Since we depend on the group to protect our young, we have all evolved to respond to the threat of a child's harm with horror and disgust, he said.
And that disgust is a powerful teacher. The putrid scent of spoiled foods and the overt signs of disease probably elicit feelings of disgust to ensure we avoid the dangers of poisoning or contagion: It is a "hallmark emotion of fear," Fessler said. Researchers have found that urban legends with a yuck factor are more likely to be remembered and passed on than those without.
At the same time, neuropsychological studies have demonstrated that the brain's responses to moral and physical disgust strongly overlap. It's a sign that humans mark one another's behavioral transgressions as evidence of danger, and that we'll keep a safe distance from the people who engage in them.
In his tweets, Trump routinely expresses disgust at policies he opposes, the people who espouse those policies, or the places his perceived adversaries come from. Since taking office in 2017, the president's Twitter offerings have included the words "disgust," "disgusted" or "disgusting" at least 54 times, often accompanied by words such as "sick," "vile," "dangerous" and "filthy."
Either deliberately or subconsciously, Trump's rhetoric also capitalizes on another human inclination: In general, we believe and remember bad things more than neutral or pleasant ones.
This is the natural result of the evolutionary principle known to psychologists as "negatively biased credulity." Essentially, it's the idea that when it comes to enhancing our prospects for survival, it's more important to remember and put stock in signals of danger than it is to heed the lessons of things that are neutral or pleasurable.
Believing a neighbor's account of discovering a hornet's nest, for instance, might save you from getting stung repeatedly, and perhaps from dying of anaphylactic shock. By contrast, you could forget or discount your neighbor's tale of finding an unusual rock, or even a shady grove, and never pay a price.
This discrepancy has been well established in psychology experiments: Humans are generally more likely to believe and remember people, places or things that suggest danger than we are to believe or remember information that is likely irrelevant to our immediate survival.
But for some people, that negative bias is more pronounced.
Fessler's experiments suggest that people who lean toward conservative positions are more likely to believe messages of danger than those who lean liberal.