Where does our personality come from? Are we the way we are because of nature or nurture? Is the degree to which we differ from each other in, say, how extraverted or agreeableness we are primarily due to differences in our genetic blueprint, or does it stem from differences in our environment and experiences?
Until recently, most of the information we had about the extent to which our personality traits are passed down from parent to child through genes or are the results of environmental influences came from twin studies. Twin studies seek to identify the genetic and environmental influences on physical, behavioral, or psychological traits (or phenotypes). Genetic and environmental influences are estimated by extrapolating from trait differences in identical (monozygotic) twins, who share all their genes, or fraternal (dizygotic) twins, who share about 50 percent of their genes.
One of the best known historical twin studies is the Minnesota Twin Study of Twins Reared Apart, which psychologist Thomas Bouchard and his colleagues ran from 1979 to 1999. The study looked at variance in various traits (e.g., height, weight, I.Q., rate of speech, and so on) in twins separated at birth and "randomly" adopted then reared apart (Bouchard et al., 1990).
If we assume that twins separated at birth and reared apart are not influenced by any shared environmental factors and that identical twins share 100 percent of their DNA, then the total observed variance in a given trait (e.g., IQ) in identical twins is due to environmental factors. Making these assumptions, Bouchard and his colleagues found that the total difference in IQ in identical twins is 30 percent. The team estimated 70 percent heritability for IQ in the general population.
This estimate is based on several problematic assumptions. Contrary to what we assumed, twins separated at birth and reared apart probably are exposed to shared environmental factors. For example, they may well have very similar sensory experiences and emotions when exposed to the same sorts of things, say, a particular roller coaster in Disney World.
Twin studies of the heritability on personality have found that between 15 and 55 percent of the overall variance in our personality is due to genetic differences (Power & Pluess, 2015). Take the Big Five of Personality, a taxonomy that classifies us in our degree and mode of the five dimensions: openness, conscientiousness, extraversion, agreeableness, and neuroticism. About 15 to 55 percent of our total differences along these five dimensions are due to genetic differences, whereas the rest is due to the environment. For any one of the five dimensions, however, the heritability has been estimated to be about 40 to 60 percent. So, 40 to 60 percent variation in how extraverted we are (say) is due to differences in our genes, and the same goes for the other dimensions of the Big Five.
Genetic testing is a newer and more accurate measure of heritability. A study conducted by Power & Pluess (2015) found that differences in our genes account for about 15 percent of the variance found in neuroticism and 21 percent of the variance in openness. The study also revealed that despite the appearance that neuroticism and openness are so different that they need to be classified as different types, they are in fact coded for by many of the same genes. The sets of genes (or genotypes) that code for neuroticism and openness overlap substantially.
The Power and Pluess heritability estimates for neuroticism and openness are comparable to the estimates yielded by twin studies. This is because, gene-based heritability estimates are, generally, about one-half of the estimates derived from twin studies. As the twin-based heritability estimates are 40 to 60 percent for any one of the dimensions of the Big Five, we should expect genetic testing to yield a heritability estimate of 20 to 30 percent. This matches Power and Pluess’s finding of 15 and 21 percent heritability for neuroticism and openness, respectively. These findings are rather unsurprising.
What's more surprising is that they didn’t find any statistically significant heritability for the three remaining dimensions of the Big Five: extraversion, agreeableness, and conscientiousness. This clearly isn’t comparable to the twin-based estimates for personality. What's more, no such unpredicted divergence between the gene-based and the twin-based heritability estimates have been identified for various other traits such as height, stature, speech patterns, and IQ. What's going on?
Why Are Extraversion, Agreeableness, and Conscientiousness Outliers?
One possible explanation for why extraversion, agreeableness, and conscientiousness stand out from most other traits could be the genes that code for them undergo many more mutations than the genes that code for neuroticism and openness. However, absent a story about why these particular genes would mutate a lot more than others, this explanation seems a little far-fetched.
I would like to suggest another possibility, which is this: for certain non-heritable personality traits, heritability estimates based on twin studies are inflated owing to artificially created linkages between traits (Brogaard, 2020). This kind of artificial inflation occurs when some modern cultural or technological innovation ties a non-heritable trait to one or more heritable traits, resulting in their co-variation.
Take the invention of basketball. A person who is tall and athletically built is more likely to join a basketball team at some point in his or her life than someone who is short and scrawny. Height and build are highly heritable. A twin study of the percent heritability of having-joined-a-basketball-team might indicate that variance in decisions about joining a basketball team is partly due to genetic differences (e.g., differences in the genes coding for heritable traits like height, build, strength, and stamina).
Our imaginary findings from our imaginary twin study would thus erroneously suggest that having joined a basketball team is a heritable trait. But this clearly isn’t so. Basketball is a relatively new invention, not the result of natural selection—having-joined-a-basketball-team isn’t an adaptive trait, nor is it heritable. After all, we don’t yet have a reliable and ethical way to artificially insert genes that code for heritable traits into the human genome. As such, having-joined-a-basketball-team is but a non-heritable trait which we are more likely to have when we have certain heritable physical traits (e.g., being tall and athletically built).
Might artificial inflation explain why twin studies and genetic testing yield radically different heritability estimates for personality (40 to 60 percent vs. statistically insignificant)? If so, then some of our highly heritable traits (e.g., our height, build, or IQ) must be strong determinants of how extraverted, agreeable, and conscientious we become.
The suggestion that heritable physical traits can have a substantial influence on our personality may seem ludicrous at first. In my next post, however, I will make a case for this seemingly implausible scenario.
Bouchard, T. J., Jr; Lykken, D. T.; McGue, M.; Segal, N. L.; Tellegen, A. (1990) "Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart." Science 250, 4978: 223.
Brogaard, B. (2020). Hatred: Understanding Our Most Dangerous Emotion, Oxford University Press.
Power, R. A.; Pluess, M. (2015). "Heritability estimates of the Big Five personality traits based on common genetic variants." Transl Psychiatry 5(7): e604. Published online 2015 Jul 14. doi: 10.1038/tp.2015.96