A recent study published in the journal Evolutionary Psychological Science examined cooperative and conflictual aspects of affinal relationships (i.e., relationships with in-laws). The results revealed that both men and women report more conflict with their mothers-in-law, than mothers, and that mothers report more conflict with their daughters-in-law, than daughters.
Forming lasting relationships with the kin of long-term mates facilitates bonds and investment in offspring by both sexes. Despite the numerous benefits of affinal relationships, conflict is also a central feature. Hamilton’s rule helps explain why biological kin are invested in each other’s survival, which can be observed among animals as well. For example, older female squirrels sometimes signal the presence of predators, revealing their location and sacrificing their own lives, in order to warn their biological kin. Despite mothers and fathers being unrelated in most societies, they cooperate and have a shared stake in the survival of their children, which creates interdependence between two families. Given affines (i.e., relatives by marriage) have shared genes with only one parent, their interests will typically align with that parent.
Jessica D. Ayers and colleagues write, “This leads affines to experience similar kinds of conflict as that of unrelated parents and may motivate behaviors that favor the interest of their genetic kin even at a cost to their affinal kin. Affinal conflict, therefore, can be thought of as an extension of the genetic conflict between mothers and fathers.”
Given the shorter interbirth interval human females have relative to nonhuman apes, women face the challenge of caring for multiple offspring at the same time, and benefit from having positive affinal relationships. Ancestral women may have relied on the help of biological kin for childcare; however, this would not be possible if genetic relatives were not geographically close. This would have necessitated the support of affinal relationships in childcare. And because affines have a genetic stake in descendants, this would incentivize them to provide the same quality of childcare the mother’s genetic kin would.
While female affinal cooperation is centred around kin care and the maintenance of social bonds, male affinal cooperation is concerned with gaining and maintaining social relationships, mate acquisition, self- and other-protection, and the investment of resources in children.
Mothers and daughters-in-law tend to have discrepancies in fitness interests which can result in conflict. “For example, a mother-in-law’s son/daughter-in-law’s partner’s resources are finite, so daughters-in-law favor her husband allocating resources to her and her family, while the mother-in-law favors her son allocating resources to his other family members.”
Conflict tends to exist in the same domains as cooperation, including childcare, material resources for kin, and spending time with children. Conflict in these domains could result in the acquisition of other social relationships, which may become a subsequent source of conflict. For example, “conflict over childcare may lower the quality of mother- and daughter-in-law relationships, causing the daughter-in-law to seek additional social relationships to help with child care—which may worsen her relationship with her mother-in-law.”
Mate acquisition and retention, and self-protection could also be sources of conflict in affinal relationships, given these interfere with mother- and daughter-in-law fitness interests.
In this study, Ayers and colleagues examined cooperation and conflict among in-laws. A total of 308 participants were recruited from online platforms. Participants responded to demographic questions (e.g., age, sex, education, marital status), and provided the names/initials of their affinal and genetic kin which would appear later on in the study. Participants answered numerous questions about these relationships, such as the length, and the overall cooperation and conflict levels across various domains (e.g., resources, social relationships, safety). The researchers derived a ‘ratio of conflict in interactions’ metric to assess whether these relationships were mostly characterized by conflict or cooperation.
The results suggest differences in perceptions of conflict with genetic and affinal kin. Both men and women reported more conflict with their mothers-in-law than mothers, and mothers reported more conflict with daughters-in-law than daughters.
As well, fathers indicated more conflict with daughters than daughters-in-law. It could be that these conflicts centre around mate choice and the addition of a new affinal relationship in a father’s coalition network. If a father believes his affinal son would strengthen the coalition, then it would be expected that this conflict would lessen over time.
The domains in which participants reported the most conflict included material resources and kin care, which are both critical for long-term reproductive success.
One limitation to the current work is that participants had to have relationships with living genetic and affinal mothers to be included in the analyses. This would have excluded participants who were involved in high conflict relationships that ended, thereby, introducing bias in the results in favour of cooperation.
The study, “Mother‑in‑Law Daughter‑in‑Law Conflict: an Evolutionary Perspective and Report of Empirical Data from the USA”, was authored by Jessica D. Ayers, Jaimie Arona Krems, Nicole Hess, and Athena Aktipis.