Why do couples show such a strong similarity in their pair-bonding behavior? A study recently published in Behavior Genetics examined, and was able to distinguish, between phenotypic selection and social homogamy.

Phenotypic selection occurs when individuals actively select partners with similar characteristics. However, phenotypic preference is often confounded with social homogamy, whereby couples’ similar pair-bonding behaviors are an artifact of their shared social backgrounds.

Using a twin-partner design, the study’s authors, Briana Horwitz and colleagues—including the Office of Behavioral and Social Sciences Research's Erica L. Spotts—explored the contributions of both phenotypic assortment and social homogamy to mate similarity.

Pair-Bonding Behavior

Pair-bonding behavior refers to partner-specific attachment, trust, closeness, and parental investment, which tend to be similar among couples; therefore, individuals and their partners tend to form and maintain partner bonds of similar strength.

The benefits to successful pair-bonding, including better health, survival, and marital satisfaction, highlight the importance of better understanding the mechanisms that underlie partner similarity for pair-bonding.

While studies have concluded that phenotypic selection (partnering based on similar heritable characteristics) accounts for partners’ similar attachment representations, it does not generalize to pair-bonding behavior more broadly.

We have learned, through a genetically-informative lens, that heritable factors are at least partly responsible for shaping individual variance in pair-bonding behavior. Studies with twins have shown that genetic and nonshared environmental factors also shape phenotypes that are correlated with pair-bonding behavior, including risk of divorce and marital quality; therefore, social homogamy could also explain similarity in pair-bonding behavior.

Taken together, this work indicates that pair-bonding behavior and related phenotypes partly reflect genetically based characteristics, but the extent to which individuals’ genetically based characteristics influence their selection of mates with similar pair-bonding behavior is unclear.

Phenotypic Assortment, Social Homogamy and the Twin-Partner Design

The study by Horwitz and colleagues looks at couple similarity for pair-bonding in a sample of Swedish twins and their partners from the Twin and Offspring Study in Sweden. Biometric modeling was used to discriminate between the effects of phenotypic assortment and social homogamy using the twin-partner design.

Findings of phenotypic assortment would indicate that individuals directly select partners with similar genetic predispositions towards pair-bonding. Phenotypic assortment might also suggest the role of active genotype-environment correlation (rGE), or individuals selecting environments correlated with their heritable characteristics.

Social homogamy, on the other hand, is an environmental explanation for couple similarity in pair-bonding behaviors. According to this theory, individuals select partners with similar pair-bonding behavior because of shared social background factors, such as shared socioeconomic status, geographical region, or educational background.

The twin-partner design is a useful approach for examining the factors that account for partners’ correlated traits. The twin-partner method is typically used to assess sources of influence on couple similarity for traits related to pair-bonding such as cognitive ability and alcohol, tobacco, and caffeine use. This study is the first to use a twin-partner method to understand couple similarity for pair-bonding behavior itself.

To be included in the study, twins and their partners had to be in married (82 percent) or cohabitating relationships (18 percent) for at least 5 years. The average length of marriage/cohabitation was 19.4 years (SD = 6 years).

According to the study results, only phenotypic assortment accounted for couple similarity in pair-bonding behavior, and the findings applied to both genders. The authors stated:

“This suggests that couple similarity for pair-bonding behavior is a consequence of individuals actively selecting mates with similar genetically based characteristics, rather than the couples’ shared social background factors. One possibility is that individuals select partners with similar temperaments reflecting the need for affiliation or approval, or for social problems, resulting in couples’ similar pair-bonding behavior. Another possibility is that these genetically influenced tendencies may cause individuals to choose particular social niches from which they select partners with similar pair-bonding behavior, via active rGE.”

These findings do not undermine the impact of early attachment experiences in mate selection, but suggest that it is influenced by both social and genetic factors. Genetic and non-shared environmental factors accounted for 34 percent and 66 percent of the variance in pair-bonding behavior, respectively. Thus, while pair-bonding behavior is partly a heritable characteristic, it is also shaped by twins’ distinct experiences not shared with their co-twins.

For details, read the article:

Understanding The Role of Mate Selection Processes in Couples’ Pair-Bonding Behavior

Citation

Horwitz, B. N., Reynolds, C. A., Walum, H., Ganiban, J., Spotts, E. L., Reiss, D., Lichtenstein, P., & Neiderhiser, J. M. Understanding The Role of Mate Selection Processes in Couples’ Pair-Bonding Behavior. Behavior Genetics. January 2016, Volume 46, Issue 1, pp 143-149. DOI 10.1007/s10519-015-9766-y

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