Can mental exercise build "mental muscle"? In one study of this question, 12 littermate pairs of young male rats were used; one member of each pair, chosen at random, was raised in an "enriched" environment with toys and companions, while its littermate was raised alone in an "impoverished" environment. After 80 days, the animals were sacrificed and their brains were dissected by a researcher who did not know which treatment each rat had received. One variable of interest was the weight of the cerebral cortex, expressed relative to total brain weight. For 10 of the 12 pairs, the relative cortex weight was greater for the "enriched" rat than for his "impoverished" littermate; in the other 2 pairs, the "impoverished" rat had the larger cortex. Use a sign test to compare the environments at α = 0.05; let the alternative hypothesis be that environmental enrichment tends to increase the relative size of the cortex.16