  Background: Human populations during the last 10,000 years have undergone
rapid decreases in average brain size as measured by endocranial volume or as
estimated from linear measurements of the cranium. A null hypothesis to explain
the evolution of brain size is that reductions result from genetic correlation
of brain size with body mass or stature.
  Results: The absolute change of endocranial volume in the study samples was
significantly greater than would be predicted from observed changes in body
mass or stature.
  Conclusions: The evolution of smaller brains in many recent human populations
must have resulted from selection upon brain size itself or on other features
more highly correlated with brain size than are gross body dimensions. This
selection may have resulted from energetic or nutritional demands in Holocene
populations, or to life history constraints on brain development.
