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ABSTRACT: We report the results of a meta-analysis on forced-choice ESP studies which used targets such as card symbols, numbers, letters, and so forth. For the period 1987 to 2010, a homogeneous dataset of 72 forced-choice studies yielded a weak but significant mean effect size (ES) of 0.01 (Stouffer Z = 4.86, p = 5.90 × 10^sup -7^). There was no evidence that these results were due to low-quality design or selective reporting. The clairvoyance studies did not produce a significantly higher mean ES than the precognition studies, and target type did not make a difference to effect size. We note that effects do not vary between investigators, but we did find suggestive evidence that the number of choices per trial is inversely related to the p value. We also found evidence of a linear incline in ES values indicating that effect sizes have increased over the period 1987 to 2010. Suggestions are made that might help facilitate further increases in effect sizes.
Keywords: ESP, forced-choice, meta-analysis, paranormal, parapsychology, psi
In the 1930s, J. B. Rhine became the first major laboratory-based empirical investigator of alleged paranormal phenomena. He is most famous for his card-guessing studies (Rhine et al., 1940/1966), first using the standard 52-card deck of playing cards, and then switching to so-called Zener cards (consisting of five symbols: star, wavy lines, square, circle, and cross) named after perceptual psychologist Karl Zener (see Figure 1).
timuli in these early parapsychological studies were not only card symbols, but were also numbers, alphabet letters, words, shapes, and so forth. Collectively, tests of extrasensory perception (ESP) using these kinds of stimuli are known as "forced-choice" because the target-guess is "one of a limited range of possibilities which are known to [the participant] in advance" (Thalbourne, 2003, p. 44).
Decades later, meta-analytic studies of forced-choice studies confirmed that the collective efforts of many investigators, in a number of laboratories worldwide, over many years, had been fruitful, as indicated by accumulative effects that were statistically significant, though effects were variable in strength and tended to be weak (see Honorton & Ferrari, 1989; Steinkamp, Milton, & Morris, 1998; Tart, 1983). However, since more than a decade has passed since the last meta-analysis of the forced-choice domain (i.e., Steinkamp, Milton, &...