Maternal psychology relates to offspring fitness in many ways, including infant morbidity and mortality. The advancing field of development origins of health and disease (DOHaD) proposes a connection between maternal psychological distress, fetal development, and adverse infant outcomes creating an intergenerational connection linking these phenotypes. Under a DOHaD framework, factors that positively affect prenatal mental health should increase both maternal and offspring fitness. Evolutionary theory suggests that allomothers (i.e., helpers who are not the offspring's mother) positively influence the survival of the child. No work to date has investigated prenatal allomother influence on maternal psychology. Thus, this represents a new approach for integrating DOHaD with evolutionary theory.

Growing evidence suggests that maternal mental health has consequences for offspring phenotypes at birth and later in life. Pregnancy is a period of vulnerability for the onset of maternal affective disorders, particularly depression and anxiety (Fishell, 2010). Prenatal stress, anxiety, and depression have been tied to low birth weight and preterm birth (Dunkel Schetter & Tanner, 2012; Grigoriadis et al., 2013; Grigoriadis et al., 2018; Grote et al., 2010; March of Dimes, 2015), both known causes of infant morbidity and mortality (Callaghan, MacDorman, Rasmussen, Qin, & Lackritz, 2006; Eshete, Alemu, & Zerfu, 2019). Prenatal exposures have also been linked to outcomes beyond birth, including developmental deficits and life-long chronic disease risk (Glynn et al., 2018; Glynn & Sandman, 2011; Kinsella & Monk, 2009; Leis, Heron, Stuart, & Mendelson, 2014). In a large-scale study of thousands of parent-offspring dyads, both prenatal and postnatal depression were each independently associated with offspring depression risk at age 18 (Pearson et al., 2013). Depression and other mood disorders have been tied to lower fertility of women, both in behaviors resulting in lower fertility rates and in reduced success of fertility treatment (Williams, Marsh, & Rasgon, 2007). Therefore, prenatal psychology can affect maternal fitness through fecundity/fertility, offspring morbidity and mortality at birth, and offsprings' lifelong health.

The costs of reproduction are offset by a flexible roster of allomothers across human societies, including by a child's father, aunts, grandparents, older siblings, as well as non-kin who have all been shown to benefit maternal reproductive success and child survival in some context (Crittenden & Marlowe, 2008; Kramer, 2005, Kramer, 2010; Meehan, Helfrecht, & Quinlan, 2014). Despite this flexibility being key for human reproductive success, researchers have noted that allomaternal help from grandmothers is especially consistent and high-quality (Chapman, Pettay, Lahdenperä, & Lummaa, 2018; Emmott & Mace, 2015; Hrdy, 2005; Leonetti, Nath, Heman, & Niell, 2005; Scelza, 2009; Scelza & Hinde, 2019; Sear, 2018; Sear & Mace, 2008). Many evolutionary anthropologists have focused on grandmothers because of their genetic closeness to their grandchildren (r = 0.25). Additionally, the obligate cessation of fertility of older women (compared to the facultative fertility of older men) reduces competition with daughters (in-law) (Cant & Johnstone, 2008). The longstanding debate on the evolutionary origins of menopause (Hawkes, O'Connell, Jones, Alvarez, & Charnov, 1998; Peccei, 2005) is beyond our scope here. Grandmothers are valuable allomothers because they often have more reproductive experience and expertise than other kin categories, such as older siblings. After much focus on grandmothers being critical to the weaning period, evolutionary scholarship has expanded to show grandmothers as critical throughout a woman's reproductive career including at birth (Rosenberg & Trevathan, 2002) and during breastfeeding (Myers, Page, & Emmott, 2021; Scelza & Hinde, 2019). The prenatal period is just as vulnerable, yet underexplored, perhaps due to theoretical focus on the fetus rather than the mother. Pregnancy brings about unique challenges for the mother, such as psychological and physiological transitions, new social roles, and among market-integrated societies: new forms of discrimination, financial strains, and medical systems to navigate. The connections that DOHaD highlights between mother and offspring during pregnancy implies motivations for grandmaternal influence that are not obvious and should be explored. We refer to these motivations as ‘adaptive interests’ and these influences on fitness outcomes, including decreasing mortality risk and increasing mate-quality at adulthood, as simply ‘benefits to the developing offspring’.

In this study, we explicitly measure the association of grandmaternal relationship characteristics with maternal prenatal mental health to gauge whether there exists an adaptive interest in maintaining maternal well-being during gestation for both allomothers. The literature suggests maternal grandmothers (MGMs) are associated with decreased grandchild mortality more consistently than paternal grandmothers (PGM) (Coall & Hertwig, 2010; Sear & Mace, 2008; Strassmann & Garrard, 2011). Based on this, we predict MGMs will positively influence maternal prenatal mental health, as there are direct and inclusive fitness benefits to both pregnant daughter and fetus (i.e., grandoffspring). The story is less clear for PGMs. Benefits to maternal prenatal psychology may improve sons' and grandoffspring fitness, but potentially at cost to the PGM's own daughters when finite resources are transmitted to sons' families. Thus, we are agnostic about PGMs' influence on maternal prenatal mental health as there are social and evolutionary reasons why it may be neutral or even negative despite the adaptive advantage of help to the grandoffspring during gestation.

