Specifically, CN individuals will have earlier first onset of depression (EOD) relative to AD, more negative life stress during acute depression compared with AD and PCI, and greater white matter integrity; CN will also be associated with the AA genotype of the COMT val158met polymorphism, which may confer both neuroprotection and higher stress sensitivity. PCI will have more EOD relative to AD, greater frailty, and lower white matter integrity than NC. AD will be associated with later age of depression onset (LOD), greater appetite loss, lower anxiety, smaller hippocampal volume, and greater memory impairment. To test these hypotheses, we propose the following