Evolutionary and economic theories of fertility variation argue that novel subsistence opportunities associated with market economies shape reproduction in ways that both increase parental investment per child and lower overall fertility. I use demographic and ethnographic data from Guatemala as a case study to illustrate how ethnic inequalities in accessing market opportunities have shaped demographic variation and the perceptions of parental investments. I then discuss two projects that use secondary data sets to address issues of conceptualizing and operationalizing market opportunities in national and cross-population comparative work. The first argues that social relationships are critical means of accessing market opportunities, and uses Guatemala household stocks of certain forms of relational wealth are associated with greater parental investments in education. The second focuses on a methodological issue in how common measures of wealth in comparative demographic studies conflate economic capacity with market opportunities, and how this conceptual confusion biases our interpretations of the observed links between wealth and fertility over the course of the demographic transition.