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The mode in which sexual organisms choose mates is a key evolutionary process, as it can have a profound impact on fitness and speciation. One way to study mate choice in the wild is by measuring trait correlation between mates. Positive assortative mating is inferred when individuals of a mating pair display traits that are more similar than those expected under random mating while negative assortative mating is the opposite. A recent review of 1134 trait correlations found that positive estimates of assortative mating were more frequent and larger in magnitude than negative estimates. Here, we describe the scale-of-choice effect (SCE), which occurs when mate choice exists at a smaller scale than that of the investigator's sampling, while simultaneously the trait is heterogeneously distributed at the true scale-of-choice. We demonstrate the SCE by Monte Carlo simulations and estimate it in two organisms showing positive (Littorina saxatilis) and negative (L. fabalis) assortative mating. Our results show that both positive and negative estimates are biased by the SCE by different magnitudes, typically toward positive values. Therefore, the low frequency of negative assortative mating observed in the literature may be due to the SCE's impact on correlation estimates, which demands new experimental evaluation.

Genetic nurturing, the effect of parents’ genotypes on offspring phenotypes through parental phenotypic transmission, can be modeled in terms of gene–culture interactions. This paper first uses a simple one-locus, two-phenotype gene–culture cotransmission model to compute the effect of genetic nurturing in terms of regression of children’s phenotypes on transmitted and nontransmitted alleles. With genetic nurturing, interpreting heritability and hence the meaning of “missing heritability” becomes problematic. Other factors, for example, population subdivision and assortative mating, generate similar signals to those of genetic nurturing, namely, correlation between parents’ nontransmitted alleles and children’s phenotypes. Corrections must be made for these to isolate the signal of genetic nurturing. Finally, a unified causal framework is constructed for genetic nurturing, population subdivision, and assortative mating. Causal and noncausal paths from transmitted and nontransmitted alleles to children’s phenotypes are identified and investigated in the presence of genetic nurturing, population subdivision, and assortative mating. Using causal analysis, assumptions made in inferring direct and indirect effects are then clarified and evaluated in a broader causal context.

Because of maternal mtDNA inheritance, mtDNA mutations detrimental only in males are not expected to be selected against, an effect termed the \"mother's curse.\" However, if there is positive-assortative mating, equivalent to what was called \"inbreeding\" by Wade and Brandvain (2009), then selection can act to reduce the frequency of these male-specific detrimental mtDNA mutants. On the other hand, as shown here negative-assortative mating, or \"outbreeding, \"paradoxically can result in an increase in the frequency of male-specific detrimental mtDNA mutants. The implications of these findings are briefly discussed.

When gene flow accompanies speciation, recombination can decouple divergently selected loci and loci conferring reproductive isolation. This barrier to sympatric divergence disappears when assortative mating and disruptive selection involve the same “magic” trait. Although magic traits could be widespread, the relative importance of different types of magic traits to speciation remains unclear. Because body size frequently contributes to host adaptation and assortative mating in plant-feeding insects, we evaluated several magic trait predictions for this trait in a pair of sympatric Neodiprion sawfly species adapted to different pine hosts. A large morphological dataset revealed that sawfly adults from populations and species that use thicker-needled pines are consistently larger than those that use thinner-needled pines. Fitness data from recombinant backcross females revealed that egg size is under divergent selection between the preferred pines. Lastly, mating assays revealed strong size-assortative mating within and between species in three different crosses, with the strongest prezygotic isolation between populations that have the greatest interspecific size differences. Together, our data support body size as a magic trait in pine sawflies and possibly many other plant-feeding insects. Our work also demonstrates how intraspecific variation in morphology and ecology can cause geographic variation in the strength of prezygotic isolation.

When an individual’s reproductive success relies on winning fights to secure mating opportunities, bearing larger weapons is advantageous. However, sexual selection can be extremely complex, and over an animal’s life the opportunity to mate is influenced by numerous factors. We studied a wild population of giraffe weevils (Lasiorhynchus barbicornis) that exhibit enormous intra and intersexual size variation. Males bear an elongated rostrum used as a weapon in fights for mating opportunities. However, small males also employ sneaking behavior as an alternative reproductive tactic. We investigated sexual selection on size by tracking individual males and females daily over two 30-day periods to measure long-term mating success. We also assessed how survival and recapture probabilities vary with sex and size to determine whether there might be a survival cost associated with size. We found evidence for directional selection on size through higher mating success, but no apparent survival trade-off. Instead, larger individuals mate more often and have a higher survival probability, suggesting an accumulation of benefits to bigger individuals. Furthermore, we found evidence of size assortative mating where males appear to selectively mate with bigger females. Larger and more competitive males secure matings with larger females more frequently than smaller males, which may further increase their fitness.

