UK children are losing contact with nature at a "dramatic" rate, and their health and education are suffering, a National Trust report says.
Traffic, the lure of video screens and parental anxieties are conspiring to keep children indoors, it says.
Evidence suggests the problem is worse in the UK than other parts of Europe, and may help explain poor UK rankings in childhood satisfaction surveys.
The trust is launching a consultation on tackling "nature deficit disorder".
"This is about changing the way children grow up and see the world," said Stephen Moss, the author, naturalist and former BBC Springwatch producer who wrote the Natural Childhood report for the National Trust.
"The natural world doesn't come with an instruction leaflet, so it teaches you to use your creative imagination.
"When you build a den with your mates when you're nine years old, you learn teamwork - you disagree with each other, you have arguments, you resolve them, you work together again - it's like a team-building course, only you did it when you were nine."
The trust argues, as have other bodies in previous years, that the growing dissociation of children from the natural world and internment in the "cotton wool culture" of indoor parental guidance impairs their capacity to learn through experience.
It cites evidence showing that:
- children learn more and behave better when lessons are conducted outdoors
- symptoms of children diagnosed with ADHD improve when they are exposed to nature
- children say their happiness depends more on having things to do outdoors more than owning technology.
Yet British parents feel more pressure to provide gadgets for their children than in other European countries.
Anger over traffic
The phrase nature deficit disorder was coined in 2005 by author Richard Louv, who argued that the human cost of "alienation from nature" was measured in "diminished use of the senses, attention difficulties and higher rates of physical and emotional illnesses".
In the UK as in many other countries, rates of obesity, self-harm and mental health disorders diagnosed in children have climbed significantly since the 1970s.
But nature deficit disorder is not generally regarded as a medical condition.
"There's undoubtedly a phenomenon that's not good for health, which is about not giving access to outdoors or green space, safe risk-taking and so on," said David Pencheon, a medical doctor who now heads the National Health Service's sustainable development unit.
"But I wouldn't say we've identified a medical condition.
"In fact we don't want to 'medicalise' it, we should see it as part of everyday life - if you medicalise it, people say 'you'd better go to your doctor and take a pill'."
But despite growing recognition of nature deficit disorder, policies aiming to tackle it appear thin on the ground.
Mr Moss cites statistics showing that the area where children are allowed to range unsupervised around their homes has shrunk by 90% since the 1970s.
Whereas some reasons behind the parental "cotton wool culture" are not based in logic - most sexual molestation occurs in the home, for example, not in parks - the one "genuine massive danger" is traffic.
"I think the first step for any child is playing outdoors in the street; and in the 40 years since I grew up, traffic has increased hugely, and that's the main reason why none of us let our kids out on their own," Mr Moss told BBC News.
"The only solution would be to have pedestrian priority on every residential street in Britain; when you are driving along the street, if there are children playing, they have priority."
The report advocates having teachers take children for lessons outdoors when possible, with urban schools using parks.
It also says that authorities who cite "health and safety" as a reason for stopping children playing conkers or climbing trees should be aware that successive Health and Safety Executive heads have advocated a measure of risk-taking in children's lives.
The changes in childhood in previous decades are now filtering through into adulthood, where levels of obesity are also rising.
Dr Pencheon observed that although doctors are beginning to prescribe exercise instead of drugs where it is indicated, much more could be done from a policy perspective.
"One of the problems here is that the NHS is not incentivised financially to do public health," he said.
"The healthcare system is run on a rescue basis - people come to us when they're ill, we patch them up and try to get them going again - that's not the culture of a system designed to keep people healthy."
The National Trust is now beginning a two-month consultation aimed at gathering views and examples of good and bad practice from the public and specialists.
These will eventually be turned into a set of policy recommendations.
"As a nation, we need to do everything we can to make it easy and safe for our children to get outdoors," said National Trust director-general Fiona Reynolds.
"We want to move the debate on and encourage people and organisations to think about how we take practical steps to reconnect children with the natural world and inspire them to get outdoors."
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