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Imagine pouring a box of granola into a bowl. Are you considering hundreds of individual chunks or the motion of the group as a whole? Human perceptual limits suggest we cannot be representing the individuals, implying we simulate ensembles of objects. If true, we would need to represent group physical properties beyond individual aggregates, similar to perceiving ensemble properties like color, size, or facial expression. Here we investigate whether people do hold ensemble representations of mass, using tasks in which participants watch a video of a single marble or set of marbles falling onto an elastic cloth and judge the individual or average mass. We find first that people better judge average masses than individual masses, then find evidence that the better ensemble judgments are not just due to aggregating information from individual marbles. Together, this supports the concept of ensemble perception in intuitive physics, extending our understanding of how people represent and simulate sets of objects. 

Adults can calculate probabilities by running simulations and calculating proportions of each outcome. How does this ability develop? We developed a method that lets us bring computational modeling to bear on this question. A study of 40 adults and 31 4-year-olds indicates that unlike adults, many 4-year-olds use a single simulation to estimate probability distributions over simulated possibilities. We also implemented the 3-cups task, an established test of children’s sensitivity to possibilities, in a novel format. We replicate existing 3-cups results. Moreover, children who our model categorized as running a single simulation on our novel task show a signature of running a single simulation in the 3-cups task. This signature is not observed in children who were categorized as running multiple simulations. This validates our model and adds to the evidence that about half of 4-year-olds don’t evaluate multiple candidates for reality in parallel.