Evolution Promotes Mothers Who Tend, So Daughters Can Befriend

Social support is crucial to our mental and physical health. The absence of social support may be even more harmful to us than smoking. This is because humans, like many other primates, are intensely social creatures. However, while we know that social support is important, we are still working on understanding how and why social relationships are so powerful that they can shape our biology. The tend-and-befriend hypothesis outlines one potential mechanism. It proposes that female primates reduce stress through tending offspring and socializing with female friends and that this benefit may underlie the evolution of close female friendships. Connecting with female friends may initiate hormonal cascades that increase oxytocin, a hormone associated with social bonding, and reducing cortisol, a key stress hormone. However, while this hypothesis is well–supported in women, most studies focus on female friendships, rather than tending offspring. This might be because, as many parents know, the long period of raising human children may cause more stress than it reduces. But what if maternal tending is part of the mechanism that leads to female befriending?

To examine this question, I worked with an interdisciplinary team, and we published our findings in a special joint issue of the American Journal of Primatology and Developmental Psychobiology focusing on Psychobiology Development in Primates. As part of a large project combining educational outreach with research, we worked with girls participating in a summer science camp. The project, coordinated by anthropologist Kate Clancy, psychologist Carla Hunter, and engineer Jenny Amos, focuses on the role of the social environment in shaping health in adolescent girls. With parental consent, girls could choose to participate in several aspects of the project, from filling out survey information, to constructing diagrams of their social networks, to participating in body measurements or contributing hormonal or epigenetic data via saliva or cheek swabs. Working with the survey and social network data, we examined how maternal relationships, paternal relationships, and close female friendships play a supportive role by examining how they vary with depressive symptoms.

We found that these factors are all interrelated. Girls with more than two close female friends have lower depressive symptoms than girls that only have one close friend or none at all. But girls who had more close female friends also had better relationships with both parents. To tease out how all of this fit together, we considered these factors in a unified model. We predicted that each type of relationship would play an important role in buffering depressive symptoms, but that maternal open communication would be strongly associated with female friendship. Our findings supported these predictions. Female friendships and paternal relationships had the largest association with reduced depressive symptoms. But most interestingly, girls that had high-quality communication with their mothers were significantly more likely to have two or more female friendships. This suggests that the largest role that mothers had on their daughter’s mental health was not just indirectly providing emotional support, but by providing their daughters with the skills they need to successfully make or maintain close friendships with female peers.

Our findings bridge the gap between evolutionary hypotheses regarding the function of social relationships and psychological models that focus on individual development. A major theory in developmental psychology is attachment theory, which focuses on how an infant’s attachment to caregivers, particularly mothers, shapes their relationships throughout life. These early relationships act as a model, forming children’s expectations of others and shaping how they interact with others. Our research expands on this concept, suggesting that one of the ways in which these early relationships continue to shape social skills through development is by modeling the communication skills necessary to establish and maintain friendships. We can further contextualize this within life history theory, which predicts that each developmental stage has consequences for lifespan health and reproductive success. Compared to most other primates, as well as other mammals, our species spends a very long time waiting to grow up. This long period of development is hypothesized to relate to our large brain size and our reliance on social and cultural mechanisms for success. Our research suggests that at adolescence, girls are applying the communication skills that they learned from their mothers to establish and maintain female friendships. Such supportive relationships are likely to become even more crucial through adulthood. Furthermore, our findings mirror research on non-human primates, which highlight the importance of maternal relationships on adult health and social competence, as well as the importance of developmental experiences and female social bonds across the lifespan.

However, findings must be considered within their cultural context. Our participants were academically oriented girls who were attending an American STEM-focused camp. Although this population was relatively racially diverse, it skewed toward higher socioeconomic statuses. This is a population of girls whose parents are likely investing heavily in their offspring, and we might expect more variation in the role of parents versus friends if we studied girls in other contexts. Furthermore, because most of our participants had caregiving relationships with both their mothers and fathers, we restricted our analyses to girls who had these primary caregiving relationships with both parents. This excluded a few participants that were cared for by grandparents, or who reported caregiving relationships with a parent and a step-parent or had same-gender parents. Understanding how relationship dynamics vary in these families is important, but in order to sufficiently investigate that variation, we would need larger numbers of participants with each type of family organization. Finally, research on some of these dynamics may be limited by heteronormative frameworks. Although much research assumes that there may be fundamentally different relationships between mother-daughter and father-daughter pairs, there is likely a great range of variation, and caregivers may able to flexibly vary in their roles. Furthermore, our focus on a gendered dimension of friendship may obscure variations in gender identity or sexuality. Hopefully, future research may be able to further investigate such variations.

Nonetheless, our findings may provide solace for parents who may wonder if they still matter in their adolescent daughter’s lives. Our research suggests that they do, and the communication skills learn from their mothers may provide girls with the tools they need to build their own female support networks throughout their lives.