What are the health benefits of having a close friend — like identical twins close — for your entire life? Well, according to research from the University of Washington, the positive effects on life expectancy can be astounding. In 2016, UW scientists analyzed data gathered in the Danish Twin Registry, one of the world’s oldest registries on identical and fraternal twins. The data the scientists reviewed included information on nearly 3,000 same-sex twins who survived beyond the age of 10 from 1870 to 1900. With the data being over a century old, scientists could ensure that all subjects in the study had completed their natural life spans. The study found that twins enjoyed a significantly higher survival rate compared to the overall Danish population, an advantage that peaked for male twins when they were in their mid-40s and for female twins in their early 60s. At those ages, male twins were more likely to be alive by 6 percentage points — meaning that in a group of 100 Danish men back then, if 84 were still alive at age 45, for twins the number was 90. For female twins, the difference at the peak was 10 percentage points.
While both fraternal and identical twins outperformed their non-twin counterparts, identical twins showed even greater gains in life expectancy over fraternal twins, leading scientists to theorize that identical twins perhaps form deeper bonds due to an enhanced ability to predict their sibling’s needs. The strength of social bonds in relation to health outcomes isn’t unique to twins, though. A similar effect has been observed between married, or otherwise partnered, couples and single people, which is known as the marriage protection effect. Both examples show the vital need humans have for strong social connections, and connections don’t get much stronger than the bond — and DNA — shared between identical twins.
You’ve heard of identical and fraternal twins. The former, known as monozygotic, happens when twins originate from the same egg, while the latter, called dizygotic, happens when two separate eggs are fertilized at the same time (the most common type of twin). However, there’s also an extremely rare third type of twin known as semi-identical, or sesquizygotic. First documented in 2007, this type occurs when two sperm fertilize the same egg. Sesquizygotic twins share the same placenta and will have somewhere between 50% to 100% of the same DNA (essentially on a spectrum between normal siblings/fraternal twins and identical twins). Although the twins also share the same amniotic sac, the two fetuses can actually be different sexes, something that’s impossible with identical twins. To this day, only a handful of sesquizygotic twins have been identified.