How do we come by our political attitudes? Are some people “born conservatives” (or liberals)? Why do we identify as Democrats vs. Republicans vs. independents? For an interesting take on this question see the recent scientific paper “The genetics of politics: discovery, challenges, and progress”, which I found via a blog post by Razib Khan (who offers some commentary of his own). Personal genomics, genetics, and related topics are personal interests of mine, as is politics (in a more general as opposed to strictly partisan sense), so the intersection of these areas is definitely a blog-worthy topic for me.1
Note that this is a review paper; it doesn’t present any new results, but simply summarizes previous research in the field. The significance of the paper is rather that the possibility of genetic influence on political attitudes isn’t a new idea sprung on an unsuspecting world. There’s been enough work done in this field over the years, and enough interesting results, to make it worth summarizing the research and collecting all the references.
The interest comes from the fact that there’s now a large body of evidence that genetic differences among people account for a good deal of the differences in the various political attitudes that people have. Figure 1 of the paper (available at the above link to the paper’s abstract or at Khan’s post) shows this graphically, displaying estimates of the relative proportion of variation in political attitudes attributable to genetic factors, shared environment (basically, the family environment), and nonshared environment (basically, anything else other than genetic factors and the family environment).
It’s worth my explaining a bit about how these sorts of estimates are made and what they do and don’t mean. The relevant term of art is “heritability”: An attitude, behavior, or other trait (e.g., height) is heritable in a particular population (e.g., all U.S. adults) to the extent that the variation in the trait within the population is due to variation in the genetic makeup of that population. So, for example, according to the graph referenced above whether people identify as liberal or conservative is estimated to have a heritability of almost 0.6, meaning that almost 60% of the variation in how people identify (within whatever population was surveyed) is estimated as being due to differences in their genetic makeup.
Making such estimates requires measuring both how people’s attitudes vary in a given population and how their genetic makeup varies. The first task can be done through polling and similar measures. The second task has traditionally been harder, as until relatively recently there’s been no direct way to compare genetic makeups between individuals. This will become much easier in the future as genome sequencing becomes much cheaper and genetic sequences between individuals within a population can be directly compared.
In the meantime researchers have used various indirect techniques, including comparing trait variations between people who are identical twins (and thus share all of their genes) relative to variations between people who are fraternal twins (who on average share only 50% of their genes). For example, if identical twins tend to both identify as liberal significantly more often than do fraternal twins then that’s an indication that this trait is likely genetically influenced to some extent.2
Some other clarifications:
A trait being heritable is not the same as its being genetically determined. Just because a trait is genetically influenced doesn’t mean that it’s going to manifest itself in the same way or to the same degree in individuals with similar genetic makeups; environmental factors also can play a significant role in this. Also, some traits, like having five fingers or toes, are determined by genes but are not heritable in the strict sense because they don’t vary among people.
A heritability estimate is associated with a population, not an individual. Thus, for example, the almost 0.6 heritability estimate for identifying as liberal or conservative does not mean that the child of two liberals has an almost 60% chance of being a liberal themselves.
Because heritability is associated with a population, two populations can have different heritability estimates. For example, it’s possible that the heritability of (say) authoritarian attitudes could be different in the U.S. than in other countries.
Heritability estimates can also change over time. In particular, as environmental variation affecting a trait goes down we’d expect heritability (i.e., the proportion of trait variation due to genetic variation) to go up. A good example is height: Because developed countries have greatly reduced the incidence of malnutrition, diseases, and other environmental factors influencing height and related physical conditions, most of the variation in height in such countries now reflects genetic variation as opposed to environmental variation. (I’ll come back to this point later as it relates to political attitudes.)
Finally, a trait X being heritable does not mean that there is a “gene for X”. Thus, for example, there is not necessarily a “liberal gene” or “conservative gene”. Instead it’s possible that a given trait is influenced by hundreds or even thousands of genes, each having a very small effect; this is apparently the case for height, and possibly also for many other traits as well, including political attitudes,
But enough preliminaries—in part 2 I’ll comment on some of the findings discussed in the review paper.
Unfortunately the publisher (Elsevier) charges $39.95 to access a copy of the paper. This not-atypical pricing is a consequence of the broken economic model of scientific publishing in the age of the Internet: University professors produce research papers and then distribute those papers through commercial publishers who charge high prices for universities and others to access the journals containing those same papers. (The publishers do provide editorial services, but much of the work in reviewing papers prior to publication is also done by researchers.) Because so much research is government funded, either directly or indirectly, there’s been a push by NIH and other government agencies to make papers more generally accessible, with ensuing political controversies.
In the technical literature, including figure 2 of the paper under discussion, you’ll see identical twins referred to as “monozygotic” (MZ) and fraternal twins as “dizygotic” (DZ). A term like “MZ correlation” is just a fancy way of referring to the extent to which identical twins share a particular trait.