Who do you think has a stronger stomach: a liberal or a conservative? Who is the tougher party?

The knee-jerk answer to this question might lean toward the latter, because conservative political ideologies – on the whole – are perceived by both sides as taking a harder line. But what brain circuits are stirring beneath those hardline decisions?

An international team of researchers conducted two studies (involving more than 31,000 people in total) and found a positive relationship between sensitivity to disgust and political conservatism. “Across both samples, contamination disgust, which reflects a heightened concern with interpersonally transmitted disease and pathogens, was most strongly associated with conservatism,” the study reports.

Disgust is a sliding scale, and we’re all grossed out by different things. Some of us shudder at the thought of seeing blood. Some draw the line at foul smells. There are people who are disgusted by homosexuality and there are people are disgusted by homophobia. And there are a few groups who have almost no sensitivity to disgust at all.

Science journalist Kathleen McAuliffe knows a lot about disgust. She took us on a wonderful tour of parasites here, and in the video above she tackles the link between visceral disgust and moral disgust. It’s hard for the average person to fathom how someone can decide to kill in cold blood, and also physically carry out the act. But research has found that cold-blooded killers have damage to the brain circuits involved in the disgust response, which explains why these people are less squeamish about not just the moral quandary of taking a life but are also quite comfortable carrying out the grizzly act.

McAuliffe points to another group of individuals who have similarly impaired disgust circuits: people with Huntington’s disease. This is a genetically transmitted neurodegenerative disease, and those with it are unable to recognize expressions of disgust in others, and don’t tend to react to foul smells, sights or tastes. People with Huntington’s also have impaired fear recognition, as the two areas are closely related in the brain.

Which brings us back to conservative political ideologies, particularly immigration aversion. McAuliffe notes that there is a link between germophobia and xenophobia, as evidenced by a study of 2000 Danish people and 1200 Americans, where the data showed that opposition to immigration increased in direct proportion to the disgust sensitivity of the individual.

If you want to see where you sit on the disgust sensitivity scale, there’s a quiz for your amusement over here.

Kathleen McAuliffe’s book is This Is Your Brain on Parasites: How Tiny Creatures Manipulate Our Behavior and Shape Society.

Kathleen McAuliffe: Similar parts of the brain - there's a lot of overlap in how we process both visceral disgust and moral disgust. Psychopaths - needless to say they're these cold-blooded killers are over representative in many of our high security jails. And these individuals show damage to many of the same circuits that are involved in disgust. Another group of individuals, although they're not predatory but people with Huntington's disease it also damages some of the circuits that are involved in disgust. People with Huntington's disease tend not to be empathetic and they think that this is related to these circuits being damaged. And they're almost unique in that they experience almost no visceral disgust whatsoever. So somebody with Huntington's disease, for example, would think nothing of picking up feces with their bare hands. So there is this sort of very interesting interrelationship, at least in the brain, between visceral and moral disgust.

A little known fact is that conservatives are more disgust sensitive. There's a huge variation acrostic populations in how disgust sensitive people are. There's actually standardized scales that measure, for example, the questionnaire you fill out and it will ask you questions about like how revolted you would be If you stepped on dog poop or if you saw a cockroach on pizza or a dirty toilet. And as a result of filling out these questionnaires they've been able to look to see if there's parallels between how disgust sensitive someone is and how conservative they are. And indeed there is a correlation. And probably the reason for that is because, again, the conservatives if you kind of really breakdown their belief systems they tend to have conservative sexual values. So, for example, concepts like virginity pledges are ideas that they're fond of. They also tend to be more opposed to immigration and foreigners are a leading source, at least in centuries past, foreigners were a leading source of exotic germs for which we had no natural defenses. So it's speculated that that could be another factor behind why people who are more conservative in their political ideology why they tend to be opposed to immigration.

Conservatives also tend to be very tradition bound. They tend to be a little bit more rigid about following religious doctrine. And again, a lot of religious practices may help to protect against infection. So that's the leading theory as to why you see this association. But in general, even in large survey studies they've shown this link between germophobia and xenophobia. So, for example, there was a paper actually that's about to be published and I think they looked at 2000 Danes and 1200 Americans, representative samples from both countries. And they found that opposition to immigration increased in direct proportion to the disgust sensitivity of the individual.

Another group did a study of 25,000 Americans. The study was done at the time of the 2008 presidential election between John McCain, a more conservative candidate, of course, and Barack Obama. And they found that the more disgust sensitive the person the more likely they would vote for John McCain. And they actually showed the proportion of votes that went to McCain in each state. It was based on the average disgust sensitivity of the state based on individual respondents to the survey.