Sub-Saharan Africa (SSA) is undergoing rapid transformations in the realm of union formation in tandem with signifcant educational expansion and rising labor force participation rates. Concurrently, the region remains the least developed and most unequal along multiple dimensions of human and social development. In spite of this unique scenario, never has the social stratifcation literature examined patterns and implications of educational assortative mating for inequality in SSA. Using 126 Demographic and Health Surveys from 39 SSA countries between 1986 and 2016, this study is the first to document changing patterns of educational assortative mating by marriage cohort, subregion, and household location of residence and relate them to prevailing sociological theories on mating and development. Results show that net of shifts in educational distributions, mating has increased over marriage cohorts in all subregions except for Southern Africa, with increases driven mostly by rural areas. Trends in rural areas align with the status attainment hypothesis, whereas trends in urban areas are consistent with the inverted U-curve framework and the increasing applicability of the general openness hypothesis. The inequality analysis conducted through a combination of variance decomposition and counterfactual approaches reveals that mating accounts for a nonnegligible share (3% to 12%) of the cohort-specifc in equality in household wealth, yet changes in mating over time hardly move time trends in wealth inequality, which is in line with findings from high-income societies.

Animal body size is involved in reproduction in various ways. Carabus japonicus exhibits considerable variation in adult body size across geographical locations depending on the larval environment. To investigate the effects of body size divergence on male mating traits, spermatophore deposition and weight, copulation duration, and post-copulatory mounting were observed using male-female pairs from C. japonicus populations with different body sizes. Then, variables with high predictive power on the mating traits were identified from individual characteristics. When the male was slightly smaller than his mate, spermatophore deposition likely succeeded, suggesting that mechanical size-assortative insemination determined male body size. Although male reproductive organ size was positively correlated with male body size, spermatophore weight was not significantly affected by male body size, whereas copulation duration decreased with increasing male body size. Enlarged males, with a high capacity for spermatophore production, could increase paternity by decreasing copulation duration and increasing mating frequency. Such shifts in mating tactics would alter selection pressures of intra-and intersexual interactions (e. g., sperm competition and sexual conflict). Genital dimensions also affected mating traits other than copulatory duration. Thus, ecological heterogeneity has the potential to lead to divergences in sexual traits, such as genital morphology, through body size divergence.

Existing research on assortative mating has examined marriage between people with different levels of education, yet heterogeneity in educational assortative mating outcomes of college graduates has been mostly ignored. Using data from the 2010 Chinese Family Panel Study and log-multiplicative models, this study examines the changing structure and association of husbands' and wives' educational attainment between 1980 and 2010, a period in which Chinese higher education experienced rapid expansion and stratification. Results show that the graduates of first-tier institutions are less likely than graduates of lower-ranked colleges to marry someone without a college degree. Moreover, from 1980 to 2010, female first-tier-college graduates were increasingly more likely to marry people who graduated from similarly prestigious colleges, although there is insufficient evidence to draw the same conclusion about their male counterparts. This study thus demonstrates the extent of heterogeneity in educational assortative mating patterns among college graduates and the tendency for elite college graduates to marry within the educational elite.

Nearly half of all marriages in Pakistan are consanguineous, with 29 and 21% of women marrying first cousins on their father’s and mother’s sides, respectively. Despite its high prevalence, little is known about the change over time in consanguineous unions in Pakistan. Examining the patterns of the marriage market is particularly important given the substantial improvement in women’s education as women’s education is associated with the decline in consanguineous unions across the world. Our analysis, based on four waves of nationally representative Pakistan Demographic and Health Survey (PDHS) (1990-91, 2006-07, 2012-13, and 2017-18) of currently married women ages 15–49 shows that the prevalence of consanguineous unions has remained stable over time. In fact, a slight increase in cousin marriages is observed in 2006-07. Further, women’s education is negatively associated with cousin marriages. Historically, men in the country have had higher levels of education than women, leading to common occurrences of hypergamous unions (where the husband is more educated than the wife). However, with the advancement of women’s education over time, there has been an observed increase in educational hypogamy, where the wife is more educated than her husband. However, the rise in educational hypogamy has not changed the marriage market norms related to cousin marriages. Moreover, use of modern contraceptive methods is not associated with consanguineous unions. We found an inverse relationship between the children ever born (so far) and cousin marriages. Women in consanguineous marriages tend to have fewer children than women in non-consanguineous marriages. Contrary to common expectations, women in both hypogamous and hypergamous relationships tend to have more children than women whose spouses have the same level of education as them. Overall, the results show that consanguinity patterns are stable, and there is no evidence that the societal changes such as improvement in women’s education and urbanization over time have led to a substantial decline in cousin marriages in Pakistan.

Prior work has examined the relationship between educational assortative mating and wives' labor market participation but has not assessed how this relationship varies by race/ethnicity. Using data from the National Longitudinal Survey of Youth 1979, we estimate group-based developmental trajectories to investigate whether the association between educational assortative mating and wives' income trajectories varies by race/ethnicity. The presence, prevalence, and shapes of prototypical long-term income trajectories vary markedly across racial/ethnic groups. Whites are more likely than Blacks and Hispanics to follow income trajectories consistent with a traditional gender division of labor. The association between educational assortative mating is also stronger for Whites than for Blacks and Hispanics. White wives in educationally hypogamous unions make the greatest contribution to the couple's total income, followed by those in homogamous and hypergamous unions. Black and Hispanic wives in hypogamous unions are less likely than their peers in other unions to be secondary earners. These findings underscore the need for studies of the consequences of educational assortative mating to pay closer attention to heterogeneity across and within racial/ethnic groups.