Each allomother relationship characteristic (social support, geographic proximity, communication) captures a unique aspect of the grandmother-mother relationship that may impact maternal mental health. Social support is the care provided by or potentially available from a known individual; this support can be emotional (e.g., listening), informational (e.g., guidance), or instrumental (e.g., provisioning) (Dunkel Schetter & Brooks, 2009). While social support is often studied in psychology, anthropologists describe such acts of care with different terminology, such as allomaternal care and often through a lens of cooperative breeding (e.g., Emmott, Myers, & Page, 2021). Previous research has shown that instrumental and emotional support can impact perinatal outcomes differently (Bedaso, Adams, Peng, & Sibbritt, 2021; Emmott & Mace, 2015). Emotional support buffers against negative psychological and physiological states and improves psychological resilience in both pregnant and non-pregnant cohorts (Bedaso et al., 2021; Reblin & Uchino, 2008; Seguin, Potvin, Stdenis, & Loiselle, 1995; Suls & Wallston, 2003). Instrumental support has been shown to buffer stress during pregnancy (Collins, Dunkel-Schetter, Lobel, & Scrimshaw, 1993) and improve well-being only if emotional support is also present (Morelli, Lee, Arnn, & Zaki, 2015). Other studies have tied greater levels of social support to a range of positive birth outcomes in the U.S. (Collins et al., 1993; Elsenbruch et al., 2007; Feldman, Dunkel-Schetter, Sandman, & Wadhwa, 2000) and to reduced infant mortality in Mexico (Kana'Iaupuni, Donato, Thompson-Colón, & Stainback, 2005) likely through moderation of HPA-axis. These studies on birth outcomes often use psychometric scales of perceived social support that were designed to capture a combination of instrumental and emotional support. While instrumental support is often the metric used in allomother research, emotional support may serve a more critical role to buffer stress during pregnancy.

Ethnographic studies on the relationship between allomaternal support and offspring fitness often do not measure any kind of social support directly, but rather use survival or geographic proximity as a proxy for grandmaternal involvement (Callaghan et al., 2006; Eshete et al., 2019). Greater geographic proximity between grandmother-mother-offspring is usually associated with increased offspring survival (Chapman et al., 2018; Engelhardt, Bergeron, Gagnon, Dillon, & Pelletier, 2019). However, these connections are often studied in historical populations where geographic proximity was a pre-requisite for all forms of social support. Today, individuals are able to offer financial, informational, and emotional support from great distances. Geographic proximity may still be a good proxy variable for instrumental support and an important variable to include for its extensive use in the anthropological literature. By analyzing other variables in tandem with geographic proximity, we gain a broader picture of grandmother-mother relationships.

Communication is a critical way for emotional and informational support to be delivered. In a study of immigrant Mexican women aged 35–50, higher levels of communication with family were related to increased perceived emotional support (Vega, Kolody, Valle, & Weir, 1991). However, not all communication is good and an excessively involved caregiver can be burdensome. For example, one study found grandmother-mother verbal conflict to be independently associated with negative maternal parenting and child behavioral problems (Barnett, Mills-Koonce, Gustafsson, Cox, & Investigators, 2012). Here, we explore the independent contributions of communication, which allows us to consider the positive and negative ways allomothers may influence maternal psychology.

This project focuses on Latina women living in Southern California. ‘Latino/a’ is an ethnic category describing people with heritage from Latin America; the broad term thus includes extensive cultural diversity. We explore grandmother-mother relationships among Latinas in the U.S. because of certain trends associated with family life. Latinos in the U.S. have the highest rates of three-generation homes (i.e., grandparents and grandchildren who live together) compared to other ethnic groups (Cohn & Passel, 2018; PEW Research Center, 2010). Simultaneously, Latinos have the highest rates of cross-border families in the U.S. due to being the largest immigrating minority (PEW Research Center, 2020). High rates of both shared homes and cross-border families creates the opportunity to differentiate between the effects of geographic proximity and communication. The cross-cultural importance of family among Latino cultural groups (familismo) means that individuals often make family life a priority at greater rates than the broader U.S. population (Campos et al., 2008). Additionally, the cross-cultural values of machismo and marianismo among Latinos structure gender roles that cast women as primary caregivers (Nuñez et al., 2016), see Supplemental Materials (SM) for notes on these cultural values. Finally, working in a post-demographic transition society may make grandparental care more valuable, as Coall and Hertwig (2011) argue, a low-fertility, low-mortality context makes grandparental care even more valuable. Fewer offspring means fewer opportunities to pass on ones' genes, so the evolved sense to invest in each grandoffspring may be greater.