When Michael Sam kissed his boyfriend on television to celebrate his successful draft into the NFL, some viewers were disgusted. The charged comments that followed demonstrated that when it comes to public displays of gay affection, some people have a gut reaction to recoil. But why?

The answer to that question is not fully known, but scientists are beginning to establish an understanding of how biology and the environment may interact to form such reactions. 

First, it's important to understand that disgust in humans can be good. We should recoil from the truly gross things that can harm us—festering wounds, rancid meat, and feces, to name a few examples, are dangerous incubators of infection. "Disgust is apart of what is referred to as the behavioral immune system, which protects us from dealing with items and individuals that might make us sick, that might kill us," says Patrick Stewart, a political scientist at the University of Arkansas (and not of X-Men, Star Trek fame).

"At this point what we believe is not that you have a gene for [being against] gay marriage."

But it's not adaptive when healthy groups of people are reacted to in the exact same way as if they were pathogens. 

Here's the state of the science of disgust right now. Conservatives are thought to have a greater propensity to be disgusted than liberals do. Many studies corroborate this idea (see here, here, and here). One illustrative study in 2011 found that people who were more physiologically disgusted, a reaction that was measured by skin reactions, by this photo of a man eating a mouthful of worms were also more likely to self-identify as conservative. They were also more likely to have a negative response to gay marriage.

Disgust reactions against gays are then correlated with negative attitudes toward same-sex marriage. That isn't to say that conservatives are less tolerant of or hardwired to dislike gays. "I think the best way of looking at this is conservatives are wired to be a little more easily disgusted and fearful," Stewart says. "They are a little bit more wired to defensiveness in their environment." This is particularly apparent for sexual issues—not just gay marriage but for topics like pornography as well.

But Stewart has also found that even liberal minds harbor these implicit associations.

In a recently published experiment, Stewart and his colleagues demonstrated that the presence of a disgusting odor decreased support of gay marriage. Random samples were sorted into either into an odor group (the researchers added a vomit-like smell to a room) or control groups (no odor). The participants were then asked about their feelings on an array of social and political issues. The results are clear: In the disgust condition "participants exposed to the smell … reported increased subjective disgust and more politically conservative attitudes concerning gay marriage, premarital sex, pornography, and Biblical truth." The disgusting odor had no impact on opinions on nonsexual political matters, such as tax cuts or immigration.

Those results are a bit of a head scratcher—how can an odor change a person's political beliefs? It's best to think of it like this: The disgusting smell temporary made the liberal minds more conservative, shifted the liberal minds to the right. In that more conservative state, the participants reacted more conservatively to gay marriage. It's a result that confirms the complexity of the issue. "What we we're finding here is that the environment plays a huge role," Stewart says.

To gain a better understanding of this complicated and still-emerging science, I called Kevin B. Smith, a psychology professor at the University of Nebraska (Lincoln). Smith is a frequently cited researcher on this topic and is the coauthor of Predisposed: Liberals, Conservatives, and the Biology of Political Difference. Below is an edited transcript of our conversation.

In terms of biology and psychology, is the disgust one feels toward something objectively gross—like vomit—the same disgust that people might feel toward gay people? Is it the same emotion?

In terms of how you experience it biologically and psychologically? Yes. When we hook people up to psycho-physiological equipment, how they experience disgust in terms of activation of their sympathetic nervous system or what have you, it's very very similar.

How would you categorize, or how would you define, the link between conservatism and disgust?

Well, I'm not sure if it is a link from conservatism per se. I think it's a link between disgust and attitudes. Most of the research points towards attitudes on sex-related policies: gay marriage, abortions, premarital sex, sex education in schools—those sorts of things. [Though, these attitudes are used to define people labeled "conservative."]

One of the things we know is that disgust is a primary emotion, and its evolved purpose is to stop humans from ingesting pathogens. Stuff that disgusts us, we don't eat and we avoid and that helps keep us healthy. And as part of our evolution that mechanism seemed to get adopted for social purposes.

The only honest answer, I think, to your broad question is, we're not really sure. But one of the ways that this plays out is that people who have these strong disgust reactions tend to be in favor of tighter social regulations of sexual relations.

So what's the larger goal of the research?

We're trying to figure out where ideology comes from. Why are some people liberals, why are some people conservatives? And ultimately, why are so people so attached to their ideology that they believe in it so much they will go to extreme lengths to support what is essentially an abstract belief system, up to and including violence?

Traditionally the answer to that has been from the environment, from experience. You know, mom and dad taught me at the table. You come from a good family of liberals, you're raised in a particular culture. It's your surrounding environment. In other words, we come into the world as political blank slates and pick up our ideology from our experiences as we go through life.

How could genes be connected to whether you are in favor of gay marriage or not? There's a big, long causal chain—there has to be—between your genes and your stance on the issue of the day. And a lot of what we are trying to do is to figure that out.

At this point what we believe is not that you have a gene for [being against] gay marriage or anything like that. But what you have is your brain, your central nervous system and your peripheral nervous system. These are information processing systems. Your genes build those information processing systems and they make them more sensitive to certain environmental stimuli and less responsive to other environmental stimuli and that affects your attitudes and behaviors.

Is there any theory of why humans can come to be seen as pathogens?

That is a huge question and I think the only honest answer to that right now is we don't know. But there are theories out there. The big one is that this mechanism evolved to help us avoid pathogens were adopted to help us avoid social things that we don't like.

So could we decrease the power of disgust in our interactions with one another?

We are at very early stages now. Right now we've got research that has come out over the past few years that suggests A: That a correlation between disgust and political attitudes exists, and B: It can be manipulated.

The question of whether the information that we can glean there could be used to alter attitudes and behaviors one way or another … that's at the very bleeding edge. One of the things we think is that when people recognize a certain set of attitudes and behaviors are anchored in biology and not just the environment, people are more tolerant of differences. So that's kind of like a speculative argument at this point. But we are starting to get some empirical evidence that is indeed the case.

If I think you are gay because you made a lifestyle choice, I'm less tolerant of you. That disgusted reaction that I have is "I know deep down in my gut it's wrong, that you shouldn't be doing that." But if I believe that sexual orientation is innate, I am much more tolerant of that regardless of what my reflexive emotional response is.

What are the findings that the field has a good consensus on?

The disgust differences are certainly a consensus. There's an emerging consensus that brain activation patterns can reliably distinguish between liberals and conservatives. I think there's an emerging consensus that liberals and conservatives process information differently, that they pay attention to different things in their environment. I think that you can pick up physiological differences in liberals and conservatives, their flight-or-flight systems react differently to particular stimuli. And all of these things I've mentioned, they are not based on single studies, they are studies based on teams around the world using different methodologies and they all seem to triangulate similar conclusions.

What are the wrong conclusions people sometimes draw from this line of research?

The big one is determinism, which drives us all nuts. We've had people write about our research and say, "There's a conservative gene," or "Conservatives are more afraid," or "Liberals don't pay attention to danger." And we are very, very wary of these deterministic absolutes because the data don't support it and nothing in our research suggests it.

One of the things we try to make really clear in the research that we do: Nothing that we have found suggests in the slightest that biology is deterministic. In other words, that just because you have this disgust reaction doesn't mean you are going to go one way or the other. What it does is pushes the probabilities one way or the other. That seems like a subtle difference but it is a big difference.