A breakthrough study published in Nature Genetics asserts identical twins — also called monozygotic twins that develop from one fertilized egg — may not be genetically identical, after all. The finding challenges the assumption scientists hold dear when studying the effects of nature versus nurture, particularly when looking at the impact of diseases: that twins have minimal genetic differences, so any difference in their experience of a bodily phenomenon stems from environmental factors. 

“So if you take identical twins raised apart and one of them developed autism, the classic interpretation has been that that is caused by the environment. But that is an extraordinarily dangerous conclusion,” the co-author of the paper and head of Iceland’s deCode genetics, Kari Stefansson, tells The Guardian.

The new research suggests that identical twins had, on average, 5.2 genetic mutational differences between them. It also found 15% of monozygotic twins had an even higher number, beginning early in their development. This means some identical twins, right from their conception, have different DNA, which may affect how they experience diseases. Experts have welcomed this research, saying it upends the classical twin research model medical scientists use in determining the effect of biology versus environment on individuals’ experience of illness.

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The research may also change how we understand and refer to the phenomenon of twinning — it may be more accurate to refer to such twins as monozygotic, instead of identical, Stefansson adds. The current research is one of a handful that challenges the identical-twins-are-identical assumption, but researchers note more investigation is needed to pinpoint the exact mutations (and when they occur) if the scientific community is to change how it studies disease.

For now, we know monozygotic twins are not identical — but how, why, where, and when needs more digging.