On genes and personality
If one thinks about humanity in broad terms – and who does not from time to time? – one inevitably wonders: what makes each of us different from everyone else? In the 18th century, the question was put in terms of “nature versus nurture”. Today, we speak about “genetics” versus “environment” (which includes individual experience) but the basic question remains: how much of our individuality is determined by our genetics? A recent big investigation has found some links between specific genes and particular personality traits and this article describes it. I will begin, however, with a short trip back into the past.
Traditionally, the matter of individual differences was framed by within-family comparisons. Even before the first human civilizations, 5000-6000 or so years ago, some (perhaps many) people noticed that children often resembled their parents. Such likenesses are faint in babies but by the time offspring are children or teenagers the resemblances can be strong. The most obvious are in physical traits but often one sees certain personality traits as well. In Greek civilization, with its interest in character and fate, from about 2500 years ago, we can be certain that people noticed such parent-offspring resemblances in character. (Probably this was also true in the great Indian and Chinese civilizations of the time but I have no knowledge of those literatures.)
Do parent-child similarities in personality reflect shared genetic factors or similar home environments? Certainly, we know that parental behaviors in the home can affect how children develop psychologically. If a child sees either parent as admirable, there may be some half-conscious mimicking of certain traits. If there is antagonism, the child may reject that aspect of their parent but nevertheless unconsciously adopt one of the rejected traits. The point is that mere resemblance between members of successive generations in their behaviors or personalities does not necessarily indicate a genetic basis for those similarities.
Indeed, in the mid-20th century, in America and Europe, there was a strong tendency to discount genetic inheritance as a factor in shaping personality, character or abilities. Environment, in the form of parental influence, was seen as everything. This may have reflected the rejection of eugenicist ideas, which dominated earlier decades, that all aspects of mentality were a matter of inheritance. That strain of thinking went back to the British thinker Francis Galton in the late 19th century. He was a cousin of Charles Darwin and, unsurprisingly, a strong believer in Darwinian evolution. He also believed ardently in heredity as the source as “genius”, which he clearly felt he possessed. He coined the term “eugenics” and propagandized for measures to foster the preferential breeding of presumably superior humans.
Galton’s creed of strong heritability of mental traits soon migrated across the Atlantic, in the early 20th century, and was promoted by a few influential American scientists, in particular Charles Davenport, the director of the Cold Spring Harbor Laboratory on Long Island, New York. It then migrated back across the Atlantic to Germany in the 1920s and was taken up, with lethal effects, in Nazi Germany in the 1930s, with mass killing (tens of thousands of victims) in institutions of those deemed physically or mentally “unfit”. In this form, eugenics went from being a debatable and contentious doctrine to an odious one and was widely rejected in the Western world in the 1940s1.
Nevertheless, over the succeeding decades, there was a stream of observations that supported the possibility of some genetic influence on the development of certain cognitive abilities and certain personality traits. (We will return to the matter of cognition in a later article.)
The strongest evidence for such involved observations on identical twins who had been separated early, sometimes just after birth, and raised in different households. (The reason for such separations was often that the biological parents could not care for one or both children.) Identical twins have essentially identical genetic material and thus, if there is a strong effect of genetic inheritance on personality, such separated twins should show strong similarities in traits and behaviors, even when raised apart. Many observations of such pairs have, in fact, shown that they often show such similarities, sometimes almost uncanny ones, in major personality traits or unusual gestures or reaction patterns to particular things.
Of course, there might be some ascertainment bias in such observations. Often, the researchers were looking for strong resemblances, with differences of lesser interest. Nevertheless, some of the observations were striking and have been supported by comparable observations of non-identical siblings raised in separate homes. Pairs of siblings share, on average, 50% of their genes2. Hence, if there is a strong genetic component to certain personality traits, one would expect siblings who had different family environments to share certain personality traits more often than two unrelated individuals. Hence, such family studies indicate an element of biological inheritance in shaping personalities.
Nevertheless, such kinds of observations are merely tantalizing and not proof of a genetic basis for personality traits. They do not reveal which genes are involved nor how their variants can alter particular personality traits. Identifying the actual genetic differences that affect particular personality traits would be an important first step in making this a scientific story, not one simply of anecdotal human interest.
Enter the technique of “Genome-wide association studies” or GWAS for short. When successful, it provides strong evidence that a particular gene or genetic element is involved3. Specifically, it allows the investigator to relate a single DNA base pair in or near a specific gene, with high probability, to a genetic difference affecting a particular trait, whether a physical one (such as a disease condition) or a psychological one (such as a personality trait).
When the Human Genome Project, the huge scientific effort to determine the sequence of the human genome, was first planned in the mid-1980s, the capability to connect single base pair changes in the whole genome to specific phenotypic (trait) differences would have seemed like science fiction. Only twenty years later, however, it was a reality though not without its problems. Today, however, it is working well.
GWAS technology is immensely complex and this newsletter is not the place to describe it’s methodologies or weaknesses (but see Supplementary reading.). The principle is fairly straightforward, however.
