In 2009, Abdelmalek Bayout was facing a nine-year prison sentence in Trieste, Italy, for stabbing and killing a man who had taunted him on the street. In order to reduce the sentence, his lawyer presented an unusual honest argument.
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 maintained, he could not be held 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.
The old question
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 true 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 problem of “missing heritability”
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 humans share ninety 9.9% 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.
But even when combining a variety of different DNA variants, the effects on personality remain smaller than expected. Current heritability estimates range between 9% and 18% for the “Big 5” personality traits, well below the 40% suggested by twin studies. What explains this “missing heritability”?
Perhaps by increasing the number of participants in these studies and improving their design—as we better understand how different genes interact—more robust genetic effects will be discovered.
For now, however, when comparing heritability estimates from twin studies and genome-wide association studies, it’s hard to know which is more accurate, Aysu Okbay says. “It’s probably somewhere in between.”
And what about “parenting”?
If “nature” may be less influential than previously thought, it might be tempting to attribute more of our personality to “nurture”: the circumstances in which we grew up, the people around us, the life events that shape our individual stories. However, understanding how the environment shapes our personality turns out to be just as complex.
Since studies show that personality can change over time, you might think that winning the lottery or losing a leg would cause a profound transformation. But it turns out that the big one-time events in life have a practically insignificant impact on who we are.
Factors such as how we are raised or our social interactions also explain only a small part of personality differences, studies repeatedly show. And although marriage may make a person slightly less outspoken, and having children may marginally reduce extraversion, these events, considered individually, do not largely determine who we become.
Exposure to certain types of trauma during childhood has been linked to the emergence of psychopathologies and poorer cognitive functioning later in life, which can manifest in personality variables such as greater neuroticism. However, adversities experienced in adulthood appear to have much lesser consequences.
“That has been the big surprise in this field of research… that if a major traumatic event happens to you in adulthood, it doesn’t leave such a deep mark,” says Brent Roberts, professor of psychology at the University of Illinois in Urbana-Champaign, USA.
The trauma narrative is very popular in popular culture: the belief that we grow personally from negative experiences. But “trauma doesn’t make you who you are,” Roberts says.
Daniel Levey’s study, for example, suggests that the CRHR1 gene, related to regulating the body’s response to stress, is strongly linked to neuroticism in the tissues of the nervous system.
This gene had previously been associated with psychiatric disorders such as depression, anxiety and obsessive-compulsive disorder (OCD), all of which are also related to neuroticism. This suggests that this personality trait is closely linked to the way the body naturally responds to stress.
Another highly anticipated study, currently undergoing peer review, provides evidence for theories that place the origin of personality in the prefrontal cortex, the brain region responsible for complex functions such as planning and decision making.
According to this study, associations with all of the “Big 5” traits (except agreeableness) are enriched in genes that are expressed in this part of the brain.
Interestingly, the study notes, since dopaminergic neurons were not “among the most enriched neuronal types,” this could challenge neurobiological theories that attribute a predominant role to dopamine in traits such as extraversion and openness.
Still, many limitations and unknowns remain, even in well-studied areas of behavioral genetics, such as the relationship between violence and the so-called “warrior gene.”
Some studies indicate that, in certain groups of men, the presence of certain moderating genes along with environmental risk factors (such as an abusive upbringing) could increase the likelihood of violent behavior in certain situations. But the results are far from conclusive.
Until now, attempts to reduce human behavior to a few genes or specific life events have failed. It turns out that human beings are much more complex.
What emerges clearly is the great plasticity of the human condition, says Jana Instinske. “It’s not that, if you have a certain genetic predisposition, you are going to always behave in the same way throughout your life.”
And what about the first environment we experience, floating in the amniotic sac? A growing field of research suggests that maternal stress during pregnancy could influence the temperament of the fetus, as part of a hypothetical phenomenon known as “fetal programming.”
For example, a 2022 study found that mothers who experienced greater stress fluctuations had babies who showed more fear, sadness, and distress at three months of age. Why this occurs is not yet fully understood, although one of the proposed mechanisms is epigenetic—that is, changes in the expression of genes, rather than the DNA itself.
Taken together, researchers have concluded that, in addition to being polygenic, personality differences are “polyenvironmental.” Al i Just as multiple DNA variants throughout the genome contribute to a given trait, each of our life experiences has a small effect that, together, ends up having a greater impact.
Furthermore, genetic and environmental factors interact in ways that we do not yet fully understand. For example, the environment seems to be able to activate or deactivate certain genetic predispositions. “A genetic predisposition does not mean that, in all environments, people behave the same way,” explains Instinske.
A way forward
These are extremely complex problems but, at least in the field of genetics, scientists say they are making progress thanks to the latest genome-wide association studies.
The key? Vastly increasing the number of participants, with recent research analyzing the genetic data of hundreds of thousands or even millions of people at a time.
“Only now do we have enough individuals and genetic samples,” says Aysu Okbay. “With so many small effects, you need really large samples to be able to detect them.”
Studies over the past decade have identified hundreds of DNA variants associated with each of the “Big 5” traits. “Much of the current focus is on obtaining (the genomes of) more and more people, so we can discover more genes and build on what others have done before,” says Daniel Levey.
However, Levey adds, more studies are needed with people of non-European ancestry. “We are losing very important cultural differences by focusing almost exclusively on a single group.”
We are still far from understanding exactly what small variations in our genetic code tell us about how personality is formed. But interesting findings are already beginning to emerge.
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