We followed the nondistressed vocalization dynamics of 30 mother–infant dyads observed in a naturalistic setting using multiple time points between 3 and 11 months to identify subtle relationships between age, sex and maternal behavior ending by 1 year of age with diverging trajectories of nondistressed vocalization. We observed no mean differences between boys and girls in frequency or duration of nondistressed vocalizations at any one time period. However, while these parameters were essentially static for boys, girls showed a quadratic developmental curve, declining in frequency and duration between 6 and 8 months and climbing above their early starting point by 9–11 months. Mothers of boys showed a linear decrease in the duration of their speech over the 9 months of our study. In contrast, mothers of girls showed quadratic patterns of ultimately increasing vocalization frequency and duration, over the months 3–11 of development. Finally, boys' and girls' vocalization contingent to maternal speech revealed no differences. Mothers of boys, however, did not change significantly over time, while mothers of girls showed an increase in contingent responsiveness from 3–5 months to 9–11 months and from 6–8 months to 9–11 months. A similar pattern was followed for object-related maternal vocal responses.