Centre-screen, a lion is basking in the sun. Three young cubs tumble, prowl and pounce around him - play-hunting, play-fighting, and occasionally launching themselves at their father, until he loses patience and brushes them away with a mighty paw. Into this picture edges David Attenborough, speaking softly so as not to disturb the family group. ‘Play’, he breathes sonorously, ‘is a very serious business’.

It certainly is. Those lion cubs are learning some of the most important lessons of their lives. They’re developing the physical control and coordination they’ll need for the hunt; they’re establishing the social pecking order within their family pack; and they’re discovering - in a safe, controlled environment - what it’s like to take risks… and what happens when you step over the line. What’s more, they’re enjoying it. The glorious thing about play is that it is fun: the young of every species is designed by nature to learn fundamental physical, social, emotional and conceptual lessons through sheer enjoyment.

As Robert Louis Stevenson put it:

Happy hearts and happy faces
Happy play in grassy places -
That is how, in ancient ages,
Children grew to kings and sages.

Unless, of course, they are denied the opportunity to play outside, or lured away from Stevenson’s ‘grassy places’ to some sort of virtual unreality.

Over the last 15 years or so, age-old play activities (running, climbing, pretend-games, making dens and so on) have been replaced for many children by a solitary, sedentary screen-based lifestyle. Scottish researchers recently found that today’s two-year-olds are as sedentary as the average office worker and that, once acquired, this couch potato lifestyle is difficult to break. As children grow older, the word ‘play’ all too often means sitting down at a PlayStation. Games are something they buy for their Gameboy. And many spend up to six hours a day staring at the TV.

Last year, the neuroscientist Susan Greenfield, president of the Royal Society, urged us to look carefully at the effects of a sedentary screen-based existence on our children and remember that ‘the human brain is exquisitely sensitive to any and every event. We cannot complacently take it as an article of faith that it will remain inviolate.’ One aspect of modern life we should look at very hard indeed is the 21st century erosion of real play.

Children who engage in nothing but screen-based leisure activities miss out on a huge amount of essential learning. Fundamental concepts and knowledge about the world are learned not from screens, but through hands-on, real-life experiences. Making mud-pies or mixing ‘petal perfume’, paddling in puddles or messing in the sandpit, riding a homemade go-kart or climbing a tree - all these activities add to children’s vital store of understanding.

As they’ve become less common, children’s performance on Piagetian tests of conceptual development (common sense understanding of the world) has plummeted. Research from King’s College London by Michael Shayer and Philip Adie shows today’s children lagging, by the age of 11, two to three years behind their counterparts 15 years ago. When I asked Dr Shayer about these results, he pointed out that - as well as a reduction in children’s leisure time outdoor play - this period also saw a sea-change in attitudes to play-based learning in school: ‘1990 was the year sand and water began to move out of the infant classroom.’ What took their place was the subject-centred National Curriculum, including an emphasis on the explicit teaching of science, and assessment on pencil and paper tests.

Children’s performance on science tests has improved steadily over the last 15 years, which at first seems at odds with Shayer and Adie’s results. But as universities report a steady and worrying decrease in students wishing to study science, we need to wonder whether performance on pencil and paper tests is less important in terms of creating scientists than the underpinning concepts acquired from real-life engagement with the world. Some fundamental aspects of learning are perhaps better caught than taught.

As well as conceptual development, there are many other obvious advantages to be gained from play. Vigorous outdoor activities - running, climbing, skipping, kicking a football around - develop children’s physical coordination and control. Pretend play helps them consolidate their understanding of the social world, providing a theatre for language development. (As Vygotsky put it, ‘Children talk a head higher in play’). And imaginative play - transforming a box into a ship, a broomhandle into a horse - doesn’t just develop the imagination; it’s also an initiation into the world of symbolic thought, which underlies academic achievement.

The children’s play expert Tim Gill believes one of the greatest losses is what he calls ‘everyday adventures’, the small but significant experiences that happen when children play out together, away from adult eyes. He believes these are an unpredictable but essential part of growing up - opportunities to make judgements, take risks, learn how to make friends and elude enemies. But they depend upon the freedom to be out and about, not closeted in the home, and children today have less and less freedom to roam beyond the confines of house, garden and school yard.

