“OUR sires’ age was worse than our grandsires’. We, their sons, are more worthless than they; so in our turn we shall give the world a progeny yet more corrupt.” That was the way of the world according to Horace, a Roman poet, writing in about 20BC.

He has no shortage of contemporary successors. Doomsayers of the past two centuries have blamed, among other things, novels, the radio, jazz, rock ‘n roll, television, horror films, Dungeons & Dragons, video games, the internet, smartphones and social media for the sad decline of the young. John Protzko, a psychologist at the University of California, Santa Barbara, though, wondered whether things might not be quite so gloomy as they seemed. To try to bring some rigour to the question, he went hunting for examples of a cognitive experiment called the marshmallow test. This test, first performed at Stanford University in the 1960s, measures how good young children are at self-control—specifically, whether or not they can defer a small but immediate reward, such as a marshmallow, in favour of a bigger one later. It was one of the first examples of a standardised psychological test, so it gave him plenty of historical data to work with.

The set-up is simple. A child is taken into a room and presented with a choice of sugary snacks. A researcher explains that the child can choose his favourite treat and eat it whenever he likes—but, if he waits 15 minutes, he can have two instead. The researcher then leaves the room. Age is the strongest predictor of successfully resisting the temptation to scoff the treat straightaway. Among children of the same age, however, doing well on the test is associated with plenty of good things later in life, from healthy weight to longer school attendance and better exam results.

Dr Protzko examined data from 30 studies spanning the past 50 years (though the original Stanford study was not one of them). At the same time, he polled 260 experts in child cognitive development, inviting them to predict what he might find. Just over half thought that children would have become worse at delaying gratification—perhaps thinking about a plethora of recent of studies into the supposedly deleterious effects of modern technology. Another third predicted no change.

Only 16% of the experts made the correct prediction. This is, that children have become steadily and significantly better at the test over the past half century. In 1967, the average waiting time before succumbing to temptation was around three minutes. By 2017, that had risen to eight minutes—an increase of about a minute a decade. And that increase seems to be happening at all levels of ability. The most impulsive children are improving at the same rate as the most prudent.

The rate of increase caught Dr Protzko’s eye as well. That rate, a fifth of a standard deviation every decade, is about the same improvement as has been seen in IQ tests over the past 80 years. (Standard deviation is a measure of variation about a mean value. About two-thirds of a normal distribution lies within one standard deviation of the mean.) The cause of this increase in IQ, which is dubbed the Flynn effect after the psychologist who brought it to the world’s attention, remains mysterious—as does whether Dr Protzko’s results are related to it. IQ is associated with the ability to delay gratification, but the correlation is far from perfect.

What is clear, though, is that Horace and his successors are not only wrong (they must be, or civilisation would have collapsed into the mud long ago), but that, over recent years, youth has actually been improving, at least in some respects. “Talking down the young,” Dr Protzko observes, “seems to be a sort of human cognitive tic.” He is now interested in working out why.