Assortative mating develops naturally if mate preferences and preferred mate traits are heritable

A study in Australia ran a computer simulation that showed how assortative mating (the preference for romantic partners similar to oneself) arises spontaneously when heritable traits and heritable preferences for mates become associated through generations. The simulation showed that the heritability of mate preferences and preferred traits is sufficient to produce assortative mating without any other mechanisms. The paper was published in Psychological Science.

Assortative mating is the tendency for individuals to choose partners who are similar to themselves in important traits, such as education, height, personality, or values. It is observed in humans and many animal species, making it a widespread pattern in nature. People tend to resemble their partners more than would be expected by chance. While this similarity can make communication and cooperation easier—whereas a large mismatch in vocabulary, cognitive capacities, or interests can make communication difficult—the study suggests these benefits are not necessary for the pattern to emerge.

In humans, assortative mating frequently occurs regarding socioeconomic status. It can also happen for psychological traits, such as intelligence or mental health vulnerabilities. Biologists distinguish between positive assortative mating, where similar individuals pair up, and negative assortative mating, where opposites attract, although the former is much more common. Positive assortative mating tends to increase genetic similarity within families and can reduce genetic diversity in small populations, an outcome usually considered undesirable.

Study authors Kaitlyn T. Harper and Brendan P. Zietsch propose that when individuals are driven by heritable preferences to choose partners with certain heritable traits, associations form between individuals’ traits and corresponding preferences because offspring inherit both the trait from one parent and the mate preference from the other.

In simple words, if a person who likes (for example) their partner to be tall (a partly heritable preference) has a child with a tall person (a heritable trait), that child may inherit the preference for tall individuals from one parent and genes that make them tall from the other, creating a genetic correlation. Through further generations, this tendency strengthens. In this way, assortative mating—i.e., the preference for individuals similar to oneself—may arise through generations without any other mechanisms.

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The study authors wanted to see whether this described mechanism is sufficient to create assortative mating (i.e., to make individuals choose partners similar to themselves). They ran a simulation of multiple generations of individuals using an agent-based model programmed in R. Agent-based modeling is a computational approach that simulates individuals interacting under a specified set of rules.

In this case, agents in the model (simulated individuals) used ideal preference values to evaluate the traits of potential partners. These traits were set to be either neutral regarding the organism’s chance to reproduce (fitness neutral) or were designed to create selection pressure, where different traits conferred different chances of reproduction.

The authors ran 10 versions of each model to assess how varying the number of preferences used in mate choice from 1 to 10 affected the strength of the associations that developed between partners’ traits. They ran each simulation for 100 generations.

Results showed that genetic correlations formed between preferences and preferred traits, as well as between partner traits, over generations. In other words, assortative mating emerged as a natural outcome, demonstrating that the heritability of preferences and preferred traits is sufficient to produce it.

“We have demonstrated that even in the absence of adaptiveness or complex social dynamics, assortative mating is likely to arise naturally when preferences and preferred traits are heritable, which is true for virtually every quantitative trait,” the study authors concluded.

The study contributes to the scientific understanding of mate selection mechanisms in living organisms. However, it should be noted that the study was completely based on a simplified computational model that operated under certain predefined rules. Real-world results might differ.

The paper, “Assortative Mating Is a Natural Consequence of Heritable Variation in Preferences and Preferred Traits,” was authored by Kaitlyn T. Harper and Brendan P. Zietsch.