Second-hand adventures courtesy of television or computer games don’t teach how to move in real space, interact with real people, or take real risks: if it all gets too dull or too scary, the child can just switch off. Thus they don’t prepare children for the real-life risk assessments we make on a day-to-day basis - judging speed and distance when crossing the road or assessing how far to trust other people with our own safety. Without the preparation of play and other independent activities involving relatively ‘safe’ risks, psychologists believe some children eventually become excessively reckless and others excessively timid.

And the changes affect social development too. In the past, it was taken for granted that most children would learn how to make friends, play as part of a group, and resolve minor conflicts through their playtime activities. When this takes place out of adult view, children can take responsibility - and make mistakes - without incurring immediate adult judgement. But many children’s playmates now are screen-based virtual friends, from whom they don’t learn social skills. And practically all real juvenile socialising goes on under the eagle eye of adults - who are naturally swift to intervene if things look dicey. Some children are thus being labelled ‘naughty’ very early in their social careers (and then going on to fulfil the prophecy), while others are learning to call for help at the first sign of danger. Many experts on child development believe that increases in bullying recorded over the last decade may be partly due to over-supervision, helping to create both bullies and victims.

All these changes in children’s play habits have happened over a single generation, helped along by two side effects of modern life. First, the development of technological (and entirely indoor) screen-based entertainment has provided a seductive alternative. Second, a huge increase in parental anxiety has led to restrictions on children’s physical activity and their freedom to play outdoors. Part of this anxiety is rational - for instance, the increase in traffic means the outdoor environment becomes less safe every year, and with more parents out at work there are fewer ‘eyes on the street’ to watch out for children’s welfare - but part of it is highly irrational, and itself a consequence of our multimedia culture.

The fact that we can now view (and repeatedly re-view) distressing images of terrible events as if they are actually happening in front of us has a much greater effect on mental stability than hearing or reading about them. Neuroscientists have found that horrific pictures directly affect the emotional centres in the brain and the more frequently they’re viewed, the more they induce feelings of anxiety. Psychologists researching the World Trade Centre attack on the people of America found that the more TV people watched, the more likely they were to suffer psychological effects, even though they had no personal connection with the atrocity.

Television coverage of horrific news, such as the Soham murders, forces parents to confront their own worst fears over and over again. The fact that this coverage continues remorselessly, 24/7, in the corner of one’s own living room - or in the bedroom just before sleep - makes it even more powerful. So, even though we know that the dangers of child abduction are no greater today than they were 50 years ago, media-induced anxiety overcomes reasoned argument.

Technological culture is thus helping to make parents irrationally over-protective. Research has found that, compared even to the 1990s, today’s children have a smaller and more specified area in which they can play freely, are monitored much more closely by their parents, and have their play curtailed at the first hint of danger. Younger children often scarcely venture out at all, but remain cooped up like battery chickens with their technological toys for company, perhaps the most inactive generation in human history.

And as researcher Michael Shayer suggested, changes in attitudes to primary education over the last couple of decades mean opportunities for learning through play within the classroom have all but disappeared. Child development expert Tina Bruce points out that ‘play cannot be pinned down and turned into a product of measurable learning… [It] is a process enabling a holistic kind of learning, rather than fragmented learning.’ So play has no place in a system dominated by tests and targets, and a curriculum broken into discrete subject areas.

What’s more, as parental paranoia and over-protectiveness grows, children’s recreational play on school premises is directed and supervised as never before. Alarmed by threats of litigation over accidental injury, and by the swelling tide of parental complaints about bullying, headteachers now keep an ever-tighter rein on what happens in the playground. Many schools have reduced the length of playtimes in order to minimise the potential for problems, and plans for a new school in Peterborough don’t include a playground at all.

All this adds up to a steady and remorseless erosion of children’s opportunities to play - and, as a society, we should be deeply concerned at the loss. If so many fundamental lessons are best caught through play, rather than formally taught, we could be rearing a generation which will be neither bright nor balanced enough to keep our technological cultural show on the road. If lion cubs stopped playing - and learning through play how to be lions - the species wouldn’t last very long. Play is, indeed, a very serious business.

Sue Palmer is author of Detoxing Childhood: What Parents Need to Know to Raise Happy, Successful Children (Orion Books).