Reply to Liu and Jiang: Maintenance of postreproductive cognitive capacity by inclusive fitness

Our paper examines genes influencing cognitive decline in postreproductive humans, which arises from neurodegenerative and cardiovascular disease (1). Among these polymorphic genes, we found many protective alleles that are derived from and are unique to the human lineage. Liu and Jiang (2) add an important observation about potential effects of some of these loci earlier in life. Data from three studies indicate that these genes do not influence cognitive performance during childhood (3) or in the general population (4, 5). Our paper (1) does not address early phenotypes, as the establishment of early cognitive function and protection against late degeneration need not involve the same mechanisms. However, Liu and Jiang’s (2) findings suggest that many alleles protecting cognition in elders have no strong effects on early cognition. This observation supports the hypothesis that postreproductive cognitive protection may have evolved by inclusive fitness.