Scientists at the Garvan Institute of Medical Research in Sydney found the personalities of identical twins differ because different environmental influences are at work.
The research has implications for everyone, because it shows that environmental factors along with genetics play a role in determining who we are.
Led by Professor Susan Clark, researchers spent eight years studying teenage twins to find out why identical twins can be so different.
What they found is that while identical twins have the same genes, the biochemical reactions that turn those genes on and off are controlled by the environment.
“In this study of over 500 twins, we found that what we call the epigenome is altered in identical twins,” Professor Clark said.
“What this means is that even though the DNA sequence is identical, what is above that DNA sequence, the epigenome, chemical modifications to the DNA are actually subtly different between individuals.
“And we could see this clearly in the identical twins.”
Eamonn Hanna, 15, is an identical twin but that does not mean he shares the same personality as his brother Alexander.
“I prefer to play sport, but he prefers skating and he’s more into the dreads and the hippy kind of side of things, but I prefer looking good, feeling fresh,” Eamonn said.
“Very different. Very, very different. Alex is really loud and out there but I’m more quiet and still out there.”
Professor Clark says the research helps to answer the question of nature versus nurture.
“It’s clear that we are born without personalities, we’re born very much with the inheritance from our parents, but certainly these things can change with different environments,” she said.
The scientists still do not understand what triggers the biochemical reactions that modify genes.