Why do we pick similar mates, or do we?

Humans often mate with those resembling themselves, a phenomenon described as positive assortative mating (PAM). The causes of this attract broad interest, but there is little agreement on the topic. This may be because empirical studies and reviews sometimes focus on just a few explanations, often based on disciplinary conventions. This review presents an interdisciplinary conceptual framework on the causes of PAM in humans, drawing on human and non-human biology, the social sciences, and the humanities. Viewing causality holistically, we first discuss the proximate causes (i.e. the ‘how’) of PAM, considering three mechanisms: stratification, convergence and mate choice. We also outline methods to control for confounders when studying mate choice. We then discuss ultimate explanations (i.e. ‘the why’) for PAM, including adaptive and non-adaptive processes. We conclude by suggesting a focus on interdisciplinarity in future research.

Female education, marital assortative mating, and dowry: Theory and evidence from districts of India

We study marital assortative mating in education and its relation to dowry in India. There are four main results and contributions of this paper. First, instrumental variable estimates using Indian Human Development Survey-II data suggest existence of positive assortative mating in education levels of husband and wife. Second, this association is weaker in dowry-prominent districts suggesting that in districts with strong patriarchal norms, high dowry transfers could substitute for lower bride's education. Third, we study the independent effect of husband's and wife's education and its interaction on dowry. Estimates suggest that dowry rises with the groom's education and falls with the bride's schooling years. However, the joint effect of husband-and-wife education on dowry is negative, implying that though dowry rises with groom's education, the rate of increase is smaller the more educated the bride is. Finally, to explain the empirical results, we propose a theoretical model of assortative mating in the presence of dowry.

Blue-eyed een erefer blue-eyed women: the role of life history strategies and sociosexuality

Previous research (Laeng et al., 2007) conducted on Norwegian samples showed that blue-eyed men rate blue-eyed women as more attractive, while brown-eyed men and all the women show no differences in attractiveness assessments with respect to eye colour. Correspondingly, positive assortative mating was found for blue, but not brown eyes, and it most often occurred in blue-eyed men. We aimed to replicate this blue-like-blue effect in the Croatian population, which differs in the ratio of eye colour phenotypes (blue eye colour is the most prevalent in Norway while brown is the most prevalent in Croatia). Additionally, we examined whether this effect is moderated by life history strategies and sociosexuality. Our hypothesis was that the effect would be larger in those blue-eyed men who exert a slower life history strategy and who are sociosexually restrictive. One hundred and twenty-eight participants assessed the attractiveness of blue-eyed and brown-eyed models, whose eye colours were experimentally manipulated in such a way that participants were shown models with natural or artificially changed eye colours. The blue-like-blue effect was replicated in the context of preferences, although it was smaller than in the original study. However, unlike the original study, in a sample of 138 participants no assortative pairing by eye colour was found between participants and their romantic partners. Finally, the hypothesis about the moderation was supported for life history strategies, but not for sociosexuality. In addition to the rationale for the blue-like-blue effect based on the paternity uncertainty account, which was offered by the authors of the original study, we discussed other accounts of this phenomenon.

Distribution of facial resemblance in romantic couples suggests both positive and negative assortative processes influence human mate choice

Previous research suggests that humans show positive assortative mating, i.e. tend to pair up with partners that are similar to themselves in a range of traits, including facial appearance. Facial appearance can function as a cue to genetic similarity and plays a critical role in human mate choice. Evidence for positive assortative mating for facial appearance has largely come from studies showing people can match pictures of couples’ faces at levels greater than chance and that facial photographs of couples are rated to look more similar than those of non-couples. However, interpreting results from matching studies as evidence of positive assortative mating for facial appearance is problematic, since this measure of perceived compatibility does not necessarily reflect actual physical similarity, and may be orthogonal to, or even negatively correlated with, physical similarity. Even if participants are asked to rate facial similarity directly, it remains unclear which, if any, face shape cues contribute to an increased perception of similarity in romantic couples. Here we use a shape-based assessment of facial similarity to show that the median similarity of long-term couples’ face shapes is only slightly greater than that of an age-matched control sample. Moreover, this was driven by the most similar 40% of couples, while the most dissimilar 20% of couples actually showed disassortative mating for face shape when compared to the control sample. These data show that a simple measure of central tendency obscures variability in the extent to which couples display assortative or disassortative mating for face shape. By contrast, a more fine-grained analysis that considers the distribution of variation across couples in the extent to which they resemble each other suggests that both positive and negative assortative processes influence human mate choice.

