title: Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean?
abstract: Positive associations between human intelligence and brain size have been suspected for more than 150 years. Nowadays, modern non-invasive measures of in vivo brain volume (Magnetic Resonance Imaging) make it possible to reliably assess associations with IQ. By means of a systematic review of published studies and unpublished results obtained by personal communications with researchers, we identified 88 studies examining effect sizes of 148 healthy and clinical mixed-sex samples (>8000 individuals). Our results showed significant positive associations of brain volume and IQ (r=.24, R(2)=.06) that generalize over age (children vs. adults), IQ domain (full-scale, performance, and verbal IQ), and sex. Application of a number of methods for detection of publication bias indicates that strong and positive correlation coefficients have been reported frequently in the literature whilst small and non-significant associations appear to have been often omitted from reports. We show that the strength of the positive association of brain volume and IQ has been overestimated in the literature, but remains robust even when accounting for different types of dissemination bias, although reported effects have been declining over time. While it is tempting to interpret this association in the context of human cognitive evolution and species differences in brain size and cognitive ability, we show that it is not warranted to interpret brain size as an isomorphic proxy of human intelligence differences.
authors.affiliation: Department of Applied Psychology-Health, Development, Enhancement and Intervention, Faculty of Psychology, University of Vienna, Vienna, Austria; Department of Basic Psychological Research and Research Methods, Faculty of Psychology, University of Vienna, Vienna, Austria; Department of Psychology, School of Science and Technology, Middlesex University Dubai, Dubai, United Arab Emirates. Electronic address: jakob.pietschnig@univie.ac.at.
authors.forename: Jakob
authors.initials: J
authors.lastname: Pietschnig
authors.affiliation: Georg Elias Müller Department of Psychology, Georg August University Göttingen, Göttingen, Germany.
authors.forename: Lars
authors.initials: L
authors.lastname: Penke
authors.affiliation: Tilburg School of Social and Behavioral Sciences, Tilburg University, Tilburg, The Netherlands.
authors.forename: Jelte M
authors.initials: JM
authors.lastname: Wicherts
authors.affiliation: Department of Basic Psychological Research and Research Methods, Faculty of Psychology, University of Vienna, Vienna, Austria.
authors.forename: Michael
authors.initials: M
authors.lastname: Zeiler
authors.affiliation: Department of Basic Psychological Research and Research Methods, Faculty of Psychology, University of Vienna, Vienna, Austria; Georg Elias Müller Department of Psychology, Georg August University Göttingen, Göttingen, Germany; Department of Psychology, University of Zürich, Zürich, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Voracek
mesh_terms: D001921:Brain / Q000033:anatomy & histology*; D006801:Humans; D007360:Intelligence / Q000502:physiology*
keywords: In vivo brain volume; Intelligence; Meta-analysis; Meta-regression; Reporting bias