How long you live, called longevity, is jointly determined by genetic and environmental factors. Researchers distinguish between two different types of longevity: average longevity and maximum lon­gevity. Average longevity is commonly called aver­age life expectancy and refers to the age at which half of the individuals who are born in a particular year will have died. Average longevity is affected by both genetic and environmental factors.

Average longevity can be computed for people at any age. The most common method is to compute average longevity at birth, which is the projected age at which half of the people born in a certain year will have died. This computation takes into account people who die at any age, from infancy onward. Thus an average longevity of 78 years at birth means

Longevity, Health, and Functioning 109

that 78 years after a group of people are born, half of them will still be alive. When average longevity is computed at other points in the life span, the cal­culation is based on all the people who are alive at that age; people who died earlier are not included. For example, computing the average longevity for people currently 65 years old would provide a pre­dicted age at which half of those people will have died. People who were born into the same birth cohort but who died before age 65 are not counted. Eliminating those who die at early ages from the computation of average longevity at a specific age makes projected average longevity at age 65 longer than it was at birth. In the United States, females currently aged 65 can expect to live about 20 more years; men about 16 more years.

For people in the United States, average longe­vity has been increasing steadily since 1900; recent estimates for longevity at birth and at age 65 are pre­sented in Figure 4.1. Note in the figure that the most
rapid increases in average longevity at birth occurred in the first half of the 20th century. These increases in average longevity were caused mostly by declines in infant mortality rates, brought about by eliminating diseases such as smallpox and polio and through bet­ter health care. The decrease in the number of women who died during childbirth was especially important in raising average life expectancies for women, both at birth and at age 65. Advances in medical technol­ogy and improvements in health care mean that more people survive to old age, thereby increasing average longevity in the general population.

Maximum longevity is the oldest age to which any individual of a species lives. Although the bibli­cal character Methuselah is said to have lived to the ripe old age of 969 years, modern scientists are more conservative in their estimates of a human’s maxi­mum longevity. Even if we were able to eliminate all diseases, most researchers estimate the limit to be somewhere around 120 years because key body

systems such as the cardiovascular system have limits on how long they can last (Hayflick, 1998). Genetic theories also place the human limit around 120 years (Barja, 2008).

Whether this estimate of maximum longevity will change as new technologies produce better artificial organs and health care remains to be seen. An important issue is whether extending the life span indefinitely would be a good idea. Because maximum longevity of different animal species var­ies widely (Barja, 2008), scientists have tried to understand these differences by considering impor­tant biological functions such as metabolic rate or brain size (Hayflick, 1996). But no one has figured out how to predict longevity. For example, why the giant tortoises of the Galapagos Islands typically live longer than we do remains a mystery.

Increasingly, researchers are differentiating between active life expectancy and dependent life expectancy; the difference is between living to a healthy old age (active life expectancy) and simply living a long time (dependent life expectancy). Said another way, it is the difference between adding years to life and adding life to years. One’s active life expectancy ends at the point when one loses inde­pendence or must rely on others for most activities of daily living (e. g., cooking meals, bathing). The remaining years of one’s life constitute living in a dependent state. How many active and dependent years one has in late life depends a great deal on the interaction of genetic and environmental factors, to which we now turn.