It's an age-old debate: do our genes make us who we are, or is it the environment in which we were raised?
There's long been agreement that both "nature" and "nurture" play some role in determining many aspects of our physical and mental selves, from our height and weight to our intelligence and disposition. But as to which plays the bigger role in shaping us, scientists have never seemed to agree.
That debate may now be over, thanks to a sweeping analysis of studies conducted around the world for more than five decades. The analysis -- involving more than 14.5 million twin pairs from 39 countries -- indicates that nature and nurture are virtually tied.
Across all of our traits, in other words, genes and environment exert equal influence.
"I’d say this settles the debate," Prof. Danielle Posthuma, a statistical geneticist at the Vrije Universiteit Amsterdam and one of the researchers, told The Huffington Post in an email.
Study design. For the analysis, Posthuma and her colleagues surveyed 2,748 twin studies published between 1958 and 2012, looking at more than 17,000 traits.
The studies compared variation in identical twins, who share all of their genes, to differences between fraternal twins, who share half. The research investigated both physical traits (like weight, height, and metabolism) and psychological traits (such as temperament/personality, intelligence, and the likelihood of suffering from anxiety or depression).
The researchers found that across all traits, the average heritability -- the amount of variation attributable to genetic differences -- was 49 percent. Environmental factors and/or measurement errors accounted for the other 51 percent. The researchers also found that for about two-thirds of the traits, genetic variance was additive -- it resulted from the cumulative effect of many genes.
Some traits turned out to be more heritable than others. For instance, cleft lip was found to be 98 percent heritable, risk for having bipolar disorder about 70 percent heritable.
The way forward. The researchers say they hope that their research will provide scientists with a resource for understanding the heritability of certain disorders, and help guide future gene-mapping projects. They've created an online tool, called MaTCH, which allows users to find the heritability of a specific trait.
"For some traits, for example anti-social behavior or autism, we used to think that environment played a bigger role in causing this, (for example autism was blamed on cold mothers)," Posthuma said in the email. "I think knowing that a trait is influenced by genes may help in understanding the disorder."
The analysis was published online May 18, 2015 in the journal Nature Genetics.