How many times do you click on your email icon in a day? Or look at Facebook, or Twitter? And how many times when reading on the internet do you click on a link navigating away from the text that was the original object of your enquiry? The web, it seems, is like an electronic sweet shop, forever tempting us in different directions. But does this mental promiscuity, this tendency to flit around online, make us, well, thicker?

The power of modern electronic media – the net, mobile telephones and video games – to capture the attention of the human mind, particularly the young mind, and then distract it has lately become a subject of concern. We are, say the worriers, losing the ability to apply ourselves properly to a single task, like reading a book in its entirety or mastering a piece of music on an instrument, with the result that our thinking is becoming shallower. Sir Tom Stoppard aired a version of this view last week when he warned that the printed page was in danger of being “swept away” on a tide of technology, with the moving image assuming ever-greater precedence in the lives of young people

“I am aware, as everybody has to be, that there’s more competition for one’s attention nowadays,” he said. “The printed word is no longer as in demand as when I was of the age of pupils, or even at the age of the teachers teaching them.”

It was not a question of new media bad, old media good, he added, but the trend was there. And he is in good company. Barack Obama remarked of his over-fondness for tinkering with his BlackBerry: “Information becomes a distraction, a diversion, a form of entertainment, rather than a tool of empowerment.”

Nicholas Carr, the American science writer, has mined this theme for his new book, The Shallows, in which he argues that new media are not just changing our habits but our brains. It turns out that the mature human brain is not an immutable seat of personality and intellect but a changeable thing, subject to “neuroplasticity”. When our activities alter, so does the architecture of our brain. “I’m not thinking the way I used to think,” writes Carr. “I feel it most strongly when I’m reading.”

Years of internet use have, he suspects, dented his ability to read deeply, to absorb himself in books. Dead trees are no longer providing him with the “high” required by his electronically trained mind: “My brain wasn’t just drifting. It was hungry. It was demanding to be fed the way the net fed it.”

He describes getting fidgety when faced with a long text. “When we go online we enter an environment that promotes cursory reading, hurried and distracted thinking, and superficial learning. We are evolving from being cultivators of personal knowledge to being hunters and gatherers in the electronic data forest.”

Carr cites research by Gary Small, a professor of psychiatry at UCLA, who concluded that constant exposure to modern media strengthens new neural pathways while weakening older ones. Just five hours of internet use is enough to awaken previously dormant parts of the brain’s pre‑frontal cortex, concluded Small. For Carr this is proof that the net can rewire the mind.

While not wanting to be a Jeremiah, Carr sees dangers. Deep thought, the ability to immerse oneself in an area of study, to follow a narrative, to understand an argument and develop a critique, is giving way to skimming. Young users of the internet are good at drawing together information for a school project, for example, but that does not mean they have digested it.

Hyperlinks are, he says, a particular problem. They are the “road bumps” in text that tempt you to click on to something else rather than finishing a passage. But is a changing mind a more stupid one? Jake Vigdor and Helen Ladd are researchers at Duke University, North Carolina. In a study spanning five years and involving more than 100,000 children they discovered a correlation between declining test scores in both mathematics and reading and the spread of home computers and broadband.

“The decline in scores was in the order of 1 or 2 per cent but it was statistically significant,” says Vigdor. “The drop may not be that great but one can say that the increase in computer use was certainly not positive.”

The cut-off year for the study was 2005, when socialising was more primitive. Since then, Facebook and Twitter have become enormously powerful consumers of young people’s time. Vigdor and Ladd concluded that the educational value of home computing was best realised when youngsters were actively supervised by parents.

This tendency to skim is compounded by the temptation of new media users to “multi-task”. Watch a youngster on a computer and he could be Facebook‑ing while burning a CD and Tweeting on his mobile phone. Modern management tends to laud multi‑tasking as an expression of increased efficiency. Science, on the other hand, does not. The human brain is, it seems, not at all good at multi‑tasking – unless it involves a highly developed skill like driving.

David Meyer, a neuroscientist at the University of Michigan, says: “The bottom line is that you can’t simultaneously be thinking about your tax return and reading an essay, just as you can’t talk to yourself about two things at once. People may think otherwise but it’s a myth. With complicated tasks you will never, ever be able to overcome the inherent limitations in the brain.”

Paying attention is the prerequisite of memory: the sharper the attention, the sharper the memory. Cursory study born of the knowledge that the information is easily available online results, say the worriers, in a failure to digest it. Perfect for our soundbite culture, but not so good for producing an informed, subtle-minded electorate. In addition, the brain needs rest and recovery time to consolidate thoughts. Teenagers who fill every moment with a text or Tweet are not allowing their minds necessary down-time. All rather worrying, but is it that bad?

We have been here before, of course. The Ancient Greeks lamented the replacement of the oral tradition with written text; and the explosion in book ownership resulting from the printing press was, to some, the work of Satan. In the 18th century, the French statesman Guillaume-Chrétien de Lamoignon de Malesherbes railed against a new device that turned people into “dispersed” individuals, isolated in “sullen silence”. He was talking about the newspaper.

Champions of modern media point to the increased ability of young people to find and manipulate information – to marry video, stills and text for educational purposes.

The net is also supposed to consume the lives of young people; yet the only reliable studies about the time spent online, collated by the World Health Organization, suggest children spend between two and four hours in front of screens, including television screens, and not six or seven, as often suggested. Moreover, there is evidence that youngsters who use sites like Facebook and MySpace have more rewarding offline social lives than those who do not.

The Byron Review on children and new technology, commissioned by the last government, included a “study of studies” by Prof David Buckingham of the University of London’s Institute of Education. He concluded: “Broadly speaking, the evidence about effects [of new media] is weak and inconclusive – and this applies both to positive and negative effects.

“Of course, this does not in itself mean that such effects do not exist. However, it is fair to conclude that directly harmful effects are significantly less powerful and less frequent than they are often assumed to be, at least by some of the most vocal participants in the public debate.”

Certainly, the tired old media do not seem to be doing that badly. An annual survey conducted by Nielsen BookScan shows that sales of children’s books in 2009 were 4.9 per cent greater than in 2008, with more than 60 million sold. The damage, if any, done by excessive computer time may not be so much to do with what is being done online as what is being missed – time spent with family or playing in trees with friends.

And as for the claim that new media is turning us into shallow multi-taskers, here are some wise words from the 18th century and the fourth Earl of Chesterfield. “There is time enough for everything in the course of the day, if you do but one thing at once,” he said, “but there is not time enough in the year, if you will do two things at a time. This steady and undissipated attention to one object, is a sure mark of a superior genius; as hurry, bustle, and agitation, are the never-failing symptoms of a weak and frivolous mind.”