Pygmalion in the laboratory

Testers and bystanders can inadvertently lead subjects to answers in laboratories and in classrooms, in face-to-face tests of human beings and other animals. Many modern investigators avoid leading by using blind tests scrupulously. This article shows how to design blind tests and illustrates common methodological errors that allow leading to confound experimental results. The object is to help experimenters, editors, and readers detect and avoid a common experimental error that often has profound theoretical implications.