Data from: Rhesus macaques use probabilities to predict future events

Humans can use an intuitive sense of statistics to make predictions about
uncertain future events, a cognitive skill that underpins logical and
mathematical reasoning. Recent research shows that some of these abilities
for statistical inferences can emerge in preverbal infants and non-human
primates such as apes and capuchins. An important question is therefore
whether animals share the full complement of intuitive reasoning abilities
demonstrated by humans, as well as what evolutionary contexts promote the
emergence of such skills. Here, we examined whether free-ranging rhesus
macaques (Macaca mulatta) can use probability information to infer the
most likely outcome of a random lottery, in the first test of whether
primates can make such inferences in the absence of direct prior
experience. We developed a novel expectancy-violation looking time task,
adapted from prior studies of infants, in order to assess the
monkeys' expectations. In Study 1, we confirmed that monkeys (n = 20)
looked similarly at different sampled items if they had no prior knowledge
about the population they were drawn from. In Study 2, monkeys (n = 80)
saw a dynamic ‘lottery’ machine containing a mix of two types of fruit
outcomes, and then saw either the more common fruit (expected trial) or
the relatively rare fruit (unexpected trial) fall from the machine. We
found that monkeys looked longer when they witnessed the unexpected
outcome. In Study 3, we confirmed that this effect depended on the causal
relationship between the sample and the population, not visual mismatch:
monkeys (n = 80) looked equally at both outcomes if the experimenter
pulled the sampled item from her pocket. These results reveal that rhesus
monkeys spontaneously use information about probability to reason about
likely outcomes, and show how comparative studies of nonhumans can
disentangle the evolutionary history of logical reasoning capacities.