Infants do not learn words at a constant rate. During the second year of life, a dramatic increase in the speed of word learning is observed. Different mechanisms explaining this vocabulary spurt have been proposed, either through endogenous factors such as learning capacity or exogenous factors, such as frequency of word usage. We demonstrate that occurrence statistics alone is not sufficient to explain the acceleration in vocabulary growth, discuss other potential exogenous contributions such as phonological complexity and suggest that a change in word learning capacities is necessary. A model implementing an increased ease of learning is introduced and illustrates this endogenous approach by replicating the non-linear vocabulary growth characteristics of language acquisition.