To examine effects of input and learner characteristics on morphology acquisition, 60 adult English speakers learned to inflect masculine and feminine Russian nouns in nominative, dative, and genitive cases. By varying training vocabulary size (i.e., type variability), holding constant the number of learning trials, we tested whether learners required a "critical mass" of vocabulary to generalize case marking patterns to new nouns. Cattell's Culture-Fair IQ Test mediated the effect of type variability on success in generalizing case marking to new vocabulary: only participants with above-median Culture-Fair Test scores showed the predicted critical mass effect of better generalization with larger training vocabulary. These results demonstrate how individual differences in central executive functioning and attention allocation capacity can affect adult second language learning.