As it turns out, this series is going to be longer than I originally intended two articles ago. The plan now is that “Part 3” in that outline is going to be “Part 3, 4 and 5”.
The Iron Law of Heritability
From Thomas Bouchard 2004:
“As Rutter (2002) noted, ‘Any dispassionate reading of the evidence leads to the inescapable conclusion that genetic factors play a substantial role in the origins of individual differences with respect to all psychological traits, both normal and abnormal’ (p. 2). Put concisely, all psychological traits are heritable.”
Bouchard then points to the general heritabilities of commonly used psychological traits:
I call this “The Iron Law of Heritability”, though this is not a commonly used term. It is meant as a label for the common understanding among psychologists that all general psychological traits are to some degree heritable.
So when we explore any kind of psychological variation, the question is not “is that variation down to genetics”, the question is “HOW MUCH of the variation we see is down to genetics”, because it is always going to be some.
Heritability of Political Views Within the United States
There is a general misunderstanding of how twin studies are done. The common conception of twin studies is that identical twins are separated at birth, and then you see how similar they are in various traits when they grow up in different environments.
This has the obvious problem in that the range of environments of adopted twins may not reflect the range of environments for the whole population.
Another, better, way to do twin studies is simply to compare identical and non-identical twins reared in the same environment. Non-identical twins share roughly 50% of their genes + additional similarity from assortive mating, whereas identical twins share almost 100% of their genes.
So instead of controlling for genes and looking at the impact of environmental variation, twin studies can control for environment and look at the impact of an increase in genetic similarity.
For example, IQ scores. What they do is look at how similar the IQs of non-identical twins are, and then compared that to how much MORE similar the IQs of identical twins are; to see how much an impact an increase of 50% genetic similarity has.
For example, if the IQs of non-identical twins correlate with each other at 0.5, and the IQs of identical twins correlate at 0.8, then that implies the general heritability of IQ in the populations examined is 0.6, or 60%. This is because a 50% increase in genetic similarity produced a 30% increase in IQ similarity.
But because of assortive mating, this is going to be an underestimate, because couples are not randomly selected; couples tend to be more genetically similar to each other than they are to the general population….[ ]