Last night a rare purple emperor butterfly briefly flew into my South London living room. It was a magical moment, one that instantly took me back to my childhood and the only other time I’ve ever seen one.

I could see the flowers in our front garden it rested on, wings open; the smell of the suntan lotion my sister had on and the sound of my dad’s old Olympus as he snapped picture after picture. We felt singled out; we glowed; we talked about it to anyone who would listen.

I can remember watching my first barn owl, too, quartering a field in low, late afternoon sun on silent wings. I can still recall the time a grass snake slithered over my bare feet; the fiery orange belly of the great crested newt I caught in our pond; and the exact texture of the bark of my favourite tree, the one I always used to climb and play spaceships high in its branches.

And yet it’s becoming harder and harder for children to have these experiences. It’s hardly surprising: the roads are busier, 24-hour rolling news means we’re all too aware of ‘stranger danger’ (although today’s children are no more at risk than in the past) and computers and games consoles have made the indoors a lot more alluring than it was when we were young.

But we must at least question the trend to keep kids inside and supervise them at all times – for their sakes. There is a critical weight of evidence building that reducing outdoor play and free contact with the natural world is taking a heavy toll on the next generation.

‘Nature deficit disorder’ is a term coined by Richard Louv to describe “the human costs of alienation from nature, among them: diminished use of the senses, attention difficulties, and higher rates of physical and emotional illnesses.”

The evidence that we need contact with nature is growing, both for adults and children, as the National Trust recently summed up in their fascinating report, Natural Childhood:

• Child psychologist Aric Sigman has found that children exposed to nature scored higher on concentration and self-discipline; improved their awareness, reasoning and observational skills; did better in reading, writing, maths, science and social studies; were better at working in teams and showed improved behaviour overall
• A recent UK study endorsed by Natural England found that those with close access to green space lived longer than those with no green space, even when adjusted for social class, employment, smoking etc
• Exposure to nature has been found to reduce symptoms of ADHD in children threefold compared to time spent indoors
• Not only are children who regularly spend time outdoors less likely to suffer from childhood obesity, but the physical benefits of outdoor play continue into later life. There is clear evidence to show that a child’s attitude towards exercise lays the foundation for their habits as an adult.
• Researchers from Bristol University and University College London have discovered that a “friendly” bacteria commonly found in soil activates brain cells to produce serotonin in a similar way to antidepressants
• A National Trust survey has revealed that 80% of the happiest people in the UK said that they have a strong connection with the natural world, compared with less than 40% of the unhappiest
• So clear is the link between increased contact with nature and better mental health that in 2007 the charity MIND launched a campaign to incorporate nature into mainstream NHS treatments
• Studies have shown that even in cases where the only variable is the view of green space from a window, incidences of crime are reduced by as much as 50%.
• According to MIND, research has shown that ‘green exercise’ (gardening, walking, conservation work, running or cycling) can be as effective as antidepressants in treating mild to moderate depression.
• A study published in the journal Science by environmental psychologist Roger Ulrich examined people recovering from surgery in hospital. All other things being equal, patients with windows looking out on leafy trees healed, on average, a day faster, needed significantly less pain relief and had fewer complications than those who saw a brick wall
• A study carried out at the University of Essex found that as little as five minutes of ‘active time’ spent outdoors boosted mental health. “We believe there would be a large potential benefit to individuals, society and to the costs of the health service if all groups… were to self-medicate more with green exercise,” said researcher Jo Barton.

What’s more, increasing alienation from nature threatens nature itself: as David Attenborough has said, “No one will protect what they don’t care about; and no one will care about what they have never experienced.”

To protect wild places and creatures, and give our children the best possible start in life, we need to help them develop a felt connection to the wildlife and nature on their doorsteps – wherever they live.

Click here for a list of upcoming family-friendly events in the London area and help your kids go wild this summer.

Melissa Harrison is a nature writer and novelist whose book, Clay, has been chosen for Amazon’s Rising Stars programme and shortlisted for the Portsmouth First Fiction award. She regularly writes and speaks on the subject of outdoor play for organisations including the National Trust and Urbanwoot. Follow her on Twitter here: @m_z_harrison