Overestimation of cognitive function was associated with age, hippocampal atrophy, plasma glial fibrillary acidic protein, and subsequent dementia conversion.<h4>Conclusions</h4>Underestimation of cognitive function may reflect functional cognitive changes linked to mental and physical ill health, while overestimation of function may be a marker of neurodegenerative changes. This evidence concerns the gene GFAP and dementia.