n 1907 a young psychologist named Oskar Pfungst published a report on a horse. It is one of the best short demonstrations of experimental method ever written, and it is free to read.

the document

Pfungst, Clever Hans (the Horse of Mr. von Osten). The German original, Das Pferd des Herrn von Osten (Der Kluge Hans), appeared in 1907; Carl L. Rahn's English translation in 1911, with an introduction by Carl Stumpf, in whose Berlin institute Pfungst worked. The translation is in the public domain and is transcribed at Project Gutenberg, ebook 33936.

what to read first

The book is long. The core is the experimental chapters, in which Pfungst varies one thing at a time: whether the questioner knows the answer, whether the horse can see the questioner, how far away the questioner stands, and whether blinkers are fitted. Hans's accuracy collapses exactly when he cannot see someone who knows the answer. Then comes the part people forget: Pfungst played the horse himself, with human participants as questioners, and found that they gave the same small cues without knowing it [paraphrase, not a quotation].

Why it lasts. Pfungst did not show that von Osten was a fraud; the evidence is that he was sincere. He showed that sincerity is not a control. That is the lesson behind blind testing, behind experimenter expectancy, and behind the "Clever Hans predictors" of shortcut learning.

Reference

1. Pfungst (1911). Clever Hans (the Horse of Mr. von Osten), trans. C. L. Rahn. New York: Henry Holt. German original 1907. Free edition at Project Gutenberg. www.gutenberg.org/ebooks/33936

see also