The origin of the possible: 12-month-olds' understanding of certain, likely, and unlikely events

To predict and prepare for near-future outcomes, infants must respond to the variability in their probability. Adults achieve that with modal concepts that quantify over multiple possibilities, but whether and how infants can do the same is unclear. In two preregistered habituation experiments, we asked whether infants can distinguish outcomes based on physical probability level (100% vs. 66% in Experiment 1. 66% vs. 33% in Experiment 2). 12-month-olds were habituated to events with 66% probability, and their proportion of looking at 100% (Exp 1., N=35) and 33% (Exp 2., N=24) events were measured before (i.e., baseline) and after habituation (i.e., test). We found that infants' proportion of looking at events with 33% probability (in Exp 2), but not at events with 100% probability (in Exp 1), increased from baseline to test. Thus, 12-month-olds distinguish likely events from unlikely ones but not from necessary events.