The cause of individual differences in behaviour and personality is one of the most hotly debated subjects in the history of psychology. Over time theories have shifted between those which emphasise the role of the environment in moulding the character of a person and those which propose that differences between people are largely innate. At one extreme lies environmentalism, which posits that people are like "blank slates" on which their character is written by the environment, and at the other genetic determinism, the idea that individual differences in psychological characteristics are determined completely by genetics.
Francis Galton, great grandson of Erasmus Darwin and second cousin of Charles Darwin, was the first to have attempted to study the inheritance of behaviour empirically. Inspired by the magnitude of his cousin's scientific breakthrough he became obsessed with studying, in his own not so humble words, the "heredity of greatness". Among his observations he noticed that some sets of twins were identical whereas others were only as similar as regular full siblings. He took several measures of physical and psychological characteristics of identical and non-identical twins and from his results he estimated that identical twins were around twice as similar as non-identical twins. Since the environment was assumed to be equally as similar for identical and non-identical twins Galton derived that around half of the variance in behaviour can be accounted for by heredity. Although his experiments had holes in their methodology, his estimates turned out to be impressively accurate for someone who had no knowledge of DNA.
Unfortunately, for all of Galton's genius, inherited or otherwise, he had one very bad idea which tainted his legacy. Eugenics. Galton believed that civilisation could be bettered by "improving the human stock" by giving "...the more suitable races or strains of blood a better chance of prevailing speedily over the less suitable than they otherwise would have had". This idea spread and gained popularity among political parties across Europe and north America. Eugenics programmes provided positive enforcement such as financial incentives to those deemed most fit for reproduction and negative enforcement including forced sterilisation of those deemed unfit such as those with disabilities, low IQ scores, criminal records and minority racial identities. This was carried out in countries including the USA and, most notoriously, Germany under Nazi rule.
As a result of the horrors of the 20th century the public and academic institutions alike, quite understandably, developed a distaste for studies into the heritability of psychological traits and the subject was shelfed. This created an academic vacuum ready to be filled with environmentalist theories of human nature, such as Sigmund Freud's theory of psychosexual development and B.F. Skinner's radical behaviourism. Psychology remained in the grips of environmentalism until the 1960's when this consensus was challenged by a new sub-field of genetics called "behavioural genetics" and a novel methodology which was beginning to stack up mountains of evidence to the contrary
Behavioural genetics is a field of study which aims to discover the influence of genetics on behaviour. In this regard it is similar to the related field of evolutionary psychology however, where evolutionary psychology investigates the reasons why differing behaviours evolve in different species, behavioural genetics aims to quantify the contribution of genetics to individual differences in behaviour within the same species.
The genetic influence on individual differences is estimated by a statistic called heritability. Heritability describes the proportion of the variance of a trait within a population which can be attributed to genetics. This is not the same as saying that heritability measures how much of the trait is caused by genetics rather it describes how much of the differences in a trait between individuals in a population is caused by genetics. Heritability estimates are only just that, estimates, the accuracy of which can be affected by factors such as genes with a non-additive effect on a trait, homogeneity of environment and age of testing the trait.
Heritability estimates paint a picture of the genetic contribution to individual differences in traits of the population they were tested in however heritability estimates of the same trait may vary between populations in different environments. An example of this is the fact that heritability of body weight is higher in more affluent countries than in poorer countries. This is because, in poorer countries, people are more restricted in their food consumption because of lack of availability so their body weight will be more dependent upon how much nutrition they have access to than their genetics whereas in affluent countries, where almost everyone can afford to eat more than they need, body weight will depend more on genetically influenced factors such as metabolism, frame and appetite.
Another caveat is that heritability shows correlation of genetics with traits, not causality. Heritability tells us nothing about the pathway which leads from genetic variants to behaviour and it often takes tortuously indirect routes. For example, attractive people tend to score higher on measures of self-confidence than average, presumably as a result of being more well received socially. Part of the heritability of self-confidence can therefore be explained by genetic variants which, rather than affecting brain areas associated with self-confidence as one might assume, result in prominent cheekbones and a cute button nose.
