Is love blind? Assortative mating says “no”

This post is also available in Dutch.

What qualities do we look for in our search for “the one”? Evidence from assortative mating might surprise you. This blog will tell you why…

Have you ever wondered how chaotic and overwhelming the world we live in is? So many things happening at the same time, with so many variables we can’t control. And love is no exception… especially in 2020!

Never in the history of humanity have romantic relationships been so diverse and complex. There’s diversity in gender, numbers (in the committed form of Polyamory, or the not-so-committed form of Serial-Dating), and the ways to meet a potential partner. In today’s modern societies we enjoy, more than ever, the choice and the freedom to search for the person who completes us. With so many possible routes, how random can our romantic trajectory get? Perhaps not as random as you may think.

The truth is opposites do not (always) attract.

When a person looks for a partner, there are a number of personal traits that potentially weigh on a decision. They can be physical traits, professional interests, hobbies, and so on. What we observe in humans and other species is that we tend to look more for the things we have in common. This tendency tells us that love is not exactly a lottery, that the mating behavior which humans show is not random. We call this tendency assortative mating.

The scientific community agrees that assortative mating in the animal world is most often in the positive direction. In positive assortative mating, traits we share are valued more than the traits we don’t when it comes to mating. One clear example of this is height. For those who venture(d) into dating apps, you have probably experienced some rejections for not fulfilling one’s height standards. Specially here in the Netherlands! Of course, there are examples of couples who differ very much in their heights. See for example Jamie Cullum and Sophie Dahl. But these cases tend to be the exception.

Very rarely can scientists observe examples of negative assortative mating, in which partners tend to look precisely for the opposite of themselves. But they do exist! A replicated example of negative assortative mating is found in populations of white-throated sparrows, in which mates tend to look for partners with a different color than their own.

How does assortative mating shape future generations?

The genetic material we inherit from our parents explains a lot about the way we look and behave. So if there’s assortative mating, it will inevitably have an impact on the next generation. Does this mean that future generations remain more similar in their genetics than one would expect, i.e. more homogamic? With current human genetics research, we can see it more evidently than ever before.

Take again the example of height. Genetic research shows that couples with similar height tend to share very similar versions of DNA which code for that trait. If couples tend to have the same DNA versions, then these same versions will remain the same across generations. In a population of tall people with preference for tall people, you can expect that the future generations will also be tall.

Perhaps this is the reason why the Dutch population is on average much taller than every other in the world. The preference for taller partners dictated the preservation of “tall” genes, and the improvement in diet and other environmental conditions in the development towards adulthood of recent generations propelled the possessors of such genes to become even taller.

Assortative mating is keeping us similar, but not really turning us into clones.
Image courtesy of Andrew Martin and Pixabay (CC0 1.0).

So you can see that assortative mating is not explained only by similar genetics. The environment surrounding us since we were little embryos also relates to assortative mating, in a much more complex manner. Environment can be shaped by assortative mating, but it can also shape it.

A good example of that is the case of body-mass index (BMI). Like height, our BMI is also related to our genetic material, and couples are more likely to have similar BMI. But unlike height, which doesn’t change when we reach adulthood, BMI is more malleable. So the genetics shared by couples doesn’t explain as much why they tend to have similar BMI, which means that environment is responsible for that missing part.

If changes in environment can lead to changes in traits during adulthood, assortative mating can happen in different ways: partners may choose each other based on characteristics they share by the time they fall in love; but they can also differ in their traits when they meet, and become more similar over time.

Imagine you started dating someone with a much healthier and more rigid diet than yours, and you become convinced that you should give it a try. Then it might be the case that you and your partner end up with a more similar BMI than in the beginning of your relationship. That, on other hand, could never happen with height.

What kind of impact does assortative mating have on society?

The evidence we have for assortative mating in couples’ height or BMI also tends to happen to social aspects of our lives. Religion, cultural background or socio-economic status also tend to be similar among couples. This makes sense given that, for many reasons, we tend to be drawn more or end up spending more time with people who are similar to us.

It becomes quite intriguing if we look at the possible impact that these choices may have on our society. If people with more resources tend to pair with each other, and those with less resources as well, would you say that assortative mating perpetuates socio-economic inequality?

Perhaps, but it’s quite unfair to blame the ones who fall in love for that. Falling in love is definitely not a rational and calculated decision. So don’t blame yourself if your romantic choice hasn’t led to a more equal society. Whether you see a lot of yourself in your partner, or it seems like your partner came from a different world, focus on what makes that person the “one”.

Original language: English
Author: João Guimarães
Buddy: Monica Wagner
Editor: Rebecca Calcott
Translator: Ellen Lommerse
Editor Translation: Floortje Bouwkamp

Credits: Top image courtesy of lauramusikanski and Morguefile (CC0 1.0).

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