Updating the Ganzfeld Database: A Victim of Its Own Success?

The existence of psi—anomalous processes of information transfer such as telepathy or clairvoyance—continues to be controversial. Earlier meta-analyses of studies using the ganzfeld procedure appeared to provide replicable evidence for psi (D. J. Bem & C. Honorton, 1994), but a follow-up meta-analysis of 30 more recent ganzfeld studies did not (J. Milton & R. Wiseman, 1999). When 10 new studies published after the Milton-Wiseman cutoff date are added to their database, the overall ganzfeld effect again becomes significant, but the mean effect size is still smaller than those from the original studies. Ratings of all 40 studies by 3 independent raters reveal that the effect size achieved by a replication is significantly correlated with the degree to which it adhered to the standard ganzfeld protocol. Standard replications yield significant effect sizes comparable with those obtained in the past.