Two cognitive factors explain this, it found: crystallized intelligence (knowledge that accumulates with age) can offset declines in fluid intelligence (the capacity to process new intelligence).

While the latter can, and does, decline to varying degrees with age, the former can help to make up for it, with the exception of a few professions in which a high degree of fluid intelligence is essential (such as air traffic controllers, who must have the ability to make split-second decisions).

The study found that declining fluid intelligence is often offset by accumulated knowledge, and people with a high reserve in fluid capacity are more likely to have that fluid capacity act as a buffer against decline.

Experienced workers in less skilled jobs might have more fluid intelligence than their job requires, if for instance they have been performing the same tasks for years and no longer require much fluid intelligence to deal with them.

People in more skilled jobs who have years of experience behind them have more accumulated knowledge to draw on, which offsets a slowdown in fluid intelligence and keeps them productive.

Pharmacists, for example, who have worked for years in their profession have amassed a vast store of “crystallized intelligence” that keeps them productive because of their experience and the knowledge of their work that they have already amassed.

The study concluded that only a minority of workers are vulnerable to a slowdown in cognitive ability: those in jobs that require very high levels of fluid intelligence and those who experience cognitive impairment (i.e., dementia).