The Surprising Scientific Reason Behind Physical Attraction

Is it your similarities that attract you to your partner, or is it your differences? Science says our sense of smell may be important in sniffing out mates with immune systems opposite to our own.

As Valentine’s Day approaches, you may be pondering one question: what attracted you to your partner? Was it your similarities? Your shared love of classical music and long walks on the beach?

Well, science tells us that the inspiration for our relationships may not be all that romantic. We’ve all heard that opposites attract, and it may actually be your differences, specifically those related to smell, that help you pick your mate.

First, it’s important to note that attempts to understand our sense of smell are very challenging. On top of that, our natural scents are also very diverse and complicated. So studies that attempt to incorporate both are inherently difficult.

An example of one such study, involves the MHC portion of the genome. In our ongoing attempts to understand the parts that make up the genome, biologists have linked a particular group of genes called the major histocompatibility complex (or the MHC group for short) to both our body’s ability to protect itself against disease and our sense of sexual attraction.

For the study, men were given t-shirts to sleep in for several nights in a row. Women were then asked to smell the t-shirts and rank them in order of their preference. Women tended to prefer the odors of men without similar MHC genes, suggesting not only a link between the MHC group of genes and body odor, but also our preference for MHC genes that differ from our own.

Another study compared results from 90 married couples to results from 152 random, computer-selected pairings among the same people. They found a significant bias for the real partners to have larger differences in their MHC gene groups and thus a preference for differences in their immune systems.

The desire to mate with someone whose immune system differs from yours makes evolutionary sense.

The desire to mate with someone whose immune system differs from yours makes evolutionary sense. If you’re not sick at the same time, you can care for each other. Also, perhaps more importantly, our choice in partner may be linked to a subconscious desire to produce healthy children. Children obviously share genes from both parents and so may get the best of both worlds when it comes to a merging of two opposing immune systems.

Other studies have shown that we can also use sight to sense the immune system strength of potential mates. One study asked women to look at photos of men whose immune systems had been triggered by receiving a Hepatitis B vaccine. Their bodies began the production of antibodies in response to the virus contained in the vaccine. Women found the men with the strongest immune system response to be the most attractive.

This so-called “nonrandom mating," also called “genetic assortative mating,” suggests that our biology plays more of a role than we realize in choosing our partners. Most (if not all) of these studies, however, have focused on white, heterosexual couples leaving many relationships unexplored.

Interestingly this attraction of opposites is not seen for other gene groups. Studies looking at other parts of the genome find more of a preference for similarities rather than differences, although often not by a significant margin. The strongest trends in long term partnerships by far are also not genetic at all, but are instead matches in education and socioeconomic class.

So it is probably best not to rely completely on your sense of smell to help you sniff out your next mate. But, if you find you prefer your partner’s smell over the scent of any others, it’s not just your imagination—it’s science!


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Ahhh you naughty immune system