Last month, a neuroscience paper got a lot of attention for reporting that Negative Age Stereotypes Predict Alzheimer's Disease Biomarkers. It was greeted by headlines such as:
If you think elderly people are icky, you're more likely to get Alzheimer’s (Healthline) Lack of respect for elderly may be fuelling Alzheimer's epidemic (The Telegraph) Your attitude about aging may impact how you age (TIME)
The research, published in the journal Psychology and Aging, wasn't about Alzheimer's disease per se. Rather, the study claims that holding negative attitudes towards the elderly (ageism) is correlated with the later development of neurobiological changes, or biomarkers, linked to Alzheimer's. The authors, from Yale University and the National Institute on Aging, were led by Becca Levy. Levy et al. report that scores on the Attitudes Toward Old People Scale (AOPS) questionnaire predicted later hippocampal atrophy, as measured with structural MRI (n=52), and with the levels of neurofibrillary tangles and amyloid plaques present in the brain post mortem (n=74). All these measures were taken as part of the Baltimore Longitudinal Study of Aging, which is the world's longest-running aging research project - it started in 1958.
These are rather fascinating results. It would be very interesting - not to mention ironic - if ones attitudes towards old people predicted how fast your brain physically degenerated with age. What makes the claims even more remarkable is the span of time that elapsed between when people completed the attitude questionnaire and when the biomarkers were measured. Consider that Levy et al. measured hippocampal atrophy 25 years, on average, after people did the AOPS scale. For the tangles and plaques, the average interval was a full 28 years - yet the effect size was far from small: the Cohen's d was 0.45 comparing low to high AOPS scorers. That answers to a single 16-item questionnaire substantially predict neuropathological changes nearly 30 years later raises my eyebrows. That's not to say it's impossible, but I find it surprising. So what's going on here? Could a problem with the statistics have inflated the effect? On PubPeer a number of issues have been highlighted. For instance, a PubPeer poster points out that the authors used one-tailed statistical tests, which seems unjustified. Using two-tailed tests, the key tangles and plaques result (p = 0.046 one-tailed) is not significant. The hippocampal atrophy finding remains significant, however. Even if the effect is real, it's a correlation, and doesn't mean that ageism causes any biological changes. Nonetheless, Levy et al. state that
Several factors support our assumption that negative age stereotypes contributed to the Alzheimer’s disease biomarkers, rather than the reverse direction.
That Alzheimer's biomarkers cause ageism does indeed seem unlikely. Yet the authors don't discuss the possibility that both of these phenomena are caused by a third, confounding factor - a covariate that wasn't controlled for. For example, socioeconomic status or income could explain these results, if richer people are less likely to get Alzheimer's, and less likely to be ageist. Or family history of Alzheimer's might be the culprit: it is certainly a risk factor for the disease, and maybe it also causes ageist attitudes (if people assume that all old people suffer dementia like their own relatives did.) Overall, then, the headlines were misleading, and the results themselves are difficult to interpret.
Levy BR, Ferrucci L, Zonderman AB, Slade MD, Troncoso J, & Resnick SM (2015). A Culture-Brain Link: Negative Age Stereotypes Predict Alzheimer's Disease Biomarkers. Psychology and Aging PMID: 26641877