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My research draws on evolutionary and ecological perspectives and has an empirical focus in traditional forager and agriculturist populations.  The question that unifies my research is how did there get to be so many of us?

Population growth has profound effects on people’s lives today.  It also is one of the remarkable stories of our evolutionary history.  The human demographic advantage can be attributed to a number of derived traits including short birth intervals, high rates of child survival, investment in juveniles, the help mothers receive from others, and the extensive food sharing and labor cooperation that characterizes human subsistence. Studying living traditional populations allows me to observe these aspects of human behavior and demography not otherwise apparent in the fossil and archaeological record.  Longitudinal study among people pursuing different lifeways, allows me to investigate variation in human behavior and how changing socioeconomic conditions affect reproductive, developmental and demographic outcomes.


Human life history and cooperative breeding

How do human mothers manage fast reproduction and raising families of multiple dependents, while our closest relatives reproduce slowly and support only one offspring at a time?  This difference is significant because it gives humans a decisive fitness advantage over closely related species.  How then do mother meet the demands of multiple dependents?  While assisting juveniles is an unusual primate trait,  juveniles also exchange resources and labor with their mothers, siblings and others, something no other primate juvenile routinely does.  One of my primary research objectives has been to integrate these two distinctly human traits into a cohesive thesis about their evolutionary implications. 

This has raised questions about how costly are human children when we consider intergenerational transfers?   How might we model these in the evolutionary past? How do we resolve that humans both reproduce fast and grow slowly within a life history paradigm? Does the long period of human juvenile dependency present an opportunity for children as helpers?  

Maya mothers, like Pumé mothers, are strapped for enough hours in the day to meet the competing demands of providing childcare to young children and  food and other resources to older children. While the Maya are agriculturalists and the Pumé are foragers, both offer a window into how mothers in natural fertility populations cope with this fundamental tradeoff,   and the role that children play in resolving it.


Effects of economic transitions on demographic processes

Most of the world’s subsistence foragers, pastoralists and agriculturalists now have some interaction with the labor-market economy.  Researchers working with these traditional populations inevitably face considering the effects that market integration has on their study populations. The interaction between economic development and demographic processes has been extensively documented in populations that already are involved in the labor market and  cash economy.  Considerably less is known about demographic changes when permanent settlement, wage labor, vaccination programs, market foods, and modern technology are introduced for the first time. My research among the Maya, Pumé and Tanala shows that this early transitional period has far-reaching effects on fertility and mortality, and in surprising and unanticipated ways. 

Some of questions we are addressing include how do these changes affect female energetics and fertility? What aspects of economic development spur rather than constrain population growth? What effect does land availability, population growth and wealth stratification have on farming practices and deforestation?  How do these changes affect infant mortality, growth and development, age at first birth, health and nutritional status?

The Maya and Pumé are poised on the edge of economic changes.  These include the introduction of labor-saving technologies, changes in labor dynamics and energetics, access to market foods, increased fertility, decreased child mortality, rapid population growth, environmental degradation, and the development of economic stratification.