Heritability is estimated by dividing the correlation of a trait in related individuals by their co-efficient of relatedness. This is easier to express as a formula:
T = Correlation of trait
G = Co-efficient of relatedness
Heritability = T / G
Correlation of the trait refers to how similar the trait is in genetic relatives compared to the population as a whole. For dichotomous traits, characteristics which a person either has or does not have, this is calculated simply by finding how likely someone is to have trait, such as a diagnosis of schizophrenia or a penchant for skydiving, given that a genetic relative such as a sibling, cousin or parent, also expresses the trait.
For quantitative traits, characteristics which every individual has but at varying levels, correlation of the trait is calculated by how accurately you can predict the level of a trait, such as height or IQ scores, in an individual given that you know the level of the trait in a genetic relative compared to how much the trait varies from mean average in the population as a whole.
The strength of the correlation is denoted by a statistic called r-squared, the value of which can vary between 0 and 1. A correlation of r-squared=0 means that the pairs of genetic relatives are no more similar to each other than any pair of individuals in the population as a whole whereas a correlation of 1 means that the trait of one of the pair of genetic relatives perfectly predicts the trait in the other. It is extremely unlikely that r-squared would ever be exactly 0 or 1 for any trait rather it usually falls somewhere in-between.
The co-efficient of relatedness is a measure of how closely related two people are and is roughly equivalent to the proportion of genetic variants which they had inherited from the same ancestor. So identical twins will have a co-efficient of relatedness of 1, full siblings and parent child pairs will have 0.5, grandparent and grandchild pairs, cousins and nephews will have 0.25 and so on.
This formula for estimating heritability works because the correlation of the trait describes how similar the genetically related pairs are compared to the population as a whole and, all else being equal, this similarity must be caused by their shared DNA. So, we can divide this similarity in the trait by the co-efficient of relatedness of the genetically related pairs to find out what proportion of the variance of the trait is attributable to genetics.
To give a hypothetical example if full siblings had a correlation of r-squared=0.25 in their extraversion scores you could estimate that extraversion was 50% heritable by following the formula for estimating heritability as 0.25/0.5 = 0.5 = 50%. You could reach the same conclusion if the extraversion scores for full cousins had a correlation of r-squared=0.125 or if the correlation was r-squared=0.5 for identical twins.
Obviously, there are confounding factors which need to be considered when designing experiments for determining the heritability of traits. The most obvious being that related people tend to share similar environments as they usually grow up in the same family households so the similarity in traits could equally have been caused by similar upbringing. This problem is tackled in part by the use of adoption studies.
Adoption studies work by comparing the similarity in traits between adoptees and their biological family members with that of their adoptive family. For example, measuring extraversion in adoptees and comparing the similarity between their extraversion scores and that of their birth parents with the similarity between their extraversion scores and that of their adoptive parents. A similar method is comparing the similarity of adoptive siblings to that of biological siblings. Adoption studies mimic the well-established animal research technique of cross fostering where animal offspring are removed from their biological parents at birth and raised by surrogates to investigate genetic linked behaviours and physical traits.
Another method of addressing the shared environment issue is by studying twins. Identical twins have been called god’s gift to genetics because they share 100% of their DNA allowing the opportunity for a natural experimental control. Twin studies work by comparing similarity of pairs of monozygotic (identical) twins with that of dizygotic (non-identical aka fraternal) twins. Because both kinds of twin share very similar environment, including gestating in the same womb and being born on the same day, any difference between the similarity of monozygotic twins and that of dizygotic twins must be in the most part caused by genetics.
Neither of these methods are infallible on their own. For example, in twin studies, there is the assumption that non-identical twins are treated equally as similarly by their parents as identical twins, despite some evidence to suggest that this is not the case, and, in adoption studies, middle class families are overrepresented in adoptive families relative to the general population potentially resulting in an overestimation of heritability since environmental variance is restricted. But by compiling the evidence gathered from several methods researchers can account for the weaknesses of each with the strengths of the others and triangulate upon more and more accurate estimates of heritability.
