Behavioural genetics is the study of human behaviour using genetically sensitive methods such as twin studies, adoption studies and molecular genetic studies.
Twin and adoption studies compare individuals who are genetically related to differing degrees. For example, twin studies compare the similarity of identical twins who share all of their genes with that of non-identical twins who share roughly half. This allows us to calculate estimates of the heritability of a trait and the impact of shared and non-shared environmental influences on it.
Heritability is a population statistic that estimates the extent to which differences between individuals in a trait such as cognitive ability, anxiety or wellbeing are explained by differences at the level of DNA. The important point here is that heritability tells us about populations, not individuals.
Shared environmental factors are those that affect two children growing up in the same family in the same way, and could include poverty, family chaos or local schools.
Non-shared environmental factors are those that affect two children growing up in the same family differently and could include hobbies, friendships, accidents or illnesses.
For a long time, behavioural genetics was said to have a “missing heritability” problem. Twin studies could estimate how heritable an aspect of behaviour was but not specify any of the genes involved. Now genome-wide association studies (GWAS) have begun to identify specific genetic variants associated with behaviour and to combine them in polygenic scores such as the EA3 score discussed in this article.
It is important to note that a behaviour being heritable does not mean it can’t be changed, and finding genes that explain heritability makes no difference whatsoever to this. If, for genetic reasons, you are very short-sighted, you can still have perfect vision if you are prescribed suitable glasses.
It is also important to note, as US psychologist Eric Turkheimer has pointed out, that we have a missing environments problem, too.
We don’t yet know with sufficient accuracy which aspects of experience really make a difference to aspects of behaviour, including those involved in teaching and learning.
Behavioural genetics, in both its quantitative and molecular forms, can help us to understand how the environment works, as well as how the genome works.