Increasing Volunteer Representativeness by Recruiting for Credit or Pay

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Abstract
It was hypothesized that recruiting for pay or college credit would considerably increase the similarity of volunteers to their parent populations (and to nonvolunteers) in intellectual ability and personality. For the testing of this hypothesis, volunteers for research-participation were recruited from introductory psychology classes by the instructor, with class credit as a reward: 68 volunteered, 34 did not. Comparisons were made for academic ability and achievement and for “MMPI personality.” In a second experiment women were recruited by the orientation testing administrator for a three-hour paid experiment: 123 volunteered, 318 did not. Of the 123 volunteers, 29, who took tests in a manner identical to the non-volunteers, were compared for “MMPI personality.” Differences did not exceed chance expectations for either group comparison. It is concluded that tangible rewards considerably increase the representativeness of volunteers for psychological experiments, since the selection is on other than the rewards of sheer “volunteerism.”