To what extent is our personality determined from birth?

He claimed that his client’s DNA indicated the presence of the “warrior gene,” a mutation that decades of scientific research had linked to aggressive behavior. For this reason, the defense argued, he could not be considered fully responsible for his actions. The appeal was successful: one year was reduced from Bayout’s sentence.

Since the 1990s, some evidence had been accumulating about the relationship between violent behavior and a variant of a gene called monoamine oxidase A, or MAOA. By 2004, this trait had received the media nickname “warrior gene.”

However, since then, our understanding of how genes influence traits and behaviors has deepened considerably.

“At first, people thought that human behaviors 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 discredited.”

Instead, a much more nuanced picture has emerged over the past 15 years. Even traits that were believed to be highly heritable, such as height, have turned out to be much more complex to isolate in the genome than previously thought.

Now, however, new methods for conducting large-scale genetic studies are beginning to broaden the picture. By revealing more and more evidence about how our genes influence — and how they don’t — who we are, they are providing new perspectives on the very complex forces that shape human nature.

For a long time, people have been fascinated by the extent to which our temperament and the course of our lives are determined at birth.

However, the origins of “personality,” that relatively stable pattern of thoughts, feelings, and attitudes that make up an individual, have proven difficult to pin down.

The “nature or nurture” question was popularized in its current sense by the English polymath Francis Galton, who in 1875 helped develop a way to study traits in twins.

However, their methods were rudimentary, and it was not until the 1920s that scientists began to compare the similarity between identical twins, who share 100% of their DNA, and fraternal twins, or fraternal twins, who share only 50%.

Twin studies have been popular ever since. Today, scientists agree that personality consists of five broad dimensions: openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism (often called the “big five personality traits”). And numerous studies with twins have analyzed whether these personality dimensions are transmitted genetically.

A comprehensive 2015 meta-analysis that pooled more than 2,500 twin studies conducted between 1958 and 2012, covering nearly 18,000 complex human traits, found (unsurprisingly) that identical twins tend to be more similar than fraternal twins. But their personalities are far from identical.

For the 568 traits related to temperament or personality, the study concluded that 47% of the differences could be attributed to genetic differences. The rest, according to the analysis, had to be explained by environmental influences. Other studies seem to support this idea: only about 40-50% of personality differences are genetic.

Twin studies have always been an imprecise tool, often based on estimates derived from differences between twins and other family members. However, around 2010, major advances in genetics began to open promising new avenues for scientists interested in measuring personality differences.

The human genome is extraordinarily complex: it contains 23 chromosomes, each with about 20,000 genes. These, in turn, are subdivided into approximately three billion “base pairs,” the smallest unit of the genome, which are often conceptualized as pairs of letters that unfold in a specific sequence.

All human beings share 99.9% of our DNA, which means that only a tiny 0.1% of the genome explains our differences. Although this narrows the search field for scientists, it still leaves millions of base pairs to analyze. Although the 2000s brought cheaper and more accessible genomic data, locating the origin of our differences has proven much more difficult than expected.

However, in the last 15 years there has been an explosion of genome-wide association studies (GWAS), a method that examines millions of the smallest parts of the genome that can vary between humans and tries to find associations between these and different personality traits.

In the early years, these studies had difficulty consistently identifying DNA variants related to personality.

Today we know one of the reasons: human traits are “polygenic,” that is, they are influenced by many different genetic variations, each with a small effect that, together, adds up throughout the entire genome. For complex traits like personality, these effects can be distributed across thousands of DNA variants.