Are we products of nature or nurture? Science answers age-old question

The age-old question of whether human traits are determined by nature or nurture has been answered, a team of researchers say. Their conclusion? It’s a draw.

By collating almost every twin study across the world from the past 50 years, researchers determined that the average variation for human traits and disease is 49% due to genetic factors and 51% due to environmental factors.

University of Queensland researcher Beben Benyamin from the Queensland Brain Institute collaborated with researchers at VU University of Amsterdam to collate 2,748 studies involving more than 14.5 million pairs of twins.

“Twin studies have been conducted for more than 50 years but there is still some debate in terms of how much the variation is due to genetic or environmental factors,” Benyamin said.

He said the study showed the conversation should move away from nature versus nature, instead looking at how the two work together. “Both are important sources of variation between individuals,” he said.

While the studies averaged an almost even split between nature and nurture, there was wide variation within the 17,800 separate traits and diseases examined by the studies.

For example, the risk for bipolar disorder was found to be 68% due to genetics and only 32% due to environmental factors. Weight maintenance was 63% due to genetics and 37% due to environmental factors.

In contrast, risk for eating disorders was found to be 40% genetic and 60% environmental, whereas the risk for mental and behavioural disorders due to use of alcohol was 41% genetic and 59% environmental.

Benyamin said in psychiatric, ophthalmological and skeletal traits, genetic factors were a larger influence than environmental factors. But for social values and attitudes it was the other way around.

However there was no single trait in which the contribution of genetic factors was zero, Benyamin said. “Genetics contribute to all traits – the difference is, by how much.”

The studies were published between 1958 and 2012 and used “classical twin design” which compared the similarities of identical twins who share all their genes to those of non-identical twins who only share half their genes.

Traits that correlated more closely in non-identical twins compared with identical twins indicated a greater influence of environmental factors.

More than half the studies were related to psychiatric, metabolic and cognitive functions, including rates of depression, anxiety, mental and behavioural disorders related to use of alcohol and tobacco, weight and height.