The fun of splashing at the beach or exploring a creek is an iconic part of childhood for countless kids around the world. It may also be a valuable investment in lifelong mental health, according to a new study.

Spending time in nature can significantly reduce stress, improve mood, and offer other mental health benefits, yet despite the mounting scientific evidence, many people still don't do it.

That isn't always by choice, of course. Not everyone has easy access to wilderness, and while some city parks and urban forests offer similar benefits, they're not universally available, either.

For many people, it's a matter of motivation. Even when we could spare one hour to hike in a forest or relax by a riverbank, we may not feel adequately compelled. Urban environments are full of duties, diversions, and distractions that hog our attention.

And as the new study suggests, our varying motivation to visit nature as adults – and thus our varying exposure to the potential health benefits – may be rooted in our youth. This supports earlier research linking childhood experiences in nature with adulthood mental health.

"In the context of an increasingly technological and industrialized world, it's important to understand how childhood nature experiences relate to well-being in later life," says lead author Valeria Vitale, a Ph.D. candidate at Sapienza University of Rome.

Using data from 16,000 people around the world, Vitale and her colleagues found that adults who expressed being cheerful and in good spirits relatively frequently were more likely to report having played in and around "blue spaces" as children, referring to watery environments like oceans, lakes, rivers, and creeks.

Previous research has linked exposure to blue spaces with improved mental health, similar to the famous benefits of forests and other vegetated habitats known as green spaces.

And since many blue spaces are actually blue-green, with trees or other substantial plant life around the water, "childhood exposure to blue spaces is likely to play a role in predicting future frequency of green space visits as well," the authors of the new study write.

The research uses data from the BlueHealth International Survey, a multi-country survey "focused on the recreational use of blue spaces and their relationship with human health," according to its website.

In the survey, respondents were asked to recall childhood experiences in blue spaces from birth to 16 years of age, along with details such as how often they went, how far they had to travel, and how comfortable their parents or guardians seemed in letting them play there.

They were also asked about their lives today, including their emotional and psychological well-being over the past two weeks and how often they've visited green or blue spaces in the past four weeks.

Adults who remembered more childhood experiences in blue spaces tended to place more intrinsic value on natural spaces in general, the survey found, and tended to visit them more often. Both of those traits were in turn linked with better mental health in adulthood.

Intrinsic motivation is "the desire to do something because it is pleasurable, valuable, and inherently rewarding," the researchers write. In this study, people who saw more intrinsic value in nature also visited natural settings more often and experienced better mental health.

That fits with previous research suggesting intrinsic motivation can make a big difference.

Not only does intrinsic motivation seem to help create interest and engagement in a beneficial leisure activity, the researchers note, but it might also make the activity even more beneficial.

Someone intrinsically motivated to perform an activity tends to be more relaxed, more focused, and less burdened by negative emotions than someone motivated extrinsically, previous research suggests. This may boost enjoyment of the activity, further reinforcing it as a habit.

"Our findings suggest that building familiarity and confidence in and around blue spaces during childhood may stimulate an inherent joy of nature," Vitale says, "and encourage people to seek out recreational nature experiences, with beneficial consequences for adult mental health."

Of course, letting small kids play around water can be harrowing for parents and guardians, who must consider a child's immediate safety as well as long-term happiness.

"Water settings can be dangerous for children, and parents are right to be cautious," says co-author Leanne Martin, postdoctoral research associate at the University of Exeter's European Centre for Environment and Human Health.

"This research suggests, though, that supporting children to feel comfortable in these settings and developing skills such as swimming at an early age can have previously unrecognized life-long benefits."

The study was published in The Journal of Environmental Psychology.