By William B. Miller, Jr. M.D.
What determines chemistry between two people? Who has not said, “They have good chemistry together?” We hear it so often that it seems trite. Typically, we direct that remark towards a couple that looks mismatched to our judgment. We ask ourselves, “What do they see in each other?” Our usual answer is that their ‘chemistry’ is just right. This is often our only way of reconciling the force of attraction between these two.
However, what if the concept of chemical attraction has a direct biological basis? New research is revealing that it does, and many of those reasons had not been previously expected or explored. It is becoming increasingly apparent that our microbiome has a tremendous influence on us. The microbiome represents all the microorganisms that are in us and around us. Astoundingly, these are so numerous that they outnumber our innate cells by a factor of ten to one or more. The current estimate is that we harbor 100 trillion cells that we would not think of as our own. Importantly, our relationship with our microbial companions is essential to our well-being. Their influence is a crucial element of metabolic pathways. They are critical participants in glucose regulation, the mediation of our immune systems, and even partially regulate our emotional responses to stress. Our relationship with these obligatory microbial partners is intimate. We cannot survive without them, and they cannot exist as they prefer without us. It should not be totally surprising then that they can affect our social choices and even our love life.
How could germs influence our sexual choices? Those factors had remained hidden from our appraisal until recently. We simply did not have the technological means to assess it. Now we do, and current research has found that the amount of microbial life in our mouths is startling, and the transfer between kissing partners is extensive. However, the particular surprise is that although frequent intimate kissing between partners does correspond to the composition of the microbes that are shared between each, there is more afoot. It seems that there is a shared linkage in microbial composition in the mouths of sexual partners that operates regardless of kissing frequency. The implication is that there is a background connection with the microbial realm that might influence our initial choices of sexual partners. For example, the microbiota on the back of the tongue is more similar between kissing partners than unrelated individuals, but that identity does not clearly correlate with any kissing behavior or frequency. Nor does it appear to be due to specifically shared environmental factors. Something extends beyond that. Could it mean that we are attracted to one another on the basis of forces that are unapparent to our typical senses?
There is other evidence to suggest that this the case and that it is based on our immunological status. We depend on that for our survival. So it should not be shocking that our immunological status might affect our choice of mates. We all exist on a planet also inhabited by aggressive microorganisms. How might any organism cope? Our evolutionary path and that of all other complex organisms include mechanisms directed towards protecting our offspring as much as possible. This imperative significantly governs how we select mates. As part of this process, we each have a group of genes that is crucial to immunological defenses.
Experiments have showed that we are unconsciously attracted to other partners whose immunological background is complementary to our own. In order to best protect our children against an intrusive and agitating microbial realm, we tend to seek to mate with those that differ from ourselves on an immunological basis. Although this may seem odd on first consideration, it makes sense from an evolutionary perspective.
The combination of immunological capacities of healthy mates that differ confers better protection to the next generation. There are research studies that reinforce this finding. Experiments have demonstrated that sexual partners are attracted to certain body odors. For human odors, microbes matter. However, in a crucial evolutionary twist, that odor attraction is not for similarity but for opposite types. In the aggregate as a species, we seem to search for other sexual partners that are not our match immunologically. What might underlie this instinct? The answer lies within both our own personal genes and our particular microbial complement. On an evolutionary basis, our species is best protected by the mixing of opposing genes and microbes. Successful mating proceeds for many reasons, but there is an important one that has been previously obscure. That additional reason is our critical association with a vast microbial partnership that is crucial to our immunological balance. When contrasting individuals both mix genes and share microbes, the next generation gets a boost.
Even more astoundingly, the influence of the microbial sphere extends well beyond these factors. Research has demonstrated that infection with certain parasites can overtly guide sexual choice. Such an example is a parasite called Toxoplasmosis. Cats serve as an intermediate host but it also frequently infects humans. Surprisingly, this common parasite has been shown to affect human behavior, physiology, and even our physical appearance. As strange as it may seem, in carefully controlled experiments, human females perceive infected males as more dominant and more masculine than uninfected males.
What should we make of all this? The next time you gaze lovingly at your favorite other, tip your hat mentally to your hidden microbial partners. In truth, your microbiome might have made you do it.
Dr. Bill Miller has been a physician in academic and private practice for over 30 years. He is the author of The Microcosm Within: Evolution and Extinction in the Hologenome. He currently serves as a scientific advisor to OmniBiome Therapeutics, a pioneering company in discovering and developing solutions to problems in human fertility and health through management of the human microbiome. For more information, www.themicrocosmwithin.com.