A situation which offers the best of both from the two aforementioned methods is cases of identical twins separated at birth. Heritability can be estimated by comparing the variance of a trait between identical twins separated at birth with that of identical twins raised together. As monozygotic twins are genetically identical any difference in similarity between the twins raised apart and the twins raised together must be caused by the environment. Cases of identical twins separated at birth are unfortunate for the people involved and are thankfully vanishingly rare. They are also, however, very useful when it comes to the study of nature and nurture so behavioural geneticists try to document as many of such cases as they can find.
You might have already heard about twin studies or anecdotes about the remarkable similarity of twins. Twins separated at birth are often discussed in the media highlighting astounding similarities in behavioural quirks between separated twins. The Jim twins (so called because their adoptive parents both named them Jim), who were separated at birth and reunited in 1979 at the age of 39, are among the most famous examples. As well as being remarkably similar in their appearance they also both enjoyed mechanical drawing as a hobby, preferred the same subjects at school, had both married women named Linda, divorced them, remarried women named Betty, and both had sons whom they named James Allen. Anecdotes like these are often remarkable however they do not tell us much about the genetics of human psychology. Not much that is quantifiable anyway. The real discoveries from twin studies are to be found in traits measured in objective and standardised pen and paper tests carried out on large sample sizes.
One such category of psychological traits which can be objectively measured is personality. Personality traits are habitual patterns of behaviour, thought, and emotion that are stable properties of an individual's psychology. There are 5 major dimensions of personality agreed upon by most personality researchers. These include openness, conscientiousness, extraversion, agreeableness, and neuroticism. Intelligence is another measurable aspect of psychology. As well as quantitative dimensions of psychology, taking yes/no measures of life events which are in large part a result of choices made by the individual, such as obtaining a university degree, being divorced, having a criminal record, or seeking support for mental health, form another route to investigating individual differences in psychology.
The psychologist Thomas Bouchard, who had had heard about the Jim twins and invited them to be a subject of his research, started receiving correspondences from other such separated twins soon after publishing the results of his case study. He eventually collected data from over 100 pairs of twins. Having a sample of this size created the perfect opportunity for the genetics of individual differences in psychology to be studied. By providing a reference point of genetic similarity, and removing shared environment from the equation, for the first time in the history of science twin and adoption studies had made it possible to disentangle and empirically measure the respective contributions of nature and nurture, and nature was coming up larger than anyone had expected.
The findings of twin and adoption studies have been summarised in the three laws of behavioural genetics:
1. "All behavioural traits are heritable".
2. "The effect of genetics is always greater than the effect of shared environment”.
3. "There is a substantial proportion of the variance in behaviour not explained by shared genes or families".
The most startling discovery brought about by twin and adoption studies, described in the first law, is that there is not a single known, measurable behavioural trait that does not show any genetic influence. Every psychological characteristic that has been investigated in twin and adoption studies has been found to be correlated between family members and the strength of these correlations increase with the percentage of shared DNA.
Not only are all behavioural traits heritable, the second law tells us that genetic factors always have a greater effect on behaviour than the home environment. Behavioural traits in adoptees are always on average more similar to those of their biological family than their adopted ones. Therefore, the effect of genetics on behaviour must be greater than the shared environment.
The third law describes the fact that there are no behavioural traits which are accounted for completely by the combined influence of heritability and the home environment. Identical twins who grow up in the same home are very similar but not psychologically identical. Therefore, there must be other factors affecting behaviour independent of genetics or shared environment. Behavioural geneticists refer to this as the non-shared environment; the things which we experience in life which are unique to us as individuals.
The non-heritable factors which influence our behaviour can therefore be categorised into the shared environment and the non-shared environment. The influence of the non-shared environment can be calculated by subtracting heritability from the correlation of a trait in siblings sharing the same home environment. It turns out that the effect of the shared environment is close to negligible. This is evidenced by the fact that, in measures of behaviour, siblings raised apart are no less similar than siblings raised together and by the fact that adopted siblings are no more similar than strangers. The most generous estimates of the impact of the shared environment are around 10% however most estimates are closer to 0%.
