When you’re looking for that special someone, you’re probably not consciously thinking about whether their genes would mix well with yours in your hypothetical children. (If you are, don’t mention that on the first date.) Instead, preferences for certain appearances are deeply influenced by culture and by ideals promoted in the media. But how our individual preferences have been shaped by sexual selection at the population level is still poorly understood.
That may soon change as DNA sequencing continues to become cheaper and easier, and more detail on human genes, traits, and mate preferences becomes available. “There will be, 5 years from now, datasets that are unquestionably large enough to start seeing [genetic variations linking couples],” said Greg Gibson, a professor of biology at Georgia Tech. Recent research is already starting to take steps in that direction for the study of face, height, and body scent.
In a study in Proceedings of the National Academy of Sciences(1), researchers found that Mexican and Puerto Rican couples living in Mexico, Puerto Rico, and the United States were more genetically related than random pairs from the same population. Lead author James Zou, a computational biologist at Microsoft Research in Cambridge, Massachusetts, said that the group was surprised that to find that similarities in the ancestry of the couples could not be fully explained by other factors, such as education level or socioeconomic status.
This led the scientists to look more closely at parts of the genome linked with specific traits, where they found that Puerto Rican couples were more likely to share variants in a set of 49 genes involved in face development. In contrast, there was no significant genetic overlap for height or skin color, although with more couples they may well help uncover similarities, Zou said. Even the effect for facial features was small, so “we have to be careful in how we interpret the results,” he added.
“To be sure, [the researchers] did not close the loop by showing that the couples indeed resemble one another more than random people, but it is reasonable to draw this inference,” Gibson noted on his blog. He and his group investigated the facial features of 20 couples and observed that individuals in a pair tend to look more like each other than the others in the study, although it will be important to account for similar ancestries within a pair, he said.
The idea that some couples look alike is not surprising. Research has shown that people are drawn to others who are similar. Using a simple mathematical model, Zou and his colleagues showed in the new study that so-called “assortative mating,” when it occurs within populations with shared ancestries, can boost risk of diseases that are more prevalent within those ancestries.
How mating patterns contribute to disease susceptibility was a major motivation for their study, but quantitative social science will also benefit from large datasets of human genomic data and richer phenotypic data. “There are a lot of really interesting fundamental questions there, and the data and the computational methods are becoming much more available,” Zou said.
The fact that men and women look different in the face is itself a result of sexual selection (2). Genes that relate outward appearance have evolved faster than other genes. Mark Shriver at Pennsylvania State University and his collaborators are looking at the patterns of facial sex differences and other traits within various populations across the globe and how those have changed over time (3).
Whether it’s skin color or facial features, it’s important to remember that people look more different than they are genetically, Shriver said. “We’ve known this for a long time; we just know it better by studying these genes. Remembering that and realizing that may help us take a step toward becoming less racialized.”
The Stature Factor
Not surprisingly, mate choice is influenced by height. In many Western societies, women tend to be attracted to men who are at least as tall as they are, if not taller (4).
Height has long been a mysterious model trait in human genetics. Up to 80% of a person’s stature is attributable to genes, but relatively few genes are known to affect height, and each has only a small effect.
According to a recent article in Genome Biology (5), analysis of the genotypes of 32,000 couples in the UK’s Biobank showed that ~5% of the heritability of height is attributable to a tendency to prefer a partner of similar height. What’s more, genetic variations underlying both a person’s height and their own preference for a partner’s height are 89% similar.
In general, “genetic contribution to the choice of height in a mate is so low that it’s very difficult to detect something significant unless you have lots of samples. I don’t think it would have worked if we had had only 1000 couples,” said the study’s senior investigator, Albert Tenesa from the University of Edinburgh.
The analysis also depended on processing a huge volume of high dimension data, which was made possible by a new, freely available software program his group developed called DISSECT (6). Tenesa is now interested in identifying additional height genes, but it’s a challenge: Even with better computational tools and data from hundreds of thousands of people, the statistical power is probably not yet sufficient, he said.
Sniffing out Love
The idea that we could smell high-quality genes in a potential mate seems pretty strange. But the famous “sweaty T-shirt study” from 1995 (7) and follow-up research suggest that people are more attracted to potential mates who have a different set of major histocompatibility complex (MHC) molecules—conveyed through their body’s scent—than their own (8).
Working with Shriver, David Puts at Penn State is collecting additional evidence that a woman’s sexual response to her mate is different midway through the menstrual cycle, near ovulation, depending on how dissimilar their MHC genes are. Previous research suggested that women’s preferences for features in a mate, such as appearance and voice, change over the course of the cycle, but now more detailed genetic data are enriching this research on women’s sexual psychology. One of Puts’s predictions is that if male–female pairs have more complementary MHC alleles, then the woman would be more likely to report having a positive sexual experience around the time of ovulation.
Evidence in other animals, such as fish, mice, and non-human primates, suggests that MHC compatibility is important in mate choice. It makes evolutionary sense to be attracted to a mate whose MHC variants are dissimilar because it would give offspring a more diverse and well-equipped immune system. Still, for humans the evidence is somewhat mixed, a challenge common to many studies of complex traits. In addition, mate choice studies are typically not well-funded, said Puts.
“With carefully conducted research, we’re still likely to find effects that exist even if they’re smaller,” Puts said. “That can tell us something about how people form and maintain romantic relationships and how that might have happened over human evolution.”