¼: That’s the proportion of Americans believed to carry a variant of the APOE gene, one of the strongest risk factors for dementia. Some 47 percent of people who carry the gene develop the brain-wasting disease. But don’t despair. Seriously. Don’t despair. The research, published in the journal PLOS ONE, found that carriers were 50 percent less likely to develop dementia if they held positive beliefs about aging, compared with those who held negative views.