Behind our sexual impulses — who we feel attracted to and why — complex biological interactions help determine our feelings, our obsessions, our repulsions… Our general attraction to people with clear complexions, for example, results from an instinctual aversion to bacteria (which cause pimples and pockmarks). Science journalist Kathleen McAuliffe explains how many of the mating habits we take for granted are influenced by infectious diseases, parasites in our gut, and our bodily scent (which is actually determined by our immune system).

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

Kathleen McAuliffe:  There's a few ways in which infectious disease may impact who we find sexually attractive. So for example, in cultures where infectious disease is highly prevalent people tend to place more emphasis on beauty. So skin free of any kind of pockmarks, and also more symmetrical features. Because what happens is that if you have an infectious disease when you're young it can derail development and that's part of the reason why people's features may be a little bit more asymmetric if they're more vulnerable to infectious disease. There's also evidence that we're more attractive to people whose odors signify that they have very different immune systems from ourselves. And the way it works is this that believe it or not odor correlates with how your immune system functions. And we all vary individually in how susceptible we are to different kinds of infection and basically the research suggests that we're most attracted to people who are most different from us in terms of how their immune system functions.

So if we mate and have children our children are going to have very varied genes and as a result if say a terrible infection is spreading around you might lose one child but you're not going to lose all your children because they're going to have very varied immune systems in terms of what could make them sick and walk they're more resistant to.

I view gut bacteria as an extension of parasitic manipulation. Even though I don't think that most gut bacteria are parasites, in fact I would call them symbiotic manipulators. And the reason I'm so interested in them is because they do manipulate behavior in a fashion not totally unlike parasitic manipulators. And the way they're able to do this is there's over a thousand different species of bacteria that inhabit our guts. And there are species that turn out basically every single neurotransmitter that you have in your brain and they turn out hormones, so stress hormones and hormones that regulate our appetite and energy levels. So the research suggests that the bacteria in our gut influence everything from whether you're energetic or sluggish, happy or sad, anxious and calm, maybe even whether you're fat or thin. And there is some research now exploring what fecal implants, if you transplant feces from one person to another they're looking to see what some of the effects are.

Some examples would be there have been efforts to show that by transplanting feces from one person to another that you may even be able to influence their appetite. So far I don't think they've had too much success. There are one or two examples though of, for example, a woman who had was getting the fecal transplant actually to treat a digestive disease. It's called Clostridium dufficile. They have shown, by the way, the fecal transplant is very effective in treating some of these digestive disorders. And this particular woman wanted to get the fecal donor her she wanted it to be her donor daughter who was there in around 15 or 16 years of age and within a short period of time after getting the fecal transplant the mother suddenly for the first time in her life was starting to become overweight and she actually eventually became obese and she was convinced it was related to the transplant. And within just a year or two of her daughter being the donor the daughter became obese. So findings like that make scientists wonder if fecal transplant might actually, in the future, just as it can cause obesity maybe if you get the donor from a thin person maybe you can prevent obesity.

It's not very appetizing to contemplate it. You may be happy to hear that scientists are hoping to just purify the useful strains of bacteria and then concentrate them in a capsule. They call them crapsules. And so they're hoping that they'll be able to use these capsules instead of getting an actual fecal transplant, which they do it using that instrument that they use to do a colonoscopy. That's how they insert feces up your intestinal tract.