Why does a mechanism that originally evolved to protect us from pathogens affect our reactions to people and behavior? One possibility is that early humans were endangered by contact with outside clans that carried diseases for which they had not developed immunity. Reacting with disgust toward members of groups seen as foreign, strange or norm-violating might have functioned as a behavioral immune system.

Consistent with this, researchers at the University of British Columbia have shown that individuals who see themselves as particularly vulnerable to disease (a trait that is correlated with disgust sensitivity) tend to report more xenophobic attitudes. These studies, we might add, did not set out to study extreme hypochondriacs, clean freaks or slobs, but rather drew on samples of ordinary college students.

Moreover, similar effects were demonstrated when researchers increased individuals’ perceived vulnerability to disease (for example, by showing them pictures of a woman battling cartoon germs in a kitchen). Participants shown such pictures felt more negatively about, say, Nigerian immigrants than did participants who were shown slides of non-pathogenic dangers, like school bus accidents or electrical appliances teetering above bathtubs.

While this avoidance mechanism may have conferred a survival benefit on our ancestors, it can easily overfire, causing us to shun groups and people inappropriately and unfairly. And because it remains tightly linked to the disgust we experience in the presence of oral contaminants (like putrid meat), it makes sense that inducing disgust with a rotten smell could cause a shift in attitudes toward certain individuals or social groups.

Recent data collected by one of us (Dr. Pizarro) has also shown that political conservatives on average report being more easily disgusted than liberals. These studies used a “disgust sensitivity” scale developed by the psychologists Paul Rozin, Clark McCauley and Jonathan Haidt, which asks hypothetical questions like, “How disgusted would you be if you took a sip from a soda can and then realized that it belonged to a stranger?” Even when controlling for income, depth of religious belief and a host of other factors, conservatives tended to score higher in disgust sensitivity than liberals.

Mr. Paladino, a Tea Party activist, seems no exception to this general pattern. He has called gay pride parades “disgusting” and proposed sending welfare recipients to state-run work camps, where they would receive “life lessons” in personal hygiene.

Taken together, researchers’ findings suggest that the foul smell of Mr. Paladino’s mailer may have done more than just lend it novelty. It also probably made voters more judgmental of New York’s “career politicians” and more receptive to the mailer’s message that the next governor needed to “cut taxes” and “ferret out corruption.” And these impressions may have endured long after the odor and feelings of disgust had dissipated.

Obviously, the malodorous mailer alone can’t explain how Carl Paladino steamrolled Rick Lazio in the primary, 62 percent to 38 percent. Nonetheless, election officials should keep the psychology of disgust in mind  and be wary of Purell dispensers or awful odors mysteriously appearing at polling places this Nov. 2.