The bulk of the non-genetic influence on behaviour is therefore made up by the non-shared environment. The non-shared environment is comprised of the non-systematic events that happen to us in our life which are unique to us as individuals. Catching a virus as a new-born, a neighbour who teaches you to play chess, a sporting injury, a musical instrument discovered in the loft and everything in-between. Their effect on our behaviour is like that of a pinball game, nudging us in different directions as we move through our life. The non-shared environment is mysterious, hard to identify and measure, non-stable and random. It is for these reasons that, despite explaining a substantial proportion of the variance between individuals, the non-shared environment is not a useful predictor of behaviour; it is just too unique to each individual to be quantified.
Findings of behavioural genetics have been solid; despite the harsh scrutiny the studies had been placed under due to the controversy surrounding the topic. In fact, the statistical rigour with which behavioural genetics studies have had to be carried out to stand up to such scrutiny resulted in the findings of behavioural genetics being one of the modern psychological theories which survived the so called "replication crisis" in which more than half of the findings of many papers published in highly regarded academic journals turned out to be statistical flukes.
We must bear in mind however the caveats about heritability mentioned earlier. Heritability estimates are only applicable to the population in which they were calculated. So, although being raised in a different home environment within the same culture is not likely to make much of a difference to your behaviour that is not to say that being raised in a radically different culture would not have an impact. It is entirely possible that a person would have ended up with a different personality had they grown up in the amazon jungle rather than the city of London. Another point to bear in mind is that estimates of the impact of the shared environment are made under the assumption that there is at least adequate nurture in the home environment. Cases of abuse, neglect and deprivation do have an impact on normal psychological development.
Despite these reservations the findings from twin and adoption studies are transformative for the way we think about ourselves. The finding that DNA is a much more important and stable influencing factor on who we are than the way we were raised delivered a fatal blow to the doctrine of radical behaviourism which had psychology in its grips for the latter half of the 20th century. Behavioural genetics teaches us that people are not blank slates upon which talents and proclivities can be written by social conditioning or blocks of clay ready to be moulded by parenting or schooling. People have their own genetic selves which, given adequate nurture, they may flourish into. Just as a gardener may create the right environment for flowers to bloom but cannot change the type of flower a seed will become.
Despite the overwhelming evidence for the heritability of behaviour academia was not immediately willing to accept theories which challenge the blank slate model. The impact which so called "social Darwinism" and eugenics has had on far-right political opinions had left people with a distaste for the idea of biologically inherited differences between people. Around the time when behavioural genetics was beginning to be studied it was dangerous both personally and professionally to publish about the genetic influence on behaviour. Researchers who published studies on behavioural genetics, those that could get published at all, had on several instances been branded as eugenicists by the media and had been demonstrated against by student movements and threatened with physical violence. These incidences had arisen from concerns by the implications of behavioural genetics for inequality, but the people involved had not considered the harm which had been caused by the assumption that we are all inherently the same.
For example, if we are all born psychologically the same then mental illness must always be a result of the environment and the blame was often landed on mothers. Freudian thinking on mental health purposed that "cold" mothering styles, which was asserted as a sign of the mothers own mental illness, is the cause of psychological diseases such as schizophrenia. Adoption studies have shown that the bulk of the risk of developing a mental illness is explained by genetic factors rather than trivial differences in parenting styles. These studies were carried out by observing the rate of mental illnesses such as schizophrenia and major depression in adoptees whose biological parents had one of these conditions. It was found that children of parents with a family history of mental illness were at an approximately equally increased risk of developing a mental illness whether they were raised by their birth parents or not. Similar studies have shown that adoptees who have no family history of mental illness who were adopted by parents who did had only a very slightly increased risk of developing a mental illness themselves. This is not to say that parenting doesn't matter. Again, growing up in situations where there is abuse, neglect or malnourishment will increase the risk of mental illness but the majority of people who struggle with their mental health did not have traumatic childhoods. The flipside of this observation is also true. Troubled upbringings are, for most people, not a life sentence.
Criticisms of behavioural genetics often make accusations of eugenics and genetic determinism and beg the question of free will. Critics of this sort have a backwards view of the implications of these findings. For one thing, the non-shared environment has been found to be equally as important as genetics for many aspects of personality. This finding alone is enough to clear the name of any behavioural geneticist on trial for ushering in a new era of eugenics. Even if it were the case that differences between individuals were caused purely by genetics eugenics could only come about under authoritarian regimes which make value judgements about which traits are more desirable and what kinds of people have greater and lesser rights to live. No such judgements are made within the realms of science.
