"Probabilities of single events are a byproduct of modern statistical and data-gathering techniques (ever wonder what a "60% chance of rain today" means?). No hunter-gatherer ever encountered a probability of a single event (it either rains today or it doesn't). Instead, hunter-gatherers encountered statistical information in the same format that other species do: in the form of the actual frequencies of encountered events (e.g., It rained 6 out of the last 10 days; Not: there is a 60% chance of rain today). If so, then mechanisms designed to make well-calibrated statistical judgments might exist, but they might require information to be in a format that they can “read” to give the right output. To test this, we translated the same standard problems into frequency formats and asked for the answer as a frequency (e.g., how many people who test positive for the disease actually have it? ___ out of ___) rather than as the probability of a single event. Whereas only 12-36% of untutored undergraduates gave the correct Bayesian response when the problem was phrased in the single event format, 76-92% gave the right answer when the problem had a frequency format ..."