Why do we pick similar mates, or do we?

Abstract

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.

Keywords: assortative mating; genetics; humans.

Figure 1.

An ultimate-proximate causal framework for PAM. Primary (i.e. mate choice, stratification, and convergence) and subordinate proximate mechanisms are shown in orange boxes, with causal pathways indicated by black arrows. Adaptive ultimate processes (i.e. those relating to the fitness effects of PAM itself) are shown in light blue boxes, while non-adaptive ultimate processes (i.e. those relating to fitness effects unrelated to PAM) are shown in dark blue boxes. Selective causal pathways linking fitness effects to proximate mechanisms are also indicated by black arrows. This figure is not comprehensive, and some processes and causal pathways are excluded because they fall outside the scope of this review. For example, cultural phenomena may influence many proximate mechanisms, but they are displayed here only in the context of stratification. Cultural behaviour may also be under selection, which is not considered here.

Figure 2.

How proximate mechanisms drive PAM. The left-hand boxes show geographically separated ancestral populations (blue and red) containing economically differentiated subpopulations (small and large circles). Blues (mainly low-economic status individuals) migrate, forming a stratified admixed population, where subpopulations are differentiated by both ancestry and economic status. Below, mating takes place, mainly within (sub)populations. PAM occurs by economic status and, in the admixed population, this covaries with genetic ancestry. Where subpopulations overlap, ancestrally and economically mismatched pairs form. Generations later, admixed individuals appear. In the middle boxes, individuals choose similar or high-value mates. In the former case, phenotypically similar individuals (squares or triangles) form pairs over time. In the latter case, high-value (1) individuals pick each other, leaving low-value (2) individuals unmated. Later, low-value individuals mate in the absence of other options. In the right-hand boxes, initial mating results in both matched and mismatched pairs. Over time, the mismatched pairs converge, increasing levels of PAM.