A sample of 99 children completed a causal learning task that wasan analogue of the food allergy paradigm used with adults. The cuecompetition effects of blocking and unovershadowing wereassessed under forward and backward presentation conditions.Children also answered questions probing their ability to makethe inference posited to be necessary for blocking by a reasoningaccount of cue competition. For the first time, children’s workingmemory and general verbal ability were also measured alongsidetheir causal learning. The magnitude of blocking and unovershadowingeffects increased with age. However, analyses showed thatthe best predictor of both blocking and unovershadowing effectswas children’s performance on the reasoning questions. The magnitudeof the blocking effect was also predicted by children’s workingmemory abilities. These findings provide new evidence that cuecompetition effects such as blocking are underpinned by effortfulreasoning processes.