No evidence of positive assortative mating for genetic quality in fruit flies

In sexual populations, the effectiveness of selection will depend on how gametes combine with respect to genetic quality. If gametes with deleterious alleles are likely to combine with one another, deleterious genetic variation can be more easily purged by selection. Assortative mating, where there is a positive correlation between parents in a phenotype of interest such as body size, is often observed in nature, but does not necessarily reveal how gametes ultimately combine with respect to genetic quality itself. We manipulated genetic quality in fruit fly populations using an inbreeding scheme designed to provide an unbiased measure of mating patterns. While inbred flies had substantially reduced reproductive success, their gametes did not combine with those of other inbred flies more often than expected by chance, indicating a lack of positive assortative mating. Instead, we detected a negative correlation in genetic quality between parents, i.e. disassortative mating, which diminished with age. This pattern is expected to reduce the genetic variance for fitness, diminishing the effectiveness of selection. We discuss how mechanisms of sexual selection could produce a pattern of disassortative mating. Our study highlights that sexual selection has the potential to either increase or decrease genetic load.

Assortative mating by population of origin in a mechanistic model of admixture

Populations whose mating pairs have levels of similarity in phenotypes or genotypes that differ systematically from the level expected under random mating are described as experiencing assortative mating. Excess similarity in mating pairs is termed positive assortative mating, and excess dissimilarity is negative assortative mating. In humans, empirical studies suggest that mating pairs from various admixed populations—whose ancestry derives from two or more source populations—possess correlated ancestry components that indicate the occurrence of positive assortative mating on the basis of ancestry. Generalizing a two-sex mechanistic admixture model, we devise a model of one form of ancestry-assortative mating that occurs through preferential mating based on source population. Under the model, we study the moments of the admixture fraction distribution for different assumptions about mating preferences, including both positive and negative assortative mating by population. We consider the special cases of assortative mating by population that involve a single admixture event and that consider a model of constant contributions to the admixed population over time. We demonstrate that whereas the mean admixture under assortative mating is equivalent to that of a corresponding randomly mating population, the variance of admixture depends on the level and direction of assortative mating. In contrast to standard settings in which positive assortment increases variation within a population, certain assortative mating scenarios allow the variance of admixture to decrease relative to a corresponding randomly mating population: with the three populations we consider, the variance-increasing effect of positive assortative mating within a population might be overwhelmed by a variance-decreasing effect emerging from mating preferences involving other pairs of populations. The effect of assortative mating is smaller on the X chromosome than the autosomes because inheritance of the X in males depends only on the mother’s ancestry, not on the mating pair. Because the variance of admixture is informative about the timing of admixture and possibly about sex-biased admixture contributions, the effects of assortative mating are important to consider in inferring features of population history from distributions of admixture values. Our model provides a framework to quantitatively study assortative mating under flexible scenarios of admixture over time.

On Mate Choice in two Xerophilic Species of Land Snails, Brephulopsis cylindrica (Pulmonata, Enidae) and Xeropicta derbentina (Pulmonata, Hygromiidae)

The cases of assortative mating are known in varied species of Gastropoda, however it is unclear how widely distributed this reproductive strategy is across the class. The data concerning land snails (Stylommatophora) are especially contradictory. By using two model species of stylommatophoran mollusks, Brephulopsis cylindrica (Menke, 1828) and Xeropicta derbentina (Krynicki, 1836), we conducted a long-term observation of their copulatory behavior in Southern Ukraine. In X. derbentina, a clear pattern of the positive assortative mating with respect to body size was found, whereas assortative mating in relation to shell banding polymorphism was revealed in 2 cases out of 7. B. cylindrica snails copulate with any available mate partner, regardless of its size.

Phylogeography and hybridization of corvid birds in the Palearctic Region

Natural hybridization increases a lot phenotypic and genetic diversity and shapes intra-species patterns, which is a subject of phylogeography. We studied mitochondrial and complete genome variation in the bird family Corvidae, genera Corvus, Pica, Cyanopica, Perisoreus and Nucifraga. In the classic case of natural hybridization between carrion and hooded crows in Siberia, we found no decreased fitness of hybrids, but instead positive assortative mating which should restrict hybrid zone width. Several genetic markers were unable to discriminate between pure carrion and hooded crows. Mitochondrial DNA sequences revealed no difference between carrion and hooded crows, but instead two diverged haplogroups within the eastern part of the distribution range of the carrion crow. NGS resulted in a clear pattern of diversification of pure forms and hybrids (by using SNPs), and showed genomic regions of increased variability, the so-called “speciation islands”. Comparing European and Siberian crow hybrid zones, differences in genome regions bearing genes of melanogenesis supposedly under divergent selection were found. Comparative phylogeographic analysis of 10 widely distributed Palearctic species revealed two kinds of patterns: one with a division into two haplogroups, western and eastern, and another one without such a division. These two phylogeographic patterns might be explained by different habitat preferences: mainly open fields for the first group and forests for the second one. One glacial refuge was assigned to the latter group, while west-east group species might have survived in several refuges. One of such species, the Eurasian magpie (Pica pica) has a gap in its range in Transbaikalia, which is currently shrinking before our eyes. The two subspecies divided by this gap differ in phenotype, mtDNA and vocalization. In their young contact zone, some hybridization occurs with small introgression limited by certain post-zygotic isolation.