As the world grows ever more urban, the lack of natural outdoor space could have lifelong consequences for our mental health. Growing up around greener spaces has consistently been linked to better health outcomes, and new research suggests these benefits could last a lifetime.
It's one of the first epidemiological studies to show an association between less contact with the natural world in childhood and worse mental health in adulthood.
Collecting data from nearly 3,600 individuals in four different European countries, researchers at the Barcelona Institute for Global Health have found that these childhood experiences are associated with feelings of nervousness and depression in adulthood.
The study coordinator, Wilma Zijlema, says this shows the importance of both green and blue spaces (both water and land), not only for a "nature-appreciating attitude", but "a healthy psychological state in adulthood".
Answering questions about their childhood - such as whether they had hiked in national parks or played in their backyard - the participants were then asked to take a psychological test, determining their feelings of nervousness, depression and fatigue in the past month.
The results show that participants who scored lower in the mental health tests also had less exposure to nature in childhood, and this was true regardless of how much time they spent in nature as adults. What's more, these participants didn't seem to place as much significance on natural spaces in general.
"In general participants with lower childhood exposure to nature gave a lower importance to natural environments," explains environmental researcher Myriam Preuss.
Of course, this research can only draw correlations, and has "limited capability to establish a causal relationship", according to the team. The participants reported on their childhood nature experiences retrospectively, which means they could be biased in their recollections.
"Longitudinal studies that objectively measure childhood [nature] exposure and health data are needed in order to investigate associations between accessibility of [nature], time and activities spent in nature during childhood, and mental as well as physical health throughout the life course," the authors conclude.
Nevertheless, these results do not exist on their own. A nationwide study in Denmark found that those residential areas with less greenery have a higher risk for psychiatric disorders in adulthood. Another in the US found a link between residential greenness during childhood and a lower risk of depressive symptoms in adulthood.
A study published just last year, and authored by one of the same researchers as this latest one, found that exposure to green spaces is correlated with structural changes in a developing kid's brain.
"This is just kind of a hypothesis," the author, Mark Nieuwenhuijsen told Inverse.
"I think the reason for it is, in general, our brains are still wired for when we were still living in the savannahs and jungles with a lot of nature around us. It's only the last few hundred years that we have moved into cities. Our brains are not really adjusted to that. It creates a kind of stress, and in particular, there's a lot of brain development happening at young ages."
In short, there is compelling evidence that the way we are designing our cities could harm the mental health of future generations. As such, the authors at the Institute for Global Health are calling on policymakers to ensure natural spaces for children, green schoolyards, maybe even nature curriculum as our cities develop.
Currently, 73 percent of Europeans live in urban areas. By 2050, that will be more like 80 percent. Finding out how much nature is enough is hard to say, but with cities swelling and nature shrinking, putting aside even a little space could be crucial to our continued health.
"In most countries, activities in nature are not a regular part of the school curriculum," the authors write.
"Consequently, children who do not have opportunities to interact with and gain an appreciation of nature at home, have little chance to experience contact with nature."
This study has been published in the International Journal of Environmental Research and Public Health.