Brain volume One of the main methods used to establish a relationship between intelligence and the brain is to use measures of brain volume. The advent of MRI as a non-invasive highly-accurate measure of living brain structure and function (using fMRI) made this the pre-dominant and preferred method for measuring brain volume. A large number of studies have been conducted with uniformly positive correlations, leading to the generally safe conclusion that larger brains predict greater intelligence. In healthy adults, the correlation of total brain volume and IQ is approximately 0.4 when high-quality tests are used. A large scale study (n = 29k) using
the UK Biobank found a correlation of .275. The strength of this relationship did not depend on sex, contradicting some earlier studies. A study using a
sibling-design in two medium sized samples found evidence of causality with an effect size of .19. This study design rules out
confounders that vary between families, but not those that vary within families. Less is known about variation on scales less than total brain volume. A meta-analytic review by McDaniel found that the correlation between intelligence and in vivo brain size was larger for females (0.40) than for males (0.25). The same study also found that the correlation between brain size and Intelligence increased with age, with children showing smaller correlations. Small studies have shown transient changes in gray-matter associated with developing a new physical skill (juggling) occipito-temporal cortex Brain volume is not a perfect account of intelligence: the relationship explains a modest amount of variance in intelligence – 12% to 36% of the variance. These caveats imply that there are other major factors influencing how intelligent an individual is apart from brain size. Taking into account measurement quality, and sample type and IQ-range, the meta-analytic association of brain volume in appears to be ~ .4 in normal adults. There appear to be
sex differences between the relationship of grey matter to intelligence between men and women. Men appear to show more intelligence to grey matter correlations in the frontal and parietal lobes, while the strongest correlations between intelligence and grey matter in women can be found in the frontal lobes and
Broca's area. VBM has also been used to show that grey matter volume in the medial region of the prefrontal cortex and the dorsomedial prefrontal cortex correlate positively with intelligence in a group of 55 healthy adults. VBM has also been successfully used to establish a positive correlation between grey matter volumes in the anterior cingulate and intelligence in children aged 5 to 18 years old. Grey matter has also been shown to positively correlate with intelligence in children. Reis and colleagues The findings that more intelligent individuals are more efficient at using their neurons might indicate that the correlation of grey matter to intelligence reflects selective elimination of unused synapses, and thus a better brain circuitry.
White matter Similar to grey matter,
white matter has been shown to correlate positively with intelligence in humans. Anatomical mesh-based geometrical modelling has also shown positive correlations between the thickness of the corpus callosum and Intelligence in healthy adults. White matter integrity has also been found to be related to intelligence. Although it is hard to pin intelligence on age based on cortical thickness due to different socioeconomic circumstances and education levels, older subjects (17 - 24) tended to have less variances in terms of intelligence than when compared to younger subjects (19 - 17). An analysis published in 2019 found the contours of 677 children and adolescent (mean age 12.72 years) brains had a genetic correlation of almost 1 between IQ and surface area of the
supramarginal gyrus on the left side of the brain.
Neural efficiency The neural efficiency hypothesis postulates that more intelligent individuals display less activation in the brain during cognitive tasks, as measured by Glucose metabolism. a large number of studies (N=27) have confirmed this finding using methods such as PET scans, EEG and fMRI. fMRI and EEG studies have revealed that task difficulty is an important factor affecting neural efficiency. In fact, more able individuals appear to invest more cortical resources in tasks of high difficulty. It has been proposed that the main reason for the neural efficiency phenomenon could be that individuals with high intelligence are better at blocking out interfering information than individuals with low intelligence.
Further research Some scientists prefer to look at more qualitative variables to relate to the size of measurable regions of known function, for example relating the size of the primary
visual cortex to its corresponding functions, that of visual performance. In a study of the head growth of 633 term-born children from the Avon Longitudinal Study of Parents and Children
cohort, it was shown that prenatal growth and growth during infancy were associated with subsequent IQ. The study's conclusion was that the brain volume a child achieves by the age of 1 year helps determine later intelligence. Growth in brain volume after infancy may not compensate for poorer earlier growth.
Parieto-frontal integration theory In 2007,
Behavioral and Brain Sciences published a target article that put forth a biological model of intelligence based on 37 peer-reviewed neuroimaging studies (
Jung &
Haier, 2007). Their review of a wealth of data from functional imaging (
functional magnetic resonance imaging and
positron emission tomography) and structural imaging (
diffusion MRI,
voxel-based morphometry,
in vivo magnetic resonance spectroscopy) argues that human intelligence arises from a distributed and integrated neural network comprising brain regions in the frontal and parietal lobes. A recent lesion mapping study conducted by
Barbey and colleagues provides evidence to support the P-FIT theory of intelligence. Brain injuries at an early age isolated to one side of the brain typically results in relatively spared intellectual function and with IQ in the normal range. ==Primates==