Making participation in experiments part of beginning psychology requirements may obviate volunteer-nonvolunteer differences as all students must participate. Since students are free to select experiments, biases may be introduced into any one by virtue of its kind and/or the time of the semester in which it is offered. In this study differences between those who sign up for the first experiment of the term and those who decline it were examined. Results indicate that those who sign up for the first experiment resemble volunteer Ss—especially on variables which appear rather likely to affect experimental results, viz., authoritarianism and conformity.
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