6.24 : Biological Influences on Intelligence

Intelligence is often thought to be linked to brain size, but the relationship is more complex than that. While brain size does correlate modestly with some abilities, like verbal skills, the connection is weaker for others, such as spatial reasoning. Other factors, like brain structure, also play crucial roles. For instance, despite Einstein's smaller-than-average brain, his parietal cortex, which is involved in spatial reasoning, was 15% wider, suggesting that neural density might matter more than size alone.

The research highlights developmental differences in the prefrontal cortex (PFC) across children with varying IQ levels. The PFC, responsible for higher-order functions like planning, decision-making, and emotional regulation, develops differently depending on a child's IQ. At age 7, children with superior IQs (121-145) have a thinner cortex compared to those with average (83-108) or high IQs (109-120). The cortex of superior IQ children thickens more rapidly, peaking at age 12, whereas it peaks at age 8 for children with average or high IQs. This delayed peak suggests that the PFC takes longer to mature in children with superior IQs. Researchers propose that this extended cortical thickening period in superior IQ children may indicate a prolonged critical period for developing advanced cognitive abilities.

Genetics also influences intelligence, with over 1,000 genes each contributing minor effects. Researchers have identified genetic variants, or alleles, linked to educational attainment, which can predict cognitive outcomes throughout life. Heritability studies suggest that up to 75% of intelligence variation within certain groups is genetic. However, heritability explains population trends, not individual intelligence. Environmental factors like education, nutrition, and social experiences still play crucial roles in shaping intelligence.

Supportive environments, such as good schools, proper nutrition, and enriched learning experiences, help individuals reach their full genetic potential. Even in populations with high genetic influence, improving environmental factors can significantly boost cognitive abilities, showing that intelligence results from a dynamic interaction between genes and the environment.