p-Hacking and publication bias interact to distort meta-analytic effect size estimates.

Science depends on trustworthy evidence. Thus, a biased scientific record is of questionable value because it impedes scientific progress, and the public receives advice on the basis of unreliable evidence that has the potential to have far-reaching detrimental consequences. Meta-analysis is a technique that can be used to summarize research evidence. However, meta-analytic effect size estimates may themselves be biased, threatening the validity and usefulness of meta-analyses to promote scientific progress. Here, we offer a large-scale simulation study to elucidate how -hacking and publication bias distort meta-analytic effect size estimates under a broad array of circumstances that reflect the reality that exists across a variety of research areas. The results revealed that, first, very high levels of publication bias can severely distort the cumulative evidence. Second, -hacking and publication bias interact: At relatively high and low levels of publication bias, -hacking does comparatively little harm, but at medium levels of publication bias, -hacking can considerably contribute to bias, especially when the true effects are very small or are approaching zero. Third, -hacking can severely increase the rate of false positives. A key implication is that, in addition to preventing -hacking, policies in research institutions, funding agencies, and scientific journals need to make the prevention of publication bias a top priority to ensure a trustworthy base of evidence. (PsycInfo Database Record (c) 2020 APA, all rights reserved).