Confirmation for Deterministic Hypotheses 1 Running head: CONFIRMATION FOR DETERMINISTIC HYPOTHESES Seeking Confirmation is Rational for Deterministic Hypotheses

The tendency to test outcomes that are predicted by our current theory (the confirmation bias) is one of the best-known biases of human decision making. We prove that the confirmation bias is an optimal strategy for testing hypotheses when those hypotheses are deterministic, each making a single prediction about the next event in a sequence. Our proof applies for two normative standards commonly used for evaluating hypothesis testing: maximizing expected information gain and maximizing the probability of falsifying the current hypothesis. This analysis rests on two assumptions: (1) that people predict the next event in a sequence in a way that is consistent with Bayesian inference and (2) when testing hypotheses, people test the hypothesis that they assign highest posterior probability. We present four behavioral experiments that support these assumptions, showing that a simple Bayesian model can capture people’s predictions about numerical sequences (Experiments 1 and 2), and that we can alter the hypotheses that people choose to test by manipulating the prior probability of those hypotheses (Experiments 3 and 4). Confirmation for Deterministic Hypotheses 3 Seeking Confirmation is Rational for Deterministic Hypotheses