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An argument is valid if its conclusion necessarily follows from its premises, regardless of whether the premises and conclusion are empirically true or false. This research tested the hypothesis that understanding validity of inference (including its differentiation from empirical truth) is a relatively late development. Students in Experiment 1 were asked to sort sets of deductive arguments. None of the fourth graders used validity as a basis for distinguishing arguments, while 45% of the seventh graders and 85% of the college students did so. Experiments 2 and 3 explored whether the dramatic age difference could be narrowed by (a) varying the types of arguments used, (b) explaining the concept of validity and instructing students to use it, and/or (c) providing feedback after each trial. Fourth-grade performance remained poor, while seventh-grade performance increased to nearly the level of the college students. It was concluded that the concept of validity typically develops between ages 10 and 12 but that application of that competence continues to increase over a much longer age span. Students not understanding validity commonly evaluated arguments on the basis of empirical truth of component propositions, though even fourth graders revealed an implicit awareness of logical form.