The Perks of Being a Twin May Include a Longer Life

Being a twin may add a few extra years to your life, a new study suggests.

Researchers found that identical twins in Denmark tended to live longer than fraternal twins in that country, while both types of twins typically outlived men and women in Denmark who were not twins. The findings were published in May in the journal PLOS ONE, and the researchers announced their findings with a statement last week.

The findings show that twins have a survival advantage over the general population at nearly every age, and between the two types of twins, identical twins have a survival advantage over fraternal twins, said David Sharrow, the lead author of the study and a demographer and postdoctoral researcher at the University of Washington in Seattle. 

The researchers said they suspect that the longevity boost in twins results from the social bonds between the two siblings, Sharrow said. The close relationships often shared by twins could act as a buffer against engaging in risky behaviors over their life spans, or provide a source of emotional or financial support, he suggested. [8 Fascinating Facts About Twins]

In the study, the researchers evaluated data from the Danish Twin Registry, one of the longest-running databases on twins in the world. The scientists looked at data on more than 2,900 same-sex pairs of twins who were born in Denmark between 1870 and 1900, limited to pairs in which both twins had survived to at least the age of 10.

All of these twins have since died, so the length of their lives is known. Slightly more than one-third of the Danish twins in the study were identical, while the rest were fraternal.

Longer lives

The researchers found that for both men and women, the average length of life for identical twins in Denmark who were born within the 30-year span around the turn of the 20th century was 4 to 5 years longer than that of the general Danish population from this same time period, Sharrow told Live Science. 

When analyzing the data by gender, the researchers found that female identical twins lived, on average, about 63.4 years, whereas female fraternal twins lived about 61.4 years and the general Danish female population lived about 58.8 years, Sharrow said.

In comparison, male identical twins in the study lived, on average, about 60.6 years, while male fraternal twins had a life expectancy of 59.1 years and males in the general Danish population lived about 57.5 years, Sharrow said.

It's not entirely clear why identical twins of both sexes may have slightly longer life spans than fraternal twins, but this is one of the study's most intriguing findings and deserves further investigation, Sharrow said. Previous studies have offered some clues, suggesting that identical twins may be more similar than fraternal twins in lifestyle habits that can influence health, or there may be differences in the closeness of the relationship between identical and fraternal twins, he said. [Extending Life: 7 Ways to Live Past 100]

Some limitations of the study are that it looked only at twins in one country born more than a century ago, so future research needs to determine if the findings hold up in other locations and time periods, Sharrow said. In addition, little information was available about the quality of the relationship between the twins, or about their health behaviors and lifestyles, he said.

Twin protection effect

Longevity studies that look at twins are often done to tease out the contributions of genetics (nature) and environmental factors (nurture) on life span. This new study, however, considered whether simply being a twin influenced the length of survival compared with people who are not twins, Sharrow said.

The findings suggest that being a twin offers social, psychological and physiological benefits that could improve health and extend longevity, Sharrow said. The researchers called this concept a "twin protection effect," he added.

The "twin protection effect" is a similar concept to the "marriage protection effect," which is the idea that married people generally have better health and live longer than unmarried adults, Sharrow said. But one of the criticisms of the "marriage protection effect" is that the health benefits observed could be caused by a selection effect, meaning that healthier men and women who tend to avoid risky behavior also tend to get married, while less healthy individuals might choose not to, he explained.

However, a selection bias would not apply to twins, because they don't choose to be twins and, therefore, don't select themselves into a healthier group, Sharrow said.

Moreover, people don't need to be twins to reap the health rewards of social relationships. There's a wide body of evidence that many different kinds of social relationships also offer a protection effect that could help people experience better health and longevity outcomes, Sharrow suggested. These relationships can range from having formal ties to a religious group to being part of other social networks.

Originally published on Live Science.