I’ve worked out why we loathe each other so much — why democracy throughout the West, and especially in the United States, has become so partisan, so adversarial, so angry.

It’s because we’re getting stupider.

Pundits have been writing for over a decade about increasing political polarization. The evidence is all around us, not just in the behavior of legislators, but in the attitudes of their electorate. Our readiness to believe different sets of facts on the basis of our political affiliation -- from climate change to the Kavanaugh nomination — is growing. Our cross-party friendships are dwindling.

In their new book, Prius or Pickup, Marc Hetherington and Jonathan Weiler show that, in the 1980s, around 15 percent of Democrats and Republicans confessed to “hating” their rivals. That figure drifted up to 20 percent by the end of the century, and then skyrocketed. By 2016, 48 percent of Democrats and 50 percent of Republicans admitted that they hated the other party.

What changed? Some social scientists argue that it has to do with the absence of a foreign threat. From the mid-1930s until 1989, America had overseas foes — Nazis, Japanese imperialists and then Communists. External enemies, the theory goes, strengthen a nation’s cohesion.

Others point to social fragmentation. The mid-to-late twentieth century was a time of exceptional homogeneity in the United States. The imposition of immigration controls a generation earlier meant that the proportion of native-born citizens was uniquely high. Most people knew their neighbors and watched the same three TV networks.

As Ben Sasse points out in his new book, Them: Why We Hate Each Other — and How to Heal, those social structures have fragmented. Rotary clubs and little leagues are disappearing, and political tribalism has taken their place.

The third explanation is the most common, though the least satisfying. Social media, we keep being told, has pushed us into political silos, allowing us to surround ourselves only with people who share our opinions.

I don’t know about you, but that’s pretty much the opposite of my experience. Social media allows people — indeed encourages people — to thrust their views on opponents. There is more pluralism now than there was 20 years ago, when you read either the Washington Times or the Washington Post depending on your ideology.

But I do believe that social media may have contributed to the problem in a different way. Screen addiction is shortening our attention span and lowering our cognitive ability, thus making us less able to entertain the idea that people we dislike might nonetheless have useful things to say.

The fall in IQ scores in the West is perhaps the most under-reported story of our era. For most of the twentieth century, IQ rose by around three points per decade globally, probably because of better nutrition. But that trend has recently gone into reverse in developed countries.

You hadn’t heard? I’m not surprised. Journalists and politicians won’t go near the subject and you can see why. Consider the theories offered by neuroscientists for the decline. Some argued it had to do with the rising age of motherhood, because the children of older mothers tend to have lower IQs, other things being equal. No one likes to say this, because it can come across as “older moms have dumb kids,” which is not true. (My wife and I were 44 when our youngest child was born, and my own parents were also elderly, but that didn’t make me too thick to grasp the concept of statistical distributions.)

Other theories were even more explosive. For example, that unintelligent people were having more kids, or that the fall in average scores reflected immigration from places with lower IQs.

But a new study from Norway, which examines IQ scores from 730,000 men (standardized tests are part of military service there) disproves all these ideas, because it shows IQ dropping within the same families. Men born in 1991 score, on average, five points lower than men born in 1975. There must, in other words, be an environmental explanation, and the chronology throws up a clear suspect: the rise in screen-time.

Kids brought up with Facebook and Instagram are more politically bigoted, not because they don’t hear alternative opinions, but because they don’t learn the concentration necessary to listen to opponents — a difficult and unnatural skill.

To put it another way, today’s American elections are mild compared to those of, say, 1800 or 1860. The relative tolerance that characterized the twentieth-century reflected rising education and rising intelligence, which made voters more capable of empathy with opponents. Reverse that rise and people will revert to the more primitive but easier rule-of-thumb: my tribe good, your tribe bad.