Yet one recent idea emerging from his interest in self-deception appears to have real significance: Research shows that older adults are biased toward paying attention to and remembering the positive over the negative and that they don’t dwell in negative moods, a phenomenon called the aging positivity effect. There’s been no functional explanation, and it would seem that such a bias could be dangerous by blinding people to hazards. But Trivers notes that positive moods improve immune function, and older adults have a greater need for a strong immune system to fight off tumors and other ills. So maybe we’ve evolved to cheer ourselves up as we age just to boost immunity.
He suggested the idea to von Hippel, who didn’t buy it. Why would natural selection shape old age, after we can no longer reproduce? But, Trivers argued, you can still help raise your grandchildren, who carry your genes. Von Hippel ran a test that found that in older adults, a greater positivity bias correlated with stronger immune function. So they published the findings in 2014 in Psychology & Aging. Now they’re working on a longitudinal study to see if positivity predicts later immune function.