The contribution of mate-choice, couple convergence and confounding to assortative mating

Abstract

Increased phenotypic similarity between partners, termed assortative mating (AM), has been observed for many traits. However, it is currently unclear if these observations are due to mate choice for certain phenotypes, post-mating convergence, or a result of confounding factors such as shared environment or indirect assortment. To dissect these underlying phenomena, we applied Mendelian randomisation (MR) to 51,664 couples in the UK biobank to a panel of 118 phenotypes under AM. We found that 54% (64 of 118) of the tested traits had a causal relationship between partners, with female-to-male effects on average being larger. Forty traits, including systolic blood pressure, basal metabolic rate, weight and height, showed significantly larger phenotypic correlation than MR-estimates, suggesting the presence of confounders. Subsequent analyses revealed household income, overall health rating, education and tobacco smoking as major overall confounders, accounting for 29.8, 14.1, 11.6, and 4.78%, of cross-partner phenotypic correlations, respectively. We detected limited evidence for couple-correlation convergence (e.g. increased similarity with respect to smoking and medication use), measured by stratifying couples by their time spent together. Finally, mediation analysis revealed that the vast majority (>77%) of causal associations between one trait of an individual and a different trait of their partner is indirect. For example, the causal effect of the BMI of an individual on the overall health rating of their partner is entirely acting through the BMI of their partner. In summary, this study revealed many novel causal effects within couples, shedding light on the impact of confounding on couple phenotypic similarity.

The pitfalls of regression to the mean in bivariate timeseries analysis

“…For example, if individuals positioned on opposite sides of their respective means become in absolute terms, this may create the impression of deliberate trait convergence – that is, the movement of traits towards each other – in mated pairs or other social partners. This may be interpreted as an adaptive strategy to increase similarity, as it has been species ranging from mosquitos (Aldersley et al, 2016; Garcia Castillo et al, 2021) to humans (Anderson et al, 2003; Sjaarda & Kutalik, 2022).…”

“…Correlations between two timeseries – that is, sequences of observations measured across time – are commonly interpreted as reflecting stable relationships, where one variable causes corresponding shifts in the other (Bergmüller & Taborsky, 2010; Dingemanse & Araya-Ajoy, 2015). In mated pairs or social partners, synchronised trait changes are seen as efforts to increase social cohesion (Anderson et al, 2003; Bergmüller & Taborsky, 2010; Garcia Castillo et al, 2021; Gray & Ozer, 2019; Laubu et al, 2016; Sjaarda & Kutalik, 2022). In individuals, traits that change in apparent coordination are viewed as indicative of “integrated phenotypes” originating from coevolution (Nielsen & Papaj, 2022; Sheehy & Laskowski, 2023).…”

Correction for participation bias in the UK Biobank reveals non-negligible impact on genetic associations and downstream analyses

“…Fast-growing biobanks with rich phenotypic data are therefore curated, to maximize power for genome-wide discovery. To ensure validity of findings obtained from genome-wide studies, substantial efforts are made to eliminate potential sources of bias, such as population stratification, assortative mating, measurement error or indirect genetic effects [1][2][3][4] . A particularly challenging bias -and typically not considered in genetic studiescan occur when data is collected from individuals not representative of their target population 5,6 .…”