Robert Yerkes developed a multiple-choice method to study ideational behavior in animals and humans using specially designed multiple-choice apparatuses. One example of Yerkes’ apparatus, used for testing human subjects, now resides in the Historical Scientific Instruments Collection at the Peabody Museum of Natural History of Yale University. Yerkes introduced the apparatus in 1913 to show that the multiple-choice method was an effective measure of ideational behavior in normal and abnormal people. An analysis of the machine’s construction and history brings to light Yerkes’ desire for methodological rigor in experimental design. Yerkes’ apparatus became commercially available in 1923 even though other psychological laboratories had been making their own versions of it since 1919. Sixteen years later, Yerkes’ apparatus was still on the market despite an apparent lack of demand for it.