i.e., inherited, and how much is nurture, or external influences. Similar questions exist about the heritability of life span. In other words, just how much of longevity is genetically determined and how much it is mediated by external influences, such as smoking, lifestyle, stress, and occupational exposures?

Studies do show that long-lived parents have long-lived children. Studies of adoptees confirm that their expected life spans correlate more strongly to those of their birth parents than those of their adoptive parents. One study of twins reared apart suggests about a 30% role for heredity in life span, while another says the influence is even smaller. Interestingly, neither of those studies looked at centenarians. When in fact those oldest of the old are included, recent research suggests heredity may have a stronger influence.