Learning, especially in the case of language acquisition, is not an isolated process; there is ever-present competition between words and objects in the world. Such competition is known to play a critical role in learning. Namely, the amount and variability of competing items during word learning have been shown to change learning trajectories in young children learning new words. However, very little work has examined the interaction of competition amount, competition variability, and task demands in adults. The current study assesses adults’ ability to map new word-referent pairs in varying amounts of competition and competitor variability. In addition, the effect of mapping context on retention was assessed. Results suggest that retention is weak in some cases and importantly, there are cascading effects of competitor variability in mapping on later retention of new words. Results are discussed in light of associative learning mechanisms and the implications of competition for learning.