Human brain volume

Cover imageIn vivo brain size correlates with IQ.

Effects of 148 mixed-sex healthy and patient-based samples (>8000 individuals).

The effect generalizes over age, intelligence domain, sample type, and sex.

Previous effect sizes were inflated due to reporting bias.

Brain size is not a necessary cause for human IQ differences.

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 By means of a systematic review of published studies and unpublished results obtained by personal communications with researchers, we identified 88 studies examining effect sizes of 148 healthy and clinical mixed-sex samples (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 ( = .24, 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.

Keywords

  • Intelligence;
  • In vivo brain volume;
  • Meta-analysis;
  • Meta-regression;
  • Reporting bias

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