Why the Nature-Nurture Debate is Unsolvable

Описание: One common debate in psychology is whether people's outcome and characteristics are best explained by an individual's genes or their environment. Using evidence from genetics in psychology, I explain why this question cannot be answered: genes and environments work together, so their effects cannot be separated.
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References and further reading:

Avinun, R. (2020). The E is in the G: gene–environment–trait correlations and findings from Genome-Wide Association Studies. Perspectives on Psychological Science, 15(1), 81-89

Harden, K. P. (2021). “Reports of my death were greatly exaggerated”: Behavior genetics in the postgenomic era. Annual review of psychology, 72(1), 37-60

Plomin, R., DeFries, J. C., Knopik, V. S., & Neiderhiser, J. M. (2016). Top 10 replicated findings from behavioral genetics. Perspectives on psychological science, 11(1), 3-23

Polderman, T. J., 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
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How to calculate heritability using a classic twin study:

Identical twins and non-identical twins both share the same environment. However, identical share 100% of their genes, whereas non-identical twins share 50% of their genes. Therefore, if identical twins are more similar to each-other (compared to non-identical twins) it is because of the additional 50% of shared genes.
Here is how to calculate the heritability of the personality trait extroversion (using example numbers):
1) Measure trait extroversion across many pairs of identical twins. Find the correlation between the scores of the first twin and the second twin. For example, this may be a correlation of r = 0.65. This number is called rMZ (meaning the correlation between identical twins).
2) Measure trait extroversion across many pairs of non-identical twins. Find the correlation between the scores of the first twin and the second twin. For example, this may be r = 0.4. This number is called rDZ (meaning correlation for non-identical twins).
3) Find out how much more similar identical twins are from non-identical twins. This is rMZ - rDZ. In this case, it is 0.25. This means that 25% of differences in trait extroversion are explained by 50% of the genes (100% of genes for identical twins, compared to 50% of genes for non-identical twins).
4) Double this number, to account for all 100% of genes. This gives the heritability (h^2) of 0.5, or 50%. This means that genetic factors would explain half of the reason why people differ in extroversion.
These 4 steps are expressed in this equation:
h^2=2(r_MZ-r_DZ)
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