Will we ever know what Neanderthal DNA did to make us human or less human.Harley Hans Hoglin
Perhaps they had less mass in their brains.
When comparing different species the ratio of brain weight to body weight does present a correlation with intelligence, though the actual brain weight has little or no effect. For example, the ratio of brain weight to body weight for fish is 1:5000; for reptiles it is about 1:1500; for birds, 1:220; for most mammals, 1:180, and for humans, 1:50.
Within human population, studies have been conducted to determine whether there is a relationship between brain size and a number of cognitive measures. Studies have reported correlations that range from 0 to 0.6, with most correlations 0.3 or 0.4 Some scientists prefer to look at more qualitative variables such to relate 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..
The brain is a metabolically expensive organ, and consumes about 25% of the body's metabolic energy in some species. 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. Brain size in vertebrates may relate to social rather than mechanical skill. Cortical size relates directly to a pairbonding life style and among primates cerebral cortex size varies directly with the demands of living in a large complex social network.