(Rob Dobi/for The Washington Post)

Sebastian Junger is the author of “Tribe: On Homecoming and Belonging.”

Many people balk at the idea that we are a product of evolution. On some level, they acknowledge that humans are social primates with a particular evolutionary heritage, but if you drill down far enough into their personal preferences — Why do you have a sweet tooth? Why do you vote a certain way? — their explanations veer into self-determination. They like sugar or vote Republican because they choose to — it’s what makes them them.

Given how natural selection works, it’s entirely possible that an aversion to evolutionary explanations is in itself a product of evolution. In a hostile environment, wouldn’t a belief in self-determination be adaptive? But no matter, the point is that anti-evolutionary bias makes rational discussion of the human race far more difficult.

This particular bias is prevalent at both ends of the political spectrum for different reasons. Cultural conservatives often reject the theory of evolution because it elbows aside the proposition that God created humans in his own image. Secular liberals, on the other hand, have a different agenda. They are comfortable with a godless universe but can balk at the prospect of living in a society that is deeply influenced by biological realities. Worldwide phenomena such as aggression and warfare are ascribed to culture — rather than genetics or biology — because to do otherwise would be to give up, as it were, on the struggle for social justice.

But many progressives who reject, for example, biological differences between the genders happily watch Olympic competitions that carefully separate women from men so that testosterone differentials do not result in men sweeping nearly every contest. And cultural conservatives who reject any biological explanation of human origins are happy to take medicine that works biologically to treat their illnesses and alleviate their suffering.

Given these sensitivities, the most inflammatory proposition of all may be that our political views could be substantially influenced by genetics. No other proposition so exquisitely combines the logical blind spots of both liberals and conservatives with the cauldron of political passion that bubbles within us.

Politics is an emotional topic because people feel they have arrived at their conclusions through deep moral reasoning. It’s not just a collection of policy opinions and problem-solving strategies; it’s a core philosophy that goes straight to a person’s identity. And because moral reasoning feels both noble and incontrovertible, few people want to be told that these sacred conclusions are profoundly influenced by genetics.

The effect of genetics is so strong, however, that according to empirical studies, identical twins who are raised apart are more likely to hold similar political views than fraternal twins who are raised together. According to Avi Tuschman, author of “Our Political Nature,” between 40 and 60 percent of the variance in our political attitudes is heritable, stemming from genetic differences between individuals; the rest comes from our environment, especially during our formative years. Political opinion also relates to brain structure: An experiment measuring the sizes of the anterior cingulate and the right amygdala accurately predicted a person’s political orientation 72 percent of the time. Some of these political proclivities appear to be connected to Chromosome 4 in a neurotransmitter receptor called NARG1.

The fact that political opinion is rooted to some degree in our genes and biology means that both liberalism and conservatism may be adaptive traits that got passed down through thousands of human generations because they helped us survive. But another trait that is clearly adaptive is our ability to get along. Political arguments may rage within families, communities and even nations, yet they only rarely threaten the cohesion of the group. On some level, humans seem to understand that differences of opinion are unpleasant but splitting up may be even more unpleasant — or downright dangerous. Humans don’t survive alone in nature.

Today, many people are concerned that the United States is splitting along ideological lines. With no credible enemy to encourage unity, the country finds itself in a deep political argument in which both liberals and conservatives accuse the other of not only being wrong but of being a threat to the country. The real threat to the country, of course, is bellicose, divisive rhetoric. But this is where evolution might throw Americans a lifeline.

If liberalism and conservatism are partly rooted in genetics, then those worldviews had to have been adaptive — and necessary — in our evolutionary past. That means that neither political party can accuse the other of being illegitimate or inherently immoral; we are the way we are for good reason. Every human society must do two things: It must be strong enough to protect itself from outside groups, and it must be fair enough to avoid internal conflict. A society entirely composed of liberals risks being overrun by enemies, and a society entirely composed of conservatives risks breaking apart over issues of inequality — “social justice,” as it’s now termed.

Put those groups together, however, and you have addressed the two greatest threats to human welfare: enemies and discord. The task for every society, from the earliest Homo sapiens of Africa to Americans of the 21st century, is to accommodate different values and worldviews into one ethos. It’s not easy to do, but our own genetic diversity clearly demonstrates that it’s possible. Otherwise, one set of values would have gradually dominated the other until there was no political discord at all, just a broad, flat uniformity. That may sound appealing at the moment. But in the long term, what a great loss that would be.