Psychologists have emphasized irrationality of human beings. Decision makers often judge the probability of an event according to a way different from the normative probability theory. Likewise human reasoning sometimes deviates from the prescriptions of logic. A number of fallacies, biases, and illusions in human cognition have been identified so far and a sketch of "irrational human" has been drawn by cognitive psychologists. My research tries to explain when and why human thinking and reasoning seems to deviate from the normative standards using mathematical models, computer simulations, and psychological experiments. Given that we only have finite cognitive resources and we all live in an uncertain world, we continuously confront inevitable trade-offs between the time required for computation and the accuracy of the results, or between costs and benefits. Our cognition must have adapted to enable us to survive in the environment. We have found that the deviations from the norms can be explained by modelling our behaviour probabilistically taking consideration of the cognitive environment of the organisms.

My recent focus is on conditional reasoning, causal induction, and judgements of probability. From the truth functional point of view including classical logic, a conditional sentence, "if p then q", is equivalent to "not-p or q", which has been highly controversial in philosophy, linguistics, and psychology. However, if we construct models of conditionals and human behaviour not logically but probabilistically, we can understand adaptive rationality of our cognitive activities including inductions and deductions using conditional or causal statements, and our linguistic utilization of conditionals. Some of my work also focuses on the role of the motivations in reasoning and decision making, which is extremely important in our daily life.