Hillary Clinton referred to conservatives who supported Donald Trump as “deplorables.”
In referring to working-class conservatives harmed by de-industrialization, Barack Obama said: “They get bitter, they cling to guns or religion or antipathy to people who aren’t like them or anti-immigrant sentiment or anti-trade sentiment as a way to explain their frustrations.”
And liberal academics, intelligentsia, bi-coastal elites, and employees of Facebook, Google and other tech companies think that conservatives are inferior to them in intelligence, education, hipness, and open-mindedness.  At the same time, these liberals pride themselves on being non-judgmental and valuing diversity.
As a libertarian, I don’t have a rooster in the ongoing cockfight between liberals and conservatives.  Not only do both sides seem dumb to me, but I’m dumber than all of them, as I share the pipedream of libertarians that the left and right will someday learn to restrict the use of government force to the protection of life, liberty and property.  But human nature assures that this will never happen.
A scholarly dissertation on human nature is the best-selling book, Behave:  The Biology of Humans at Our Best and Worst, by Robert M. Sapolsky.  The book includes a section on the source of our political orientations and describes the behavioral and biological differences between conservatives and liberals.
Warning:  The book is a very difficult read, especially for a dumbbell like me.
The first half of the book resembles a medical textbook.  It delves deeply into biology, neuroscience, evolutionary science, and behavioral science.  It describes the workings of neurons, dendrites, axons, and synapses; the behavioral effects of testosterone, oxytocin, vasopressin, estrogen, progesterone, and other hormones; and the biological role of such neurotransmitters as serotonin, norepinephrine, dopamine, acetylcholine, glutamate, and GABA.  It details how hypothalamic and pituitary hormones regulate glucocorticoid levels; how glucocorticoid levels can in turn negatively affect cognition, impulse control and empathy; how these impairments can lead to depression, anxiety, substance abuse, and anti-social behavior; and how all of this is related to childhood adversities, such as poverty, poor diet, and abusive parents.
The first half of the book also describes the functions of different parts of the brain, the detrimental effects of damage or stunted development on the different parts, how the human genome evolved, and how humans are similar to, and different from, other mammals.
The author then ties all of this together to explain the evolution of empathy, sympathy and moral reasoning, as well as the less desirable human traits of tribalism, authoritarianism, warfare, and a natural aversion in all of us to people who are different.
An aside:  As with other books of this genre, one can’t read Sapolsky’s book without wondering about free will; specifically, how genes, hormones, and brain wiring affect our behavior and choices, and how these biological factors are related to the circumstances of birth and upbringing.  As the study of these factors advances, new findings could raise more questions about the limits of free will and thus shake the foundations of religious beliefs and the criminal justice system.
Back to the book:  Beginning on page 444, Sapolsky explains the differences between liberals and conservatives and how biology affects their respective political orientations.
Up to this page, Sapolsky is evenhanded, apolitical, and nonpartisan.  Although he is an admitted liberal, he is self-deprecating in the preceding 443 pages about his own political beliefs and sees the plusses and minuses of both liberals and conservatives.  But in addressing political orientations, his objectivity seems to wane.  This is ironic, given that he devotes an entire earlier chapter to the reasons why people see the world as “us” versus “them.”  Apparently, even he can’t help seeing the world of politics in this way.
Sapolsky begins by citing studies that suggest that lower intelligence is a predictor of conservative ideology.  More specifically, he goes on to say, there is a link between lower intelligence and a subtype of conservatism known as right-wing authoritarianism, or RWA.  For people with poor abstract reasoning skills, RWA provides simple answers that they can understand.  This is why they often embrace populist policies that are harmful to them personally on matters of trade and other complex economic and social issues.
Conservatives tend to be uncomfortable with ambiguity, complexity, and situational subtleties, Sapolsky says.  For this reason, they have a need for predictability, structure and hierarchy.  This in turn makes them value loyalty, obedience, and law and order.
Conservatives also have a higher sensitivity to visceral disgust, which comes into play in evaluating social and moral issues.  If something disgusts them at a visceral level, then in their thinking it cannot be good.  For example, if the thought of men having sex with men is disgusting to them, they will tend to be against gay marriage, irrespective of religious beliefs.
Sapolsky says that in terms of biology, “Liberalism has been associated with larger amounts of gray matter in the cingulate cortex (with its involvement in empathy), whereas conservatism has been associated with an enlarged amygdala (with, of course, its starring role in threat perception).  Moreover, there’s more amygdala activation in conservatives than in liberals when viewing a disgusting image or doing a risky task.”
But other brain regions are also activated in conservatives when they look at disgusting images, including “the basal ganglia, thalamus, periaqueductal gray, (cognitive) kdlPFC, middle/superior temporal gyrus, pre-supplementary motor, fusiform, and inferior frontal gyrus.”  Sapolsky says it isn’t clear how all these fit together.  He’s right:  It’s not clear at all to this dumbbell.
Some behavioral geneticists have hypothesized that inherited genes account for approximately 50% of one’s political orientation.  But Sapolsky clarifies that most of the genetic findings “haven’t been replicated, reported effects are small, and these are published in political science journals rather than genetics journals.”
Interestingly, in spite of all the alleged flaws of conservatives, they are generally happier than liberals, according to Sapolsky.
The problem with Sapolsky’s take on the differences between conservatives and liberals is that it doesn’t align with reality, regardless of what brain scans show, regardless of hormone measurements, and regardless of laboratory experiments.
First, as history shows and as current politics demonstrates, liberals are just as enamored with authoritarianism and hierarchy as conservatives, but for different ends.  After all, they endorse big government, have flirted with socialism and communism, have replaced fathers with government in the raising of children, embrace speech codes on college campuses, are disgusted by religion and guns, generally prefer government-enforced collectivism and egalitarianism over individualism, and are not bashful about using a government gun to get their way.  No doubt, parts of their brain light up at such thoughts.
Second, liberals can be just as resistant to new ideas as conservatives when the ideas go against their orthodoxy.  For example, there is no way that they would ever consider that racial diversity may have a downside, or that women and men gravitate to certain professions due to innate differences.
Third, liberals can be just as irrational, illogical, unreasonable, and stupid as conservatives, as evidenced by leading liberal personalities in politics, media, and Hollywood.   For example, the liberals on “The View” are just as dumb as Sean Hannity, CNN is just as dumbed-down as Fox News, and Democrats in Congress are just as dumb as Republicans.
Given human evolution, it can’t be any other way.  Despite the remarkable advances in science stemming from the modicum of reason used by humans, our species is still stuck emotionally, socially, and genetically in the first 190,000 years, or first 95%, of human existence.  This was a time of hunter-gatherers who lived in small tribes, with an egalitarian sharing of resources, a social pecking order but no central government, and a wariness of other tribes.
It wasn’t until 10,000 years ago that humans began to domesticate animals, grow crops, and establish settlements.  And it wasn’t until the last 5,000 years, which comprise only 2.5% of human existence, that humans formed cities, empires, and nation states.  More relevant to today’s politics, the Enlightenment came about only 220 years ago, and the Industrial Revolution, only 180 years ago, or .11% and .9%, respectively, of human existence.
This means that our biology, genetics, brain wiring, and emotional make-up haven’t changed much from hunter-gatherer times and thus haven’t kept pace with scientific advances and the complexities of the modern world.  In that sense, all of us, whether liberal, conservative or libertarian, are dumb.
Take it from a dumbbell.