New Preprint: Effect Sizes, Power, and Biases in Intelligence Research

Our new meta-meta-analysis on intelligence research is now online as a preprint at

We analyzed 131 meta-analyses in intelligence research to investigate effect sizes, power, and patterns of bias. We find a typical effect of r = .26 and a median sample size of 60.

The median power seems low (see figure below), and we find evidence for small study effects, possibly indicating overestimated effects. We don’t find evidence for a US effect, decline or early-extremes effect, or citation bias.


Comments are very welcome and can be posted on the PubPeer page



Interview about the Replication Paradox

September 2015

Watch a 5 minute interview (in Dutch) in which I explain when replication research is and isn’t useful.

This interview was originally published on the website of Tilburg University:

The Replication Paradox is now published!

June 2015

My latest paper “The replication paradox:  Combining studies can decrease accuracy of effect size estimates” is now published in Review of General Psychology. You can find a postprint of the paper here. The full reference is:

Nuijten, M. B., Van Assen, M. A. L. M., Veldkamp, C. L. S., & Wicherts, J. M. (2015).The replication paradox: Combining studies can decrease accuracy of effect size estimates. Review of General Psychology, 19(2), 172-182.