People have a behavioral immune system that acts as the body’s first line of defense against germs or infectious diseases, an ASU alumnus and psychology professor from the University of British Columbia said at a Wednesday lecture.

The psychology department hosted Mark Schaller to discuss how people’s behavior changes when they’re around a person they think is carrying an infectious pathogen.

Douglas Kenrick, a professor of social psychology, said Schaller is one of the psychology department’s most successful alumni.

Kenrick said Schaller brings together two areas of psychological research that are traditionally separated: cultural and evolutionary influences on behavior.

In his lecture, Schaller said people give off superficial cues that allow others to tell if they are transmitting pathogens that could infect them.

“People are sensitive to whether [other] people have infectious diseases,” he said.

If people pick up on these cues, Schaller said, they will likely alter their behavior to avoid contact with the sick person. Schaller refers to this change in behavior as the behavioral immune system.

But sometimes people misread cues and think a healthy person is sick or a sick person is healthy, he said.

“Just as a smoke detector sounds its alarm in response to objectively harmless events, human minds also respond adversely to people who merely have a superficial resemblance to actual disease threats,” he said.

In line with this “smoke detector principle,” Schaller said people often think of those with physical anomalies — like the elderly, the disabled or the obese — as potential disease-carriers.

The social stigma associated with these groups of people is in part linked to people’s behavioral immune systems, Schaller said.

“It’s hard to make the rational argument that someone is obese because of an infectious disease,” Schaller said, but because obesity is deviant from the typical human body type, people might psychologically view that person as being more likely to be have a pathogen.

“If someone looks different from normal, [people interpret that as] a piece of information saying that person might have an infectious disease,” he said.

The behavioral immune system might also play a role in xenophobia and ethnocentrism, Schaller said.

A foreign person could be seen as carrying “exotic pathogens” or prone to violate cultural rituals and norms, he said, and would therefore be considered more dangerous to be around.

Schaller said many cultural norms serve as defenses against disease, such as hygiene rituals and cooking practices.

“A lot of spices are natural antibiotics,” he said. “In places that have a historically higher pathogen prevalence, people traditionally use more spices in cooking.”

Schaller said non-normative behavior can be costly to a person’s health.

If people disregard social norms, they could be acting in ways that put them at a higher risk for contracting pathogens. Thus, in societies with more disease, collectivism is more than likely to pesist over individualism, he said.

Even the percentage of left-handers in a society could indicate the prevalence of pathogens in the area, Schaller said.

“If you’re left-handed, you’re in the minority anywhere on the planet,” Schaller said. “People who are naturally left-handed are coerced into being right-handed. In a sense, the percent of left-handed people in a population is an indication of how much that society lets its people be different.”

Psychology graduate student Chad Mortensen said he attended the lecture because it was related to some of his research.

“It’s interesting to see a former ASU grad student come back and fill us in on what he’s done,” he said. “But it was also interesting because he shared some cutting-edge results that weren’t even published yet.”

Reach the reporter at kkfrost@asu.edu.