Do twin studies explain political disagreement?
Posted Jun 04, 2019
On June 1, Democratic presidential candidate Andrew Yang tweeted,
According to twins studies between one-third and one-half of political alignment is linked to genetics; that is most of us are born somewhat wired to be liberal or conservative. If this is the case we need to build bridges as much as possible. It’s not just info or culture.
The first part of this statement is mostly true: Twin studies (or comparisons between identical and fraternal twins) do show that identical twins tend to be more similar than fraternal twins on political attitudes. From this, we infer that genes play a role in the development of political opinions. But exactly what that role is, and how it might impact how we engage with one another, is still very much unknown.
Political Attitudes Are Partly Heritable
Political attitudes, both generally and specifically, appear to be partly heritable. But what does that mean? Heritability (as estimated from twin studies) means that people who are more genetically similar — that is, identical twins — are more similar in their behaviors compared to people who are less genetically similar — fraternal twins. Politics is no different than any human behavior that scientists have studied using the twin method: Everything seems to be at least a little bit heritable. This finding of non-zero heritability is so consistent across human individual differences that it's been called the First Law of Behavior Genetics. Heritability estimates are always about group averages in similarity, never about individuals. Although identical twins tend to be more similar in their politics than fraternal twins, that doesn't mean that all pairs of identical twins have the same political opinions, or that all fraternal twins differ substantially, or even that all pairs of identical twins are more similar than all pairs of fraternal twins.
But we also know that, like politics, no human behavior is entirely heritable. Human individual differences on average tend to be around 50% heritable, but there's a huge range. Even within political attitudes, the range in heritability goes from around 10% for "sense of civic duty" to nearly 60% for "political knowledge/sophistication" (Hatemi & McDermott, 2012, Figure 1). On the full spectrum of heritability, that makes political attitudes weakly to moderately heritable.
Heritable Doesn't Mean Unchangeable
Actually interpreting what heritability "means" is a much bigger challenge than just estimating it by comparing twins. It doesn't mean that something won't change. For example, schizophrenia is strongly heritable, yet it changes over the lifespan and can be treated without genetic intervention. A heritability estimate is a simplified proxy for what we're really interested in, which is what causes the behavior. And heritability can arise from a lot of different potential causes. Even if we find specific genes that are correlated with political attitudes, it doesn't mean that those genes cause the attitudes.
Genes do not directly cause any human behavior; they code for proteins, which swirl around in the body, building structures and engaging in chemical reactions, on their own and in response to outside stimuli. Heritability can occur either because "genes cause the thing" (through a long, long chain of events from gene to behavior), or because genes cause something else that causes the thing, or because certain genes are correlated with having experiences that cause the thing (gene-environment correlation), or because genes change how a person responds to experiences (gene-environment interaction). Or all of the above simultaneously. Heritability estimates average across all of these complex processes, and more. So saying that political attitudes are partly heritable is true, but doesn't tell us what we need to do to build bridges between people who disagree.
Information and Culture Play a Big Role
Unlike the cluster of opinions that scientists label "political attitudes," political party affiliation shows very little evidence of genetic influence. Instead, party affiliation appears to be strongly (70%) due to influences that are shared between twins who are raised together (such as family, school, or cultural experiences). That is, twins who were raised together tend to have the same political party affiliation, regardless of whether they're identical or fraternal (Hatemi & McDermott, 2012, Figure 1). One thing we know from the history of public policy interventions is that even more strongly heritable behaviors can absolutely be influenced by changes in the culture. More education causes IQ scores to go up — for example, when governments provide public schools and increase the minimum required years of schooling. Raising the minimum drinking age decreases teenage alcohol use. Raising taxes on tobacco products decreases smoking. Politics is partly heritable, and resolving disagreements is important, but one does not inform the other.
Hatemi, P. K., & McDermott, R. (2012). The genetics of politics: Discovery, challenges, and progress. Trends in Genetics, 28(10), 525-533. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.363.440&rep=rep1&type=pdf