A set of tri-zygotic quadruplets, three girls and one boy, participated in weekly observations from 1;2 to 1;10 (years;months), a period of transition from prelinguistic gesture to 50 words. In the study, one girl served as a genetic mate to her identical twin and a biological risk mate to her fraternal sister. The biological risk mates achieved milestones in lexical development at similar times; however, the genetic mates demonstrated more similarities in pattern of lexical development and in the modality of their word productions. Degree of similarity changed over the observation period. Imposed upon the natural experiment was a within-subject manipulation of the social environment: The experimenters modelled a core vocabulary via the gesture+verbal modalities to the children during each visit. The modelling resulted in increased rates of word learning for three of the children; the child with the greatest biological risk, the boy, derived the greatest benefit. The findings provide unique support for a dynamic, multi-factorial model of lexical development involving the interaction of genetics, the biological environment and the social environment. Furthermore, they illustrate the robustness of early lexical development in the face of biological risks.