Separate analyses with the APOE ε4 allele as an additional covariate confirmed that an increased risk of incident AD was associated with scoring in the top quartile of neuroticism (HR = 3.82; 95%CI =1.85–7.89) or the bottom quartile of conscientiousness (HR = 3.40; 95%CI =1.39–8.28). We also tested for interactions between each of the five factors and the APOE ε4 risk variant. We found significant interactions between APOE genotype and openness (interaction term: HR = 0.58; 95%CI =0.34–0.98) and agreeableness (interaction term: HR = 1.87; 95%CI =1.10–3.16), such that high openness was protective among the APOE ε4 carriers, whereas high agreeableness was protective among the non-carriers.
