There is nothing quite as difficult to predict as the future. In my lifetime I have already lived through an “inevitable” ice age that never materialized and “inevitable” mass starvation (through overpopulation) that also never happened. When I was in Central America I remember reading a book called Inevitable Revolutions by the historian Walter LaFeber, but more than a quarter of a century later the inevitable still had not taken place. By now, according to predictions, most of us should have been dead from AIDS, that is if variant Creutzfeldt-Jakob Disease or Ebola virus had not got us first. The repeated failure of confident predictions is therefore almost enough to make one sceptical of dire visions of the future. Only the sheer pleasure of contemplating catastrophe to come keeps the market for apocalypses alive.

One of our present concerns in the western world is the rapid aging of the population. Never have so many people lived to so ripe an old age, and this at a time when the birth rate is falling. Who is going to support the doddering old fools who will soon be more numerous than the energetic and productive young?

A recent article in the New England Journal of Medicine points out that something unexpected has happened to confound the gloomy prognostications of epidemiologists and demographers. As the percentage of people surviving into old age increases, so the proportion of them who suffer from dementia decreases. People are not only living longer, but living better. This is a phenomenon that has happened across the western world.

The article states that “in 1993, 12.2% of surveyed adults 70 years of age or older [in America] had cognitive impairment, as compared with 8.7% in 2002.” Similar results have been obtained elsewhere. In the light of this unexpected and unpredicted trend, estimates of the prevalence of dementia in England have had to be revised downwards by 24 percent. The burden of the elderly on the economy will therefore not be as great as was feared.

What accounts for the decline in the prevalence of dementia?