If you think that opposites attract, you are likely mistaken, or at least that’s what the science says. Over a hundred years ago, researchers first discovered that couples are actually more alike than 
they are different.
One of the first things researchers noted was that tall people tend to partner with other tall people. Since then, studies of this phenomena – called “assortative mating” – have found strong correlations among couples for almost all traits, including behavior, personality, and disease risk.
There are a lot of reasons for this. Love is complicated, and figuring out precisely why couples get together is tricky.
But a new study the journal Nature Human Behavior has identified genetic evidence for assortative mating, reporting that, at least for one trait, height, there is a strong correlation between the genetic variants that influence height in one partner and the actual height of the other.
In this study, researchers from the University of Queensland in Brisbane, Australia, the University of North Carolina, 23andMe and dozens of other institutions, were able to look specifically at genetic factors by using a new design and analytical approach that allowed for them to carefully control for non-genetic influences.
Isolating all the different cultural and social influences involved in why people partner up, isn’t easy, however. We may be attracted to our partners for any number of reasons. For instance, partners might have similar BMIs because people are attracted to individuals with a BMI that resembles their own. Or, they might choose a partner who is culturally similar. Alternatively, their BMIs might converge over time due to shared environment.
To isolate some of those kinds of influences the researchers in this study used data from more than 24,000 couples, including 11,908 from 23andMe, and compared an individual’s genetically-predicted height and BMI to those of their partner. Because genetics are determined at birth, these shouldn’t be correlated if partners become more similar only after meeting each other.
The researchers also controlled for population, to remove the effects of shared cultural background.
They found a strong correlation between people’s genetic predisposition for height and the actual height of their partner. The findings showed that similarity in couples’ heights can be largely explained by people selecting mates who are like them. Tall people, it seems, are attracted to other tall people.
But the findings were not as strong for BMI, where there was a weaker correlation between people’s genetic contributions to BMI and the actual BMI of their partners.
One possible implication of this is that “mate choice” may, in turn, affect the genetic architecture of traits in humans as well. Assortative mating may increase the odds of parents passing on a trait, like height, to their children.
“These results have implications for the interpretation of resemblance between relatives and for estimates of genetic parameters in populations,” the researchers said.
You can find the paper in the journal Nature.