APOE and Alzheimer disease: In humans, the direction of the APOE4 by testosterone interaction is less consistent, whereby Panizzon et al. found that low testosterone levels in men related to smaller hippocampal volumes and poorer episodic memory among APOE4 carriers only [39, 40], whereas Hogervorst et al. found that low testosterone levels related to a greater likelihood of an AD diagnosis among APOE4 non-carriers only [31].