It involves scanning the genomes of people with a particular trait, let us call it trait “A”, for individual base-pair differences scattered all over the human genome. Those single base pair differences in DNA sequence are called “single nucleotide polymorphisms” or SNPs and they are located within short but identifiable specific gene regions . With the sophisticated technology of GWAS, one then identifies those SNPs that appear in people with trait A with a much higher probability than expected if there were no connection between the SNP and the trait in question. By setting the probability of association-by-chance sufficiently low, one identifies SNPs in or near genes that are almost certainly involved with production of the trait. Given that the SNP is embedded within a small but distinct sequence that can be located within the genome, one can identify the specific gene in question.
Such identification is just the start of the investigation of how that SNP affects the trait but at this point, most of the genes in the human genome can be classified as to what their encoded gene product does (biochemically) within the cell. That knowledge in turn provides clues to how a genetic alteration, usually involving a small decrease in the amount of active gene product, might contribute to the trait under study. This is the case whether that trait is a normal physical trait in the human body such as height, or a disease condition, or a human personality trait. In effect, GWAS delivers high level correlations between an SNP and a gene and a trait. Correlations do not prove causation but in the case of GWAS findings, with the current methodologies, they can provide quite strong evidence for a connection between the genetic change and the alteration in the observed trait.
The long excursion into the methodology allows us to focus on the recent published investigation mentioned at the beginning. It was of a GWAS study of five basic human personality traits, using a very large sample of people. The personality traits – one might even call them “temperaments” – are sometimes labeled the “big five” by psychologists. They are: agreeableness, conscientiousness, extraversion, neuroticism, and openness.4
Since all of these traits must reflect the properties and activities of specific tracts of neural circuitry within the human brain, there might be many genes involved in building that circuitry and for which relatively small changes might nevertheless affect the trait at issue. The first thing needed was a large data-base of people who had been psychologically typed, for the traits in question, and DNA samples from those people. One such source, which was used in this study, was the Million Veteran Program, whose name reveals the people used: American military veterans, who, given the nature of the American military, were primarily men of European and Afro-American descent. Other sources and studies were brought in as part of a so called “meta-analysis”, which improved the sample size and reliability of the conclusions.
Even with the huge data base available for the psychological testing and for providing the samples one would not expect to identify all of the genes whose alteration might affect a given trait. However, the data-base is sufficiently large to virtually guarantee a number of gene candidates for these traits. In fact, 34 genes for the five traits in total were identified. The largest groups of genes were those for extraversion and neuroticism, with eleven in each category. The smallest gene groups were those for conscientiousness (two) and agreeableness (three). For the last category, openness, there were seven. All of these numbers are almost surely underestimates of the number of genes that can change to variants that would affect these traits.
There is much in this study of interest and I will not attempt to summarize the findings but one point deserves a comment here. Many of these genes have identified gene products with known biochemical function. This offers the possibility that their roles in producing the traits their SNPs were connected with can be solved. Nor is this study alone. An even newer publication shows how GWAS can be used to look at the genetic basis of certain traits in our best animal friend, the dog.5 Eventually it should be possible to connect up some of the findings about humans and dogs for a fuller picture of how genetic properties can underlie specific psychological and behavioral traits.
The essential point at present is that this research provides some of the strongest evidence that certain specific genetic differences amongst humans can affect specific personality traits. It also confirms that GWAS is a good technique for exploring this matter. Finally, it further undermines the old eugenics notion of single genes “controlling” certain complex traits. The work illustrates that things concerning mental abilities and genes are far more complicated than that. However, “more complicated” means more interesting and that is always a good thing.
Supplementary Reading
The Wikipedia article on Genome Wide Association Studies
This is a good short introduction to the subject and, of course, has references.Abellaoui, A. et al. (2023). 15 years of GWAS discovery: realizing the promise. Am. J. Hum Gen. 110: 179-194.
An up-to-date and clear review of the history and current state of GWAS studies.
Gupta, P. et al. (2024). A genome-wide investigation into the underlying genetic architecture of personality traits and overlap with psychopathology. Nat. Hum. Behav. Doi.org/10.1038/s-451562-024-01951-3
Two good books on the history of eugenics are the multi-author Davenport’s Dream: 21st century reflections on heredity and eugenics (2008) (eds. J.A. Witkowski and J.R. Inglis). Cold Spring Harbor Laboratory Press, Cold Spring Harbor, and E.A. Carlson (2001) The Unfit: A history of a bad idea, Cold Spring Harbor Laboratory Press, Cold Spring Harbor.
We have already looked at the fact that the word “gene” is somewhat problematical (see “On how the term ‘gene’ first gained, then lost its meaning”). Think of a gene as a specific DNA sequence that contributes to one or more traits, however directly or indirectly.
The first GWAS study was published in 2002 but suffered from insufficient sample size. The approach tends to be dated from 2007, when the first big study, with more reliable conclusions, was published.
Of course, one can identify other personality traits, some of them pathological, in people but the “big five” are the ones that psychologists often deal with and cover a great deal of human personality and behavior.
See Mahmoodi et al. (2024). Deciphering the genetic basis of behavioral traits in dogs: Observed-trait GWAS and latent-trait GWAS analysis reveal key genes and variants. The Veterinary Journal doi.org/10.1016/j.tvjl.2024.106251