Intuitions of probabilities shape expectations about the future at 12 months and beyond

Abstract

Rational agents should integrate probabilities in their predictions about uncertain future events. However, whether humans can do this, and if so, how this ability originates, are controversial issues. Here, we show that 12-month-olds have rational expectations about the future based on estimations of event possibilities, without the need of sampling past experiences. We also show that such natural expectations influence preschoolers' reaction times, while frequencies modify motor responses, but not overt judgments, only after 4 years of age. Our results suggest that at the onset of human decision processes the mind contains an intuition of elementary probability that cannot be reduced to the encountered frequency of events or elementary heuristics.

Fig. 1.

Experiment 1: infants' looking time to improbable/probable outcomes never experienced before. (a) Three identical and one different object bounced in a container, simulating a lottery. After an occlusion period, one of the objects exited, presenting a probable outcome (b) in which one of the three identical objects exited, or an improbable outcome (d), in which the unique different object exited. Afterward, the occluder faded out and infants could see all of the objects. (c) Mean looking time (SEM) during the outcome phase.

Fig. 2.

Experiment 2: infants' looking time at impossible/possible outcomes closely mirroring the probable/improbable outcomes of experiment 1. (a–c) By interposing a bar between the three identical objects and the single different object (a), the movies were transformed (b) so that the probable outcome of experiment 1 became impossible and the improbable outcome became possible (c). (d) Mean looking time (SEM) during the outcome phase.

Fig. 3.

Three- and 5-year-olds' RTs for improbable/probable events before experiencing outcome frequencies. (a) The structure of the experiment. After bouncing inside the frame, a ball exits either from the three-hole side or the 1-hole side, generating either a probable or an improbable outcome. (b) The mean RTs (SEM) in the first four trials of experiments 3 and 4 (probability condition) and in the corresponding four trials of experiment 5 (control condition).

Fig. 4.

Five-year-olds' RTs and judgments to improbable/probable events, before and after exposure to experienced frequencies. Improbable events were presented either frequently (a) or infrequently (b) between participants. The ordinate reports RTs to the appearance of the ball in milliseconds, and the abscise reports the 24 trials infants experienced, grouped in six blocks of four, separately reporting appearance from the three-hole or one-hole sides. Children were asked a probability question (PQ) before seeing the 24 movies and a frequency question (FQ) afterward. Pie charts present the distributions of the answers to the probability and frequency questions, color-coded as in the RT charts.