A study published in the June issue of the Journals of Gerontology Series A: Biological and Medical Sciences, reports that the most significant difference between identical and fraternal twins who lived into their 70s or 80s was how often they kept in touch by phone or mail.
Identical twins who communicated once a month or more lived longer, on average, than identical twins who did not maintain such frequent contact, and longer than fraternal twins no matter how frequent their communication. The twins analyzed in the study were part of the World War II Veterans Twins Registry - the largest twins registry studied to date - maintained by the National Research Council and the Institute of Medicine, which are part of the country's National Academies.
The difference in median life span between identical and fraternal twins is less than two years - 82 years versus 80.5 years - but the fact that close communication has an effect on the longevity of identical twins indicates there is something special about the relationship between identical twins. Anecdotes are common about identical twins who talk with one another daily, for example, and even twins who "sense" from a distance something affecting their co-twins. Identical twins are monozygotic, that is, they split from the same egg and carry the same genes.
"I think the difference we see is related to the closeness of the relationship between identical twins, which is qualitatively different from the relationship between fraternal twins," said study author Malcolm D. Zaretsky, a UC Berkeley researcher in the Department of Molecular and Cell Biology. "This indicates stronger ties among monozygotic twins, and argues that their genetic relatedness has an environmental influence leading to longer life span."
Social relationships and networks have been shown to improve both health and longevity, he said, and somehow, the genetic closeness of identical twins makes a close relationship even more beneficial.
"Frequent communication could provide greater support and competition for a healthier lifestyle that lasts into later life and could effect reduced susceptibility to disease or improved outcomes for illnesses. Frequent communication interacting with relatedness could have similar effects on non-twins as well," he wrote.
Several earlier studies have demonstrated that longevity is influenced not only by smoking, exercise and alcohol use, but also by social networks in people's lives, Zaretsky noted. One of the first was a 1979 study of mortality among Alameda County, Calif., residents conducted by Lisa Berkman and S. Leonard Syme of UC Berkeley's School of Public Health.
"We're really just beginning to understand how the body's defenses are compromised by life circumstance to affect health," said Syme, now a professor emeritus of epidemiology. "For these identical twins, it's not just communication and it's not just genetics. You can't explain it just with biology or with the social environment - it is somehow the combination of the two."
Zaretsky's new study is further demonstration that social interaction and social networks affect people's health and longevity more than the genes they inherit. He found, as did previous Scandinavian studies, that longevity has a heritability of only 20-30 percent. This means that genetics contributes little to how long we live - our social environment and general lifestyle are much more important.
"The take-home message is that ancestry and genes are not that important in predicting longevity," said Walter M. Bortz, M.D., past president of the American Geriatric Association and a clinical associate at Stanford University's School of Medicine. Bortz steered Zaretsky to the twins registry as a means to explore the role of genes in determining life span.
"It ain't the cards you're dealt, it's how you play the hand," said Bortz.
Zaretsky hopes to pursue his observations by probing the brains of identical and fraternal twins with functional magnetic resonance imaging (fMRI) to look for physical differences.
"I would expect the brains of identical and fraternal twins to differ, because environment is known to directly affect brain structures," he said. "I hope we will find some differences that will give us hints about what is going on."
The comparison of identical and fraternal twins is a well-established way to tease out the relative contributions of genes and environment to human behavior. Analyses of twin registries kept by the governments of Sweden and Denmark have shown that identical twins live longer than fraternal (dizygotic) twins. The only factor known to affect this longevity difference, however, is death of one twin, and this is true more for identical than for fraternal twins.
About four years ago, Zaretsky began looking at data in the World War II Veterans Twins Registry in search of behavioral differences between identical and fraternal twins that might explain the difference in live span. The registry contains data on 26,984 veteran twins - 11,832 identical and 15,062 fraternal - born between 1917 and 1927.
In questionnaires distributed to members of the registry in 1967-69 and 1983-85, the twins had been asked about various behavioral and social factors, including marital status, alcohol consumption, smoking, exercise level, whether they lived with their parents past the age of 15, and how frequently they communicated with their co-twins. The latter question was asked only in the second survey, which had a response rate of 71 percent when corrected for mortality.
Zaretsky analyzed the responses and found one factor - social communication between twin partners - that was highly correlated with longer life in identical but not fraternal twins. Other types of social interaction, including active membership in church or community groups or close relationships with other relatives or friends, did not preferentially benefit identical or fraternal twins.
"Clearly, identical twins who communicated frequently survived longer than those who did not," he said. "This was not true of fraternal twins, whether they communicated or not."
In line with the Scandinavian studies, he found that the median life span of identical twins is 82 years - that is, half live more and half live less - versus 80.5 years for fraternal twins.
Interestingly, identical twins exhibited healthier behavior, which also contributed to their longevity. More of them exercised and fewer smoked in comparison with fraternal twins.
Zaretsky noted that exercise at all levels above the lowest level of exercise increased longevity.
"This indicates that a moderate amount of exercise is highly beneficial for health, and that highly vigorous exercise is not more beneficial than moderate exercise," he said.