Daniel Z. Lieberman and Michael E. Long

Opinion contributors

In 2012 scientists at the University of Nebraskacollected a group of volunteers and showed them a collage consisting of three pleasant — a bunny, for instance — and three disturbing pictures, including a spider crawling on a human face. You might think people would focus on the happy photos and avoid the disturbing ones. But no, most people gazed at the scary ones.

Biology quickly explains that: our brain includes an early warning system to identify and prioritize threat, so we can stay alive. But scientists saw something else not as easily explainable: conservatives focused on the threatening stimulus more than liberals. The reason may upend expectations in the midterm elections.

The brains of conservatives and liberals are far more alike than different, but one exception might matter a lot just now: liberals tend to have higher levels of dopamine than conservatives. Dopamine is the neurotransmitter (brain chemical) that equips us to think about the future. In general, our fascination with new things — our comfort with change — tracks with the level of dopamine. In fact, there appears to be a genetic link between a strong dopamine system and the appearance of liberal ideology, especially if we think of that ideology as the pursuit of change — progress, or progressivism.

Threats activate conservatives more than liberals

It further follows that, being attracted to change, progressives would be more active politically than conservatives, who are generally more content with the way things are. History suggests this biological edge is real because, in the long run, through good times and bad, progressive ends usually emerge: government tends to grow, not shrink, and its influence tends to expand, not contract.

In politics, the neurochemical advantage belongs to the left, and that would be the end of the matter but for one thing: circumstances can fire up conservatives’ neurochemical motivation to match or exceed that of the progressives. As the experiment with happy versus disturbing photos demonstrated, conservatives are more reactive to threat than liberals are. In fact, threat does even more. It tends to make people of all ideologies more conservative, and you don’t need brain science to know that. You can look at history.

With occasional exceptions, polls typically find that Americans trust Democrats more when it comes to health care, but trust Republicans more on national security. Historically, terrorist attacks increase the number of people who say they plan to vote Republican.

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And if you think you can explain away that phenomenon with ideology, think again. Researchers at Cornell asked college students to fill out a survey about their political beliefs. Half the participants took the survey in an empty hallway. The other half were seated near a seemingly innocuous item, a dispenser for hand sanitizer — a reminder of the risk of infection from dirty hands. Those who sat near the hand sanitizer reported higher levels of moral, social, and fiscal conservatism.

Just an anomaly? When a separate group of students was asked to use a germ-killing hand wipe before answering the questions, researchers saw the same result. Glenn D. Wilson, a psychologist who studies the influence of evolution on human behavior, joked that bathroom signs that say “Employees must wash hands before returning to work” are campaign billboards for the Republican Party.

Aggressive Democrats will motivate GOP voters

It takes little to trigger the defensive reaction to threat, which brings us to the coalescing Democratic strategy articulated by former attorney general Eric Holder as “When they go low, we kick ‘em.” The “threat” of the left toward the right now includes confronting conservative leaders in restaurants and at their homes, using vulgarity in public to express opposition, exercising the “heckler’s veto,” and mounting campaigns to get high-profile employees fired from their jobs for expressing conservative opinions.

That these tactics are relatively rare or, to some activists, justified, misses the point. Threat rouses conservatives to action more than it rouses liberals and can push the general population further to the right.

An NPR/PBS NewsHour/Marist poll found that in July there was a 10-point gap between the number of Democrats and Republicans saying the November elections were “very important.” Following the Brett Kavanaugh confirmation hearings, which conservative leaders characterized as the destruction of a reputation by accusation, the gap evaporated into a tie.

Striking fear in the hearts of one’s opponent can be satisfying, but it is not always productive. Under ordinary circumstances, 100 Democrats marching down the street is probably a protest, while 100 Republicans is probably a parade. But that can easily change.

Daniel Z. Lieberman, MD, is a psychiatrist and professor at George Washington University School of Medicine. Michael E. Long, a writer and playwright who trained as a physicist, is a lecturer at Georgetown University. Their new book is "The Molecule of More: How a Single Chemical in Your Brain Drives Love, Sex, and Creativity – and Will Determine the Fate of the Human Race." Follow them on Twitter: @MoleculeOfMore