Intelligence can be defined as the “inferred process that humans use to explain the different degrees of adaptive success in people’s behaviour” (Good, 2020). There are many types of intelligence, such as academic, social and emotional. Personality is defined as “the characteristics or blend of characteristics that make a person unique” (Weinberg and Gould, 1999). There are many individual differences in intelligence and personality, and this may be a result of differences in the cerebral cortex. The cerebral cortex is the outer layer of the brain, which has many different functions. The association between these individual differences and differences in the cerebral cortex will be examined throughout this essay in more detail.
Many aspects of the cerebral cortex are associated with individual differences in intelligence. Most of the studies use IQ (intelligence quotient) to study intelligence. A main difference in the cerebral cortex associated with intelligence are dendrites. Goriounova et al. (2019) used human surgical tissue to demonstrate that higher IQ is associated with larger dendrites, faster action potentials and more efficient transmission of neurones. In addition, the thickness of neurones was also found to correlate with speed of information transmission. Therefore, thicker and larger dendrites are associated with higher IQ. Furthermore, variations of dendritic density in grey matter has been associated with differences in intelligence. A higher density of neurones with disorganised orientation is associated with lower IQ, and a lower density of neurones with organised and well-defined orientation is associated with higher IQ.
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These findings can be linked to people with intellect abnormalities, such as Down Syndrome. Often having a thicker cortex, due to less effective pruning of dendrites, they have lower IQ’s due to a higher density of dendrites. These findings suggest that morphology and organisation of the cerebral cortex has more of an effect on intelligence than the size of the brain. In fact, only a very weak correlation has been found between overall brain size and intelligence. Another difference in the cerebral cortex that may be associated with individual differences in intelligence is the overall volume of grey matter. A strong correlation has been found between frontal cortex volume grey matter and Spearman’s g (IQ). In addition, Einstein’s brain has created a nice case study to examine why his IQ was so high (over 160). Einstein had a smaller than average brain, and his brain was morphologically very different to a normal brain. Not only was his parietal operculum missing, his parietal cortex was 15% wider than average. Researchers believe this enlargement increased his ability to think and imagine (Witelson et al., 1999), increasing his intelligence.
Overall, these studies and findings provide an understanding for how individual differences in intelligence may be associated with differences in the cerebral cortex. However, they are not without their limitations. Many of the studies above used IQ as a test for intelligence, however, it does not measure every type of intelligence, such as creativity or emotional intelligence. Therefore, these findings are limited and lack generalisation to a wider definition of intelligence. Moreover, the findings are associations, meaning that direct cause and effect has not been identified, and the information should be interpreted with caution. It should also be noted that even though the cerebral cortex has identified the causes of a large number of individual differences in intelligence, there are other influences, such as genetics, which may equally impact intelligence.
Individual differences in personality has also been associated with differences in the cerebral cortex in a number of ways. A key theory linking personality and the cerebral cortex is Eysenck’s biological model of personality and arousal. In this model, the ascending reticular activating system (ARAS) is thought to control elements of consciousness and arousal, while connecting to various areas of the cerebral cortex. This arousal is managed through the reticulocortical and reticulolimbic systems. The four personality traits (extravert, introvert, neurotic and emotionally stable) are linked to these two systems. Eysenck believed that high arousal in the reticulocortical system led to introversion, and low arousal led to extraversion. Whereas high arousal in the reticulolimbic system led to neuroticism, while low arousal led to being emotionally stable. In other words, extraverts have ARAS that work at a lower level, meaning they display behaviour to increase arousal levels, and introverts have ARAS that work too well, so aim to lower their levels of arousal.
Even though these findings provide evidence for how the cerebral cortex is related to personality, there is a lack of consistent evidence supporting it. Matthews et al. (1999) found that relationships for these different personality types were often weak, and that there is little evidence relating neuroticism to arousal. Therefore, there is a need for contemporary and stronger research to be conducted, for it to be more reliable. Another area of the cerebral cortex associated with abnormal aspects of personality is cortical density in the prefrontal cortex. Gregory et al. (2012) found that a reducing in this is seen in psychopaths. This therefore provides an area of the cerebral cortex that is directly associated with personality differences.
In conclusion, many aspects of the cerebral cortex can be associated with individual differences in intelligence and personality. Many of the associations are back up by consistent research, however it does need to be highlighted that these are only associations, and not causation.
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