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The sheep-goat effect refers to the fact that believers in psi tend to do better than chance in psi experiments, while those who don't believe in psi or don't believe it has any relevance in psi experiments tend to score below chance in the experiments.
The expression dates back to 1942 and Gertrude Schmeidler, a professor of psychology at City University of New York.* She asked her students whether they believed in psi before giving them an ESP card test. She called believers "sheep" and non-believers "goats." Sheep scored above chance at a statistically significant level, and goats scored below chance at a statistically significant level. Her results have been replicated many times, according to Mario Varvoglis.
Some interpret this to mean that you must believe in psi for it to work. For example, Varvoglis claims:
...one's attitudes toward psi affects the likelihood that such phenomena will occur in the first place. The more an individual harbors a reductionistic view of the world, the less chance such phenomena will emerge (let alone be witnessed by them); the more one is interested in interconnectedness, and open to psi experiences, the more likely the world will "respond" by creating such experiences.
How he know this is anyone's guess.
I can understand how believers might score above chance (by looking for cues, sensory leakage, counting cards, cheating, etc.). I can understand why researchers who believe in psi tend to get positive results, while non-believers tend to get negative results (experimenter effect, cheating, differences in competence and care in setting up a proper experiment, etc.) But I have no idea why non-believers, as a group, would score significantly below chance time and time again in ESP card experiments, a phenomenon referred to as psi-missing. J. B. Rhine thought it was because the non-believers didn't like him. Possibly. But I find it hard to believe that the believers liked him.