Internal state language (ISL) research contains knowledge gaps, including dimensionality and predictors of growth, addressed here in a two-aim study. Parent-reported expressive language from N = 6,373 monolingual, English-speaking toddlers (M[subscript age] = 23.5mos, 46% male, 57% white) was collected using cross-sectional and longitudinal data in WordBank. Exploratory and confirmatory factor analyses suggested a best-fitting one-factor model of ISL. The single-factor model of ISL was then submitted to hierarchical linear modeling to evaluate predictors of ISL development. Age 2 ISL production was predicted by child sex, wherein females outperform males, and maternal education, wherein higher education contributes to higher ISL. Only maternal education emerged as a significant predictor of ISL growth. These results provide support to theory suggesting a unitary construct of ISL, as opposed to considering ISL as categorical, and further illustrate linear growth through the second postnatal year that varies as a function of child sex and maternal education.