External Validity of Laboratory Experiments: The Frequency-Validity Relationship

The question of the generalizability of laboratory experiments to the natural settings of ordinary people was investigated in a case study on the frequency-validity relationship. Previously advocated by John Locke and David Hartley, this relationship states that the mere repetition of plausible but unfamiliar assertions increases the belief in the validity of the assertions, independent of their actual truth or falsity. The external validity of this relationship was tested for a random sample drawn from telephone listings of adults living in Schwabing, Munich. Subjects were tested in their homes rather than in a university laboratory. The increase in mean validity judgments by repetition, its independence from actual truth or falsity, as well as the absolute and relative size of the effect were found to be in ex-