IQ and the brain
The correlation between brain size and IQ seems to hold for comparisons between and within families. However, one study found no within family correlation (Schoenemann et al. 2000).
The brain is a metabolically expensive organ, and consumes about 25% of the body's metabolic energy. Because of this fact, although larger brains are associated with higher intelligence, smaller brains might be advantageous from an evolutionary point of view if they are equal in intelligence to larger brains. Skull size correlates with brain size, but is not necessarily indicative.
Brain size is a rudimentary indicator of the intelligence of a brain, and many other factors affect the intelligence of a brain. Higher ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks.
A recent University of Chicago study links 2 sets of genetic variation (alleles) to brain size, race and spurts in human evolution. In particular, these genetic variations –arguably responsible for greater intelligence-were relatively common in Europe and Asia, but markedly less common in sub-Saharan Africa. The variations can be established as appearing 8-10,000 years BC, spreading to 70% and 30% of humans respectively, and are associated with agriculture and written language.
Here is a list of some species, along with their rough average brain sizes:
- Homo erectus: 980 cm³
- Homo habilis: 750 cm³
- Homo floriensis: 380 cm³
- Homo neanderthalensis : 1200-1700 cm³