Causal learning is a fundamental ability that enables human reasoners to learn about the complex interactions in the world around them. The available evidence with children and adults, however, suggests that the mechanism or set of mechanisms that underpins causal perception and causal reasoning are not well understood; that is, it is unclear whether causal perception and causal reasoning are underpinned by a Bayesian mechanism, associative mechanism, or both. It has been suggested that a Bayesian mechanism, rather than an associative mechanism, underpins causal reasoning because such a mechanism can better explain the putative backward- blocking finding in children and adults (e.g., Sobel, Tenenbaum, & Gopnik, 2004). In this paper, we report two experiments to examine to what extent infants and adults exhibit backward blocking and whether humans’ ability to reason about causal events is underpinned by an associative mechanism, a Bayesian mechanism, or both.