Nature vs nurture: How much of our personalities are determined at birth?

Laurie Clarke delves into the devilishly complex forces that shape our personalities – and the new research revealing ever more about how our genes do, and don't, make us who we are.

In 2009, Abdelmalek Bayout faced a nine-year prison sentence in Trieste, Italy, for stabbing and killing a man who had mocked him in the street. Aiming to reduce the sentence, his lawyer made an unusual legal argument.

His client's DNA, he said, indicated the presence of the "warrior gene", a mutation that decades of scientific research had tied to aggressive behaviour. Because of this, the argument went, he couldn't be held fully accountable for his actions. The appeal was successful: a year was sheared off Bayout's sentence. 

From the 1990s, evidence had accumulated of some kind of link between violent behaviour and a variant of a gene called monoamine oxidase A, or MAOA. By 2004, it had earned the media-friendly moniker of the "warrior" gene.

Since then, however, our understanding of how genes influence traits and behaviours has deepened significantly. "Initially, people thought that behaviours were influenced by a few genes with very large effects," says Aysu Okbay, assistant professor of psychiatry and complex trait genetics at Amsterdam UMC in the Netherlands. "That has been completely debunked."

Instead, over the past 15 years, a far more nuanced picture has emerged. Even traits thought to be highly heritable, like height, have proven far more complicated to isolate on the genome than once assumed.

Now, though, new methods for large-scale genetics studies are beginning to widen the picture. By revealing ever more about how our genes do – and don't – make us the people we are, they are yielding new insights into the devilishly complex forces that shape human nature.

People have long been fascinated by the extent to which our temperament and the trajectory of our lives is set at birth. Still, the origins of "personality", the relatively stable pattern of thoughts, feelings and attitudes that make up an individual, have proved difficult to pin down. (Read more about the millennia-long efforts to define personality types).

The question of "nature or nurture" was popularised in its current sense by English polymath (and the founder of eugenics) Francis Galton, who in 1875 helped pioneer a way of studying traits in twins. But his methods were rudimentary, and it wasn't until the 1920s that scientists began comparing the similarity of identical twins, who share 100% of their DNA, with fraternal twins, who share only 50%.

Twin studies have been popular ever since. Today, scientists have convened on the idea that personality consists of five broad dimensions: openness, conscientiousness, extraversion, agreeableness and neuroticism (often called The Big Five personality traits). And many twin studies have now examined whether these personality dimensions are passed down genetically.

A 2015 comprehensive meta-analysis of more than 2,500 twin studies between 1958 and 2012, covering almost 18,000 complex human traits, found (unsurprisingly) that identical twins are typically more similar than fraternal twins. But their personalities are certainly not identical.

For the 568 traits that were descriptions of temperament or personality, the study found that 47% of differences could be attributed to genetic differences. The remaining portion, it concluded, must be accounted for by environmental influences. Other studies seem to support this – only around 40-50% of personality differences are genetic.

In 1979, American psychologist Thomas Bouchard set out to track down twins separated in infancy. He found that identical twins raised apart were often strikingly similar.

Most famously, Bouchard came across identical twins called Jim who had been separated at birth and reunited at age 39. "The twins were found to have married women named Linda, divorced, and married the second time to women named Betty," he wrote in a 1990 study. "One named his son James Allan, the other named his son James Alan, and both named their pet dogs Toy." 

Critics, however, have argued that Bouchard's studies contained methodological flaws, and noted that such coincidences could easily occur between unrelated persons, if one drew from enough data.

Twin studies have always been an inexact art, often relying on........

© BBC