Has there ever been a time when the political process has been so bipartisan and divisive? Yes, actually, one has only to recall the rancorousness of the Bush-Gore or Bush-Kerry campaigns, harken back to the acrimonious campaigns of Nixon or Johnson, read historical accounts of the political carnage of both pre- and post-Civil War elections, or watch HBO’s John Adams series to relive in full period costuming the bipartite bitterness between the parties of Adams and Jefferson to realize just how myopic is our perspective.

We can go back even further into our ancestral past to understand why the political process is so tribal. But for the business attire donned in the marbled halls of congress we are a scant few steps removed from the bands and tribes of our hunter-gatherer ancestors, and a few more leaps afield from the hominid ancestors roaming together in small bands on the African savannah. There, in those long-gone millennia, were formed the family ties and social bonds that enabled our survival among predators who were faster, stronger, and deadlier than us. Unwavering loyalty to your fellow tribesmen was a signal that they could count on you when needed. Undying friendship with those in your group meant that they would reciprocate when the chips were down. Within-group amity was insurance against the between-group enmity that characterized our ancestral past. As Ben Franklin admonished his fellow revolutionaries, we must all hang together or we will surely hang separately.

In this historical trajectory our group psychology evolved and along with it a propensity for xenophobia—in-group good, out-group bad. Thus it is that members of the other political party are not just wrong—they are evil and dangerous. Stray too far from the dogma of your own party and you risk being perceived as an outsider, an Other we may not be able to trust. Consistency in your beliefs is a signal to your fellow group members that you are not a wishy-washy, Namby Pamby, flip-flopper, and that I can count on you when needed.

This is why, for example, the political beliefs of members of each party are so easy to predict. Without even knowing you, I predict that if you are a liberal you read the New York Times, listen to NPR radio, watch CNN, hate George W. Bush and loathe Sarah Palin, are pro-choice, anti-gun, adhere to the separation of church and state, are in favor of universal healthcare, vote for measures to redistribute wealth and tax the rich in order to level the playing field, and believe that global warming is real, human caused, and potentially disastrous for civilization if the government doesn’t do something dramatic and soon. By contrast, I predict that if you are a conservative you read the Wall Street Journal, listen to conservative talk radio, watch Fox News, love George W. Bush and venerate Sarah Palin, are pro-life, anti-gun control, believe that America is a Christian nation that should meld church and state, are against universal healthcare, vote against measures to redistribute wealth and tax the rich, and are skeptical of global warming and/or government schemes to dramatically alter our economy in order to save civilization.

Research in cognitive psychology shows, for example, that once we commit to a belief we employ the confirmation bias, in which we look for and find confirming evidence in support of it and ignore or rationalize away any disconfirming evidence. In one experiment subjects were presented with evidence that contradicted a belief they held deeply, and with evidence that supported those same beliefs. The results showed that the subjects recognized the validity of the confirming evidence but were skeptical of the value of the disconfirming evidence. The confirmation bias was poignantly on display during the run-up to the 2004 Bush-Kerry Presidential election when subjects had their brains scanned while assessing statements by both Bush and Kerry in which the candidates clearly contradicted themselves. Half of the subjects were self-identified as “strong” Republicans and half “strong” Democrats. Not surprisingly, in their assessments Republican subjects were as critical of Kerry as Democratic subjects were of Bush, yet both let their own preferred candidate off the evaluative hook. The brain scans showed that the part of the brain most associated with reasoning—the dorsolateral prefrontal cortex—was quiet. Most active were the orbital frontal cortex that is involved in the processing of emotions, the anterior cingulate that is associated with conflict resolution, and the ventral striatum that is related to rewards.

In other words, reasoning with facts about the issues is quite secondary to the emotional power of first siding with your party and then employing your reason, intelligence, and education in the service of your political commitment.

Our political parties today evolved out of the Paleolithic parties of the past.