Family planning is the usual modern route to producing a small family. Can human behavioral ecology provide a framework for understanding family planning behavior? Hillard S. Kaplan (Yearb. Phys. Anthropol. 39:91–135) has proposed a general theory of human parental investment based on the importance of skills development in children. As modern, skills-based, competitive market economies are established, parental investment strategies would be predicted to become oriented toward producing increasingly competitive offspring in a pattern of coordinated investment in their embodied capital—in (...) other words, skills training along with good health to ensure their long-term productivity. Parental embodied capital and resources are also expected to be associated with motivation to produce competitive offspring. The basic parental investment trade-off between quality and quantity should predict greater investment in fewer children and the adoption of family planning behavior. Data on family planning in two ethnic groups in Northeast India (Khasi and Bengali) currently experiencing early-phase transition into modern market economies from very different social and ecological baselines are examined within this analytical framework. The results show a mixture of strategies in conjunction with family planning that involve decreased as well as increased investment in the embodied capital of children among Bengali and a divergence of investments in education and health among Khasi. These mixtures of strategies provide some insight into the motivations to use family planning in the face of economic transition, given differing local cultural and ecological conditions and the opportunity structures they provide. (shrink)