How do people make rich inferences from such sparse data? Recent research has explored this inferential ability by investigating probabilistic reasoning in infancy. For example, 8- and 11-month-old infants can make inferences from samples to populations and vice versa (Denison & Xu, 2010a; Xu & Denison, 2009; Xu & Garcia, 2008a). The current experiment investigates the developmental origins of this probabilistic inference mechanism with 4.5- and 6-month-old infants. Infants were shown 2 large boxes, 1 containing a ratio of 4 pink to 1 yellow balls, the other containing the opposite ratio. The experimenter sampled from, for example, the mostly pink box and removed a sample of either 4 pink and 1 yellow balls or 4 yellow and 1 pink balls on alternating trials. Six-month-olds but not 4.5-month-olds looked longer at the 4 yellow and 1 pink sample (the improbable outcome) than at the 4 pink and 1 yellow sample (the probable outcome). (Contains 1 table, 2 figures and 5 footnotes.)