Biological and Genetic Correlates

g has a large number of biological correlates. Strong correlates include mass of the prefrontal lobe, overall brain mass, and glucose metabolization rate within the brain. g correlates less strongly, but significantly, with overall body size. There is conflicting evidence regarding the correlation between g and peripheral nerve conduction velocity, with some reports of significant positive correlations, and others of no or even negative correlations.

Current research suggests that broad-sense heritability of g is between 0.5 and 0.8, and narrow-sense heritability approximately 0.3, but the causal pathways are currently unknown. The heritability of most test performance is thus attributed to g.

Brain size has long been known to be correlated with g. Recently, an MRI study on twins showed that frontal gray matter volume was highly significantly correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors. g has been observed in mice as well as humans.

g is probably limited by the channel capacity of short-term memory. Mental power, or the capacity C of short-term memory (measured in bits of information), is the product of the individual mental speed Ck of information processing (in bit/s), and the duration time D (in s) of information in short-term working memory, meaning the duration of memory span. Hence:

C (bit) = Ck(bit/s) × D (s).