Smartphones. Facebook. Reality TV.

It's easy to look at our changing world and come to the following conclusion: we're all getting stupid.

IQ numbers would suggest the opposite.

But now, some scientists worry that we are starting to reverse those gains.

A nation of genuises

For the last century, we've been getting smarter - fast. The average IQ of the population has been increasing by about three points per decade.

Even Mensa has noticed. "Our membership is growing at about 10 per cent a year," says Teresa Wong, the organisation's recruitment officer. "And about a third of them are under 18."

Were we to test people today using IQ tests from the 50s, 75 per cent of the population would be classified as gifted (to cope with this, IQ tests are regularly made progressively harder).

That has had significant benefits.

The average IQ of the population has been increasing by about three points per decade. As we have access to information any time we want.

"We know we're less violent – by a long shot. We seem to be living longer and being healthier. We have more complex lives, more complex skills. We can drive cars, we can do all sorts of things. We're accelerating and becoming smarter," says Dr Tony Florio, a clinical psychologist and senior lecturer at the University of NSW.

The same increase in IQs over the last century has happened in nearly every other developed nation. It's known as the Flynn effect after James Flynn, the New Zealand researcher who discovered it.

There are several theories for why this has happened: we're better educated, modern education systems mould our brain in ways IQ tests tend to like, and many of us will do cognitively-taxing things like play video games during our leisure hours.

But as a society, we also got healthier. We eat better. Modern medicine means childhood diseases are less likely to stunt brain development. Some even suspect the phasing out of lead in fuels played a role. For the same reasons that successive generations are taller, they are probably smarter, too.

Going backwards

But something strange and troubling appears to be happening. In several countries – Norway, Denmark, Finland, Britain and France – IQ scores that rose for so long now seem to be dropping, at a rate of about 2.4 points per decade in recent decades.

What's going on?

Scientists first suspected that, because people with higher IQs tend to have fewer children, average intelligence would slowly fall over time. But a recent paper, published in PNAS in June, appears to rule this out by showing that IQs are declining even within families.

After 1975, on average, older brothers tended to be more intelligent than younger brothers – a reversal of the trend up to that point. So it's not genes, or natural selection.

The rise of the smartphone

Could it be that our smartphones are making us dumber?

We can look up any fact in an instant. We never need to remember facts or directions or friends' phone numbers. Is that making us more stupid?

The picture is complicated, and there is not enough evidence yet to really say one way or another. But several interesting studies point to smartphones having at least some effect on cognition.

Studies have linked smartphone use with a decreased ability to exert high levels of focus and poorer attention control – although video games seem linked to better multitasking abilities.

In one influential study, volunteers were asked to type newly-learned trivia facts into a computer. Those who were told the computer would remember the facts for them performed much worse on a later memory-recall test. The researchers dubbed it the Google Effect.

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In several countries – Norway, Denmark, Finland, Britain and France – IQ scores that rose for so long now seem to be dropping.

In another study, volunteers were given a series of questions that pushed the limits of their brain. Those who said they were heavy smartphone users tended to perform worse and be less analytical. Poor academic performance has also been linked to heavy smartphone use.

More dramatic was a 2013 experiment in which volunteers were given a camera and asked to take pictures of various objects in a museum, while just looking at other objects. A day later, they were able to better-recall seeing the objects they hadn't photographed.

Smarter – or better adapted?

Does that make us dumber? Or are our brains just adapting to the demands of the modern world?

Flynn himself argues that his namesake 'effect' does not necessarily show we're getting smarter – it's just that our brains have become more 'modern'.

"We used to have a lot more common sense, and understood things more from our own common experience," says Dr Florio.

"There is less emphasis on rote learning today. I teach students, and their handwriting is worse – because they are more keyboard dependent. I don't think kids can do multiplication in their head as easily either."