Josef Perner (Salzburg) gives a talk at the MCMP Colloquium (14 Dec, 2011) titled "Development of Counterfactual Reasoning & Emotions". Abstract: Controlling for suppositional reasoning when asked counterfactual questions children do not engage in counterfactual reasoning before 9 to 12 years. Without this control even 3 year olds can give correct answers. For instance when asked, “Suppose Carol takes her dirty shoes off before she walks across the floor, will the floor be dirty or clean?” one has no logically valid answer but one might find “clean” the more suitable option. And children tend to do so. Now when told: “Carol walked with her dirty shoes across the sparkling floor and made it all dirty,” the counterfactual question, “If Carol had taken her shoes off before walking across the floor, would the floor be dirty or clean?” can now be answered with some justification with “clean.” Children as young as 3 years do so (Harris et al 1986). However, from their responses we cannot tell whether they used counterfactual reasoning or simply applied suppositional reasoning, since both lead to the same answer. If we now change the story slightly: “Peter walked with his dirty shoes across the sparkling floor and then came Carol with her dirty shoes and walked across it. Now the floor is all dirty,” then the counterfactual question, “If Carol had taken off her shoes, …?” should be answered with “(still) dirty,” because the floor was already dirty before Carol walked across it. In contrast, suppositional reasoning would still lead to “clean,” and now only children between 9 and 12 years start giving the correct counterfactual answer. We have now established that this finding is robust for different ways of assessing counterfactual reasoning. Moreover, the onset of counterfactual emotions of regret and relief also follows the same developmental course.