# Intelligence and cortical morphometry: caveats in brain-behavior associations

## TL;DR

This paper shows that cortical thickness and contrast do not improve IQ prediction beyond brain volume, highlighting methodological issues in brain-behavior studies.

## Contribution

The study identifies multicollinearity and brain size confounding as key issues in cortical morphometry-intelligence associations.

## Key findings

- Cortical thickness and peri-cortical contrast do not reliably improve IQ prediction beyond brain volume.
- Results are consistent across child, adult, and developmental datasets, as well as across MRI scanner types and sites.
- The findings suggest a methodological explanation for conflicting results in prior brain-behavior studies.

## Abstract

It is well-established that brain size is associated with intelligence. But the relationship between cortical morphometric measures and intelligence is unclear. Studies have produced conflicting results or no significant relations between intelligence and cortical morphometric measures such as cortical thickness and peri-cortical contrast. This discrepancy may be due to multicollinearity amongst the independent variables in a multivariate regression analysis, or a failure to fully account for the relationship between brain size and intelligence in some other way. Our study shows that neither cortical thickness nor peri-cortical contrast reliably improves IQ prediction accuracy beyond what is achieved with brain volume alone. We show this in multiple datasets, with child data, developmental data, and with adult data; we show this with data acquired either at multiple sites, or at a single site; we show this with data acquired with different MRI scanner manufacturers, or with all data acquired on a single scanner; and we show this with fluid intelligence, full-scale IQ, performance IQ, and verbal IQ. But our point is not really even about IQ; rather we proffer a methodological caveat and potential explanation of the discrepancies in previous results, and which applies broadly.