Genes influence cognition, brain structure, and brain function more in old age.
Older carriers of disadvantageous genotypes exhibit exacerbated decline.
Individual-difference factors modulate age magnification of genetic effects.
Heritability studies document substantial genetic influences on cognitive performance and decline in old age. Increasing evidence shows that effects of genetic variations on cognition, brain structure, and brain function become stronger as people age. Disproportionate impairments are typically observed for older individuals carrying disadvantageous genotypes of different candidate genes. These data support the resource-modulation hypothesis, which states that genetic effects are magnified in persons with constrained neural resources, such as older adults. However, given that findings are not unequivocal, we discuss the need to address several factors that may resolve inconsistencies in the extant literature (genegene and geneenvironment interactions, study populations, geneenvironment correlations, and epigenetic mechanisms).