APOE and dementia: Stratified analysis according to APOE ɛ4 status furthermore revealed that having high cognitive reserve in two or three periods over the life course was associated with a lower risk of dementia in both ɛ4 allele carriers (RR 0.5; 95% CI 0.3–0.9) and ɛ4 allele noncarriers (RR 0.7; 95% CI 0.5–0.97).