Scientific interest in monozygotic twins has been driven mainly by questions about how much in any human outcome is due to nature and how much to upbringing (nature versus nurture). There were studies of twins separated for some reason (mostly because of the mother's death) and raised in different environments, where after decades it became clear that they share temperament, psychomotor tempo and nonverbal language - gestures. There were coincidences that favored the predictions of astrologers, and there were dozens of topics for jokes and playful banter.
According to researchers at the University of Washington in Seattle, their study, recently published in PLOS, is the first to examine relationships between twins as a significant factor affecting life expectancy. The analyses confirm that twins have lower mortality throughout life up to age 65, for both sexes, but among men these findings are more pronounced than among women. A twin has its closest friend from birth. And mutual social support is present throughout life.
"We found that in any age period, monozygotic twins survive more often than dizygotic twins, and the latter somewhat more than the rest of the population," said David Sharrow, the first author of the study, at a press conference. "The results indicate that close social relationships are an important protective factor."
All data come from the Danish Twin Registry, which represents one of the oldest follow-ups of this phenomenon. The authors selected a total of 2932 pairs of same-sex twins who lived to at least the tenth year and were born between 1870 and 1900, making it possible to track them over the entire life course and compare them with the overall Danish population.
For men, this advantage appears most prominently in their forties, when the percentage difference in mortality compared to the general population is a full six percent. Specifically: if out of 100 boys born, 84 are still alive at age 45, in the case of twins this number is a full ninety. In women, this advantage appears only after age sixty, but there it reaches a full ten percent.
The study authors are convinced that these results are the result of social support, similar to the support provided by marriage or participation in tighter social communities. Many studies show the protective influence of both institutions, even marriage that is not very harmonious, on health status. However, the authors raise the question of whether marriage is the cause of better health, or whether people with better health are those who are more likely to marry. Just as people more easily become members of various churches, associations, or clubs. This question does not apply in the case of twins, because people could not choose to be twins; and so the influence of social support applies regardless of health status.
A social network can support health in many ways: friends can participate and inspire each other toward a healthy lifestyle, and can also contribute through criticism to giving up bad habits. How useful it is to have a shoulder to lean on, where one can symbolically or literally cry, how welcome help is in times of need, illness, and life crisis.
The statistics also produced one further observation: when causes of death were assessed, compared with the general population far more ordinary causes were found than sudden deaths from external causes, whether accidents or other unexpected events. The authors also attribute this to a generally safer and therefore healthier lifestyle, toward which social support leads.
Source: Sharrow DJ, Anderson JJ: A Twin Protection Effect? Explaining Twin Survival Advantages with a Two-Process Mortality Model. PLoS One. 2016 May 18;11(5):e0154774