Henderson, N. D. 1970. Brain weight increases resulting from environmental enrichment: a directional dominance in mice. Science 169(947), 776-778.

Sets of our male and sets of four female C3HxBALB hybrid mice were raised in (1) small standard cages [280 cm2] that were barren, vs.(2) large test cages [1375 cm2] that were barren, vs.(3) large test cages [1375 cm2] that were enriched with a variety of objects for climbing, exploring, hiding and gnawing.The weight of the brain of subjects raised in a barren environment was not affected by the size of the cage, but it was significantly increased in subjects raised in large plus enriched cages. This indicates that environmental enrichment is more important than additional unstructured space for brain development.