Objectives: The apolipoprotein E (APOE) e4 allele is an established risk factor for dementia, yet this genetic variant is associated with a mixed cognitive profile across the lifespan. This study undertakes both a systematic and meta-analytic review of research investigating APOE-related differences in cognition in mid-adulthood, when detrimental effects of the allele may first be detectable. Methods: Thirty-six papers investigating the behavioral effects of APOE e4 in mid-adulthood (defined as a mean sample age between 35 and 60 years) were reviewed. In addition, the effect of carrying an e4 allele on individual cognitive domains was assessed in separate meta-analyses. Results: The average effect size of APOE e4 status was non-significant across cognitive domains. Further consideration of genotype effects indicates preclinical effects of APOE e4 may be observable in memory and executive functioning. Conclusions: The cognitive profile of APOE e4 carriers at mid-age remains elusive. Although there is support for comparable performance by e4 and non-e4 carriers in the 5th decade, studies administering sensitive cognitive paradigms indicate a more nuanced profile of cognitive differences. Methodological issues in this field preclude strong conclusions, which future research must address, as well as considering the influence of further vulnerability factors on genotype effects
