It's well understood that a difficult childhood can increase the likelihood of mental illness, but according to new research from the University of South Australia, a happy and secure childhood does not always protect a child from developing a mental illness later in life.

Conducted in partnership with the University of Canberra, the finding is part of a study published in Current Psychology, which examined how early childhood experiences relate to different developmental pathways, and how these might be associated with poor mental health.

Given that both positive and negative childhood experiences were found to manifest as anxiety or other mental health disorders into adulthood, researchers believe that it's our ability to adapt—or rather not adapt—to unexpected scenarios that might be influencing mental health.

In Australia, almost 50% of the population will experience mental illness at some point in their lives, with an estimated 314,000 children aged 4-11 (almost 14%) experiencing a mental disorder.

The national recurrent expenditure on mental health-related services is estimated at $9.9 billion or about $400 per person.

While the study reaffirmed that people who had adverse and unpredictable early life experiences had elevated symptoms of poor mental health (including depression and paranoia), it also found that children who grew up in stable and supportive environments were also at risk of experiencing symptoms of anxiety in adulthood.

Lead researcher, and PhD candidate, UniSA's Bianca Kahl, says the study highlights the indiscriminate nature of mental illness and reveals key insights about potential risk factors for all children.

"As the prevalence of mental health conditions expands, it's imperative that we also extend our knowledge of this very complex and varied condition," Kahl says.

"This research shows that mental health conditions are not solely determined by early life events, and that a child who is raised in a happy home, could still grow up to have a mental health disorder.

"There's certainly some missing factors in understanding how our childhood environment and early life experiences might translate into mental health outcomes in adulthood.

"We suspect that it's our expectations about our environments and our ability to adapt to scenarios when our expectations are not being met, that may be influencing our experiences of distress.

"If, as children, we learn how to adapt to change, and we learn how to cope when things do not go our way, we may be in a better position to respond to stress and other risk factors for poor mental health.

"Testing this hypothesis is the focus of the next research study."