As for the matter of free will, let us say that it was instead found that there was no relationship between genetics and personality. That a person’s personality was carved out purely by the environment no matter how complex and unpredictable the relationship between personality and environmental factors. Would the person be any more free if this were the case? Is it not that, under such circumstances, a person would become whatever their environment dictates like a leaf blown through life by the winds of circumstance without direction? If a person’s character is determined to any degree by their genetics it can at least be that some of the forces which move them through life come from within. The guidebook that has been passed down to them by countless generations of ancestors, written in the DNA at the core of every cell in their body. If anything can be called a self which is free to will why not this?
Most people today have come to accept that the environmentalist view of human nature is too extreme. In large part because they could not help but notice the effect of genetics on behaviour within their own families with their own eyes. Despite the gradual shift of opinion within academia, folk theories of behavioural genetics remain not quite in sync with the science. Most people accept that genetics has some influence on behaviour in a person’s young life, but it is commonly believed that these influences get ironed out as a child grows up and settles into their environment. Counterintuitively, one of the findings of behavioural genetics is that adoptees become more similar to their biological family members as they get older even if they have never met each other.
This finding suggests that the heritability of behaviour increases with age and that genetics encroaches more and more on to the territory of the environment as the individual gets older. As if everyone has a genetic destination for their personality towards which they circle closer and closer as they move through life. This echoes of humanistic theories of psychology such as Maslow's self-actualisation and of Jungian psychoanalytic theories of individuation. Perhaps the influence we see of non-shared environment on behaviour is a result of the individual moulding themselves into a shape which might fit into some niche in the impossibly complicated puzzle which is their life. As they move through life, they accumulate understanding of both the world and of themselves, an understanding we might call wisdom. With each gain in wisdom a person may pivot into positions which better suit their genetic proclivities and aversions. Strengths and weaknesses.
We know from twin and adoption studies that our behaviour is influenced by genetics. But where are the genes and how do they result in behavioural traits? The answer is not so straightforward. The hunt for the specific genes which influence behavioural traits, as well as for most other complex phenotypes, had been largely fruitless and the few successful finds only explain a fraction of the variance in behaviour. What this shows us is that the heritability of complex traits identified in twin and adoption studies, rather than resulting from a few genes with high impact as was previously believed to be the case, are a result of the cumulative effect of genetic variants across the entire genome, each with a marginal contribution to the overall heritability of the trait. The term for such traits is polygenic traits.
The physiological pathways from genetic variants to polygenic traits such as behaviour are currently not well understood. With modern genomics however we are at least one step closer to understanding the genetic architecture of behaviour. The rise of a new technology called microarrays has made it possible to look at variants across the whole genome rather than just sequencing a few single genes at a time. Massive studies using micro-array technology carry out genome-wide searches in hundreds of thousands of individuals to identify statistical associations between traits and clusters of genetic variants. These studies, called genome-wide association studies, are not able to point exactly to the specific genes involved in influencing behaviour however, with large enough sample sizes, they are able to point to the region within the genome where corelated genetic variants can be found.
One of the applications for the findings of genome-wide association studies are the production of predictive models called polygenic scores. Polygenic scores can make predictions about a trait in an individual by summing together the estimated effect size of all the genetic variants possessed by an individual which were found in GWAS to have an association with the trait. For example, by scanning the genome of an individual we can use polygenic scores to predict with some degree of certainty their likelihood of developing major depression at some point in their life or whereabouts they would be likely to score for the different dimensions on a personality test. As more people's genomes are sequenced and the sample sizes of genome-wide association studies become larger and larger the accuracy of the predictions made using polygenic scores become more accurate and ever-increasing proportions of the heritability estimated by twin and adoption studies can be accounted for.
The technology is still in its early stages however the potential for its use is massive. As behavioural genetics transitions towards behavioural genomics we can build upon the foundations of twin and adoption studies and go further than family resemblance in investigating the genetic influence on behaviour by looking directly at a person’s genome. This is an exciting moment in the history of science as behavioural genomics promises to provide us with another avenue towards knowing ourselves.
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