Lisa S. McAllister, Ph.D.

Since 2005, facilitated by the Tsimane Health and Life History Project, I have worked with a population of South American forager-farmers, the Tsimane of Bolivia. This research has two foci: (1) how changing environments affect women’s and men’s fertility preferences and fertility, and consequently women’s reproductive health and autonomy; and (2) how adolescent mothers’ resource allocation — among the competing demands of growth, reproduction and somatic maintenance — affect their and their children’s health, growth and development.

Adolescent Pregnancy and Maternal and Infant Growth and Development

My first project with the Tsimane investigates the effects of early age at first birth and high fertility on maternal growth, health and cognitive development. Women have finite resources. Adolescence is a period of continued growth, and social and cognitive development. Reproducing during adolescence reduces women’s investment in their own growth and somatic maintenance but not completely, such that during gestation and lactation their first-born child receives fewer resources than the first-born children of women who delay their reproduction.

This work explores growth patterns in Tsimane women through adolescence, testing for trade-offs in energetic investment between growth and pregnancy. My findings, to date, support previous research suggesting that early age at first birth stunts maternal growth. However, my findings further suggest that: (1) women who give birth at age 15 or younger continue their growth post-pregnancy (although they are shorter as adults than average); (2) women on faster growth trajectories, do not cease growth earlier and are often taller than average as adults, if their growth is uninterrupted by pregnancy/motherhood; (3) the first-born sons, but not daughters, of mothers aged 15 and younger suffer long-term growth deficits. My future work in this area will investigate the long-term consequences of adolescent reproduction to the growth and health of first born and later born children; and the relationship between age at first birth and maternal and child cognitive development.

Women’s and Men’s Fertility Preference in a Changing Environment

My second project with the Tsimane explores why fertility among the Tsimane remains high (TFR = 9.1) despite small preferred family sizes (IFS = 4 to 5 children), and access to contraceptives and family planning programs. This high fertility, coupled with increasing infant and child survival rates, indicates that the Tsimane population will double in size within 24 years. As the Tsimane are marginalized, face discrimination and have limited land rights, this population growth and the associated environmental degradation are unsustainable. Furthermore, high fertility is associated with negative maternal and child health outcomes and low female autonomy. It is crucial that we better understand the dissonance between these people’s small desired family sizes and fertility, particularly as existing family planning programs and improved access to contraceptives are not having the expected affects.

My research suggests that excess fertility increases with proximity to market towns that provide resource stability and healthcare. Tsimane women living near these urban centers have better maternal condition and likely greater fecundity, which is not countered by increased use of contraceptives. Furthermore, they have low returns on investments in human capital and low socioeconomic mobility: Tsimane women are stuck in their homelands where adult status and success are measured in terms of reproductive status and family size, and sociopolitical power and economic security hinge on large kin networks. Under these conditions, despite a desire for smaller family sizes, women are trapped in a reproductive role, and motivation to delay or reduce reproduction is low. Consequently, adolescent reproduction is the norm for Tsimane women despite trade-offs between maternal and fetal/infant growth, development, cognition and health. However, the evolutionary, sociopolitical and long-term economic advantages (e.g. higher fertility) of an early reproductive start may outweigh these costs. My future work in this area will continue to unravel the interplay between socioeconomic development and evolved life history strategies at the onset of demographic transition.