One of the most frequently noted observations among healthcare professionals is the considerable variation in the vitality of our rapidly growing population of elders. Why is it that some individuals survive to their Los, 80s or 90s, relatively vigorous and free from chronic illness, and maintain their cognitive abilities, while others experience considerable physical and cognitive impairment with a reduction in activities of daily living? Is it because these individuals are genetically different or because they have accumulated different experiences throughout life? In the terms of quantitative genetics, we want to know how important, relatively speaking, genes and environments are for the individual differences we see among individuals. Clearly, both genes and environments are important for aging. What is not as well delineated is whether the influence of genetic effects is the same throughout the last half of the lifespan, or whether the importance of these effects is different for different characteristics like health, memory, and functional abilities.
Twins represent an opportune natural "experiment" for studying the relative importance of genetic and environmental effects. Identical twins share all of their genes in common. Hence, any differences between members of a pair must be due to environmental differences. Fraternal twins, like other siblings, share half of their genes in common. Thus, we compare the similarity of identical and fraternal twin pairs in order to estimate how important genetic effects might be. If genetic effects are of importance, then identical twins should be twice as similar as fraternal twins. The extent to which identical twins are different provides an estimate of the importance of what is known as "nonshared environments;' that is, those individual-specific environmental factors that cause differences among family members.
Although the use of twin studies for the study of human health and behavior has a tradition dating from the late 1800s, there have been relatively few twin studies of aging. The first twin study of aging was that by Kallmann, Jarvik, and colleagues, known as the New York State Psychiatric Institute Study of Aging. After this pioneering effort, which focused mostly on cognitive abilities, mental health, and longevity, most of the recent studies have been broad multidisciplinary efforts using population-based samples of twins (Pedersen, 1996). There are currently seven twin studies of aging, four of which are based on twins in Sweden and Denmark. These studies includethe following: the Swedish Adoption/Twin Study of Aging (sATsA), which includes a sample of twins separated at an early age and reared apart; ocro Twin, comprising pairs who are 80 years of age or older (focusing on "successful agers"); the GENDER Study of unlike-sexed twins (which helps us get a handle on sex linked differences in aging); the Longitudinal Study of Aging in Danish Twins (LSADT ); the Minnesota Study of Adult Development; the Black Elderly Twin Study (BETS ); and the National Heart Lung and Blood Institute (NHLBI) study of male twin veterans. Each of these studies provides a unique contribution to the understanding of how genetic and environmental factors affect human aging. In the following, I will provide a review of some of the results, starting with cross-sectional findings and moving to longitudinal findings when these are available.
Perhaps one of the most pertinent characteristics for the gerontologist to study is the subject's self rated health, as this measure reflects the individual's own opinion of his or her status and is a predictor of survival. Self rated health can be assessed with the simple question, "On the whole, how do you rate your health?" or augmented to elicit additional information, as in "How would you rate your health compared to 5 years ago?" and "Does your health prevent you from doing things you would like to do? …