People are different in their looks but also in their traits and behaviour. For some characteristics such as hair colour or height we hardly question the role of heritability. For traits and behaviours, such as how ambitious or aggressive someone is, genetic influence seems less self-evident. To better understand the origins of individual differences, twin studies are employed. Basically, identical twins are twice as similar genetically as fraternal twins. If identical twins are also twice as similar on a trait, genetic influence seems implicated. Twin studies utilize advanced statistical methods to show how much of the variance between people is because they differ in their genetic make-up and how much is because they experience different environmental conditions (Boomsma et al., 2002). We know from twin studies that approximately half of these differences are because people have different DNA and about half is because they grow up in different families (Porsch et al., 2016).
“These studies probe associations between hundreds of thousands of genetic variants and a trait, behaviour, or diagnosis.”
Twin studies cannot tell us which genes are linked to traits and behaviours. It is important to keep in mind that people do not differ in which genes they have but in the variant of the gene. Some variants are a bit more effective in regulating certain biological processes in our bodies than others. Taking aggression again, it is feasible that variation in genes that regulate neurotransmitters and hormones have something to do with individual differences in this behaviour. To identify those genes, molecular genetic approaches such as genome-wide association studies come in handy. These studies probe associations between hundreds of thousands of genetic variants and a trait, behaviour, or diagnosis. For complex traits such as aggression, no single genetic variant has a substantial effect but researchers can use innovative methods to apply the information gained in such genome-wide association studies, provided they have genetic data from participants (Abdellaoui & Verweij, 2021). Importantly, whereas twin studies have suggested that around 50% of variation between people in most traits is due to differences in genetic make-up (Polderman et al., 2015), molecular genetic studies yield much smaller effects. The difference between both has been termed “missing heritability” and it remains a challenge to improve molecular genetic studies to increase their predictive strength.
“Genetic propensity and environmental factors operate in interplay: Aggressive children might become friends with other aggressive children, hang out in bad neighbourhoods, and thus run greater (environmental) risk to go off the rails.”
Genetic propensity and environmental factors operate in interplay which means that genetic information can be a fantastic tool to understand the influence of the environment on human development (Harden, 2021): genetic propensity might drive people to seek out particular environments or genetically-influenced traits might evoke certain environmental reactions. Aggressive children might become friends with other aggressive children, hang out in bad neighbourhoods, and thus run greater (environmental) risk to go off the rails. Genetically-influenced aggressive behaviour might evoke harsh parenting, a known environmental risk for child maladjustment. Genes also interact with environmental factors such that genetically-influenced traits might only result in problematic (or beneficial) outcomes under specific environmental conditions. For instance, a person with a higher genetic propensity for smoking might become a chain smoker if in a relationship with another smoker. A child with a high genetic risk for aggression might never be aggressive in a calm and attentive environment. Given that children’s genes that increase the risk for aggression are inherited from parents and might negatively influence parental behaviour, however, these children might be at double risk, genetically and environmentally.
“Is it possible and ethically advisable to use insights about genetic propensity and its interplay with environmental conditions?”
Is it possible and ethically advisable to use insights about genetic propensity and its interplay with environmental conditions? Utilizing information from molecular genetic approaches to capture risk for specific traits and behaviours carries a Eurocentric bias because genome-wide associations studies are mostly based on North American and Western European samples. As such, information derived from these studies would apply with greater precision to some populations compared to others. Moreover, using genetic propensity as marker of individual risk encouraged some individuals to engage in a different lifestyle than others raises various ethical concerns. In principle, it is possible to target interventions to people at greatest genetic risk to prevent negative outcomes, but the risk of stigmatization of “potentially aggressive” children, and the fact that we are still nowhere near perfect prediction of outcomes imply that such attempts would be seriously premature. It is worrying that commercial providers offer embryo screening based on genetic information that goes far beyond detection of risk for life-threatening diseases (Turley et al., 2021). Researchers who use genetic information to understand human development therefore need to consider ethical implications of their work when disseminating results. Notwithstanding such concerns, the insights from genetically-informed research might help understand not only individual differences in traits and behaviours but also variation in response to environment and interventions. As such, while not ready for direct application yet, genetically-informed fundamental social and behavioural science is very promising and a serious player when it comes to better understanding the complex interplay of antecedents of differences between people.
NOTE. Image by Qimono at Pixabay.
Abdellaoui, A., & Verweij, K. J. (2021). Dissecting polygenic signals from genome-wide association studies on human behaviour. Nature Human Behaviour, 5(6), 686–694.
Boomsma, D., Busjahn, A., & Peltonen, L. (2002). Classical twin studies and beyond. Nature Reviews Genetics, 3(11), 872–882.
Harden, K. P. (2021). “Reports of my death were greatly exaggerated”: Behavior genetics in the postgenomic era. Annual Review of Psychology, 72, 37–60.
Polderman, T. J. C., Benyamin, B., de Leeuw, C. A., Sullivan, P. F., van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics, 47(7), 702–709. https://doi.org/10.1038/ng.3285
Porsch, R. M., Middeldorp, C. M., Cherny, S. S., Krapohl, E., van Beijsterveldt, C. E. M., Loukola, A., Korhonen, T., Pulkkinen, L., Corley, R., Rhee, S., Kaprio, J., Rose, R. R., Hewitt, John. K., Sham, P., Plomin, R., Boomsma, D. I., & Bartels, M. (2016). Longitudinal heritability of childhood aggression. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics, 171(5), 697–707. https://doi.org/10.1002/ajmg.b.32420
Turley, P., Meyer, M. N., Wang, N., Cesarini, D., Hammonds, E., Martin, A. R., Neale, B. M., Rehm, H. L., Wilkins-Haug, L., Benjamin, D. J., & others. (2021). Problems with Using Polygenic Scores to Select Embryos. New England Journal of Medicine, 385(1), 78–86.