A few years ago, researchers made the unnerving discovery that in the brains of people with Alzheimer’s disease, disordered clumps of abnormal proteins had been growing for 15 or even 20 years before their diagnosis. That means these pathological-looking deposits are silently accumulating in the brains of millions of seemingly healthy individuals in their 50s and 60s.

Recently, scientists have found that a blood test can detect that silent damage with surprising accuracy. About 13 per cent of people ages 75 to 84 have Alzheimer’s disease, which means a substantial fraction of younger people ought to test positive. But are we better off knowing?