A bill that would release the 112 children in immigration detention in Australia will soon go before the House of Representatives. The bill passed the Senate last week, but it could be rejected by a government-majority House.

While it’s widely accepted that detention is bad for child asylum seekers, the long-term effects of that harm are rarely spelled out. Our recently published research sheds some light on this.

So what should members of parliament consider when casting their votes?

Growing brains are vulnerable

The brain has evolved to respond effectively to stressful situations, many of which are normal challenges of everyday life. Indeed, some researchers argue that humans’ extended period of childhood and adolescence (compared to other species) has evolved to maximise our adaptability to the varied environments and social dynamics we traverse. What defines these periods, in this context, is a changing brain, a brain trying to adapt.

This adaptability, however, comes at a cost: growing brains are more vulnerable. Repeated trauma in childhood appears to change children’s enduring hormonal function and brain development, and increases the risk of developing a range of psychological disorders.

Cortisol is often referred to as the “stress” hormone. It plays a complex and wide-ranging role in the human stress and arousal response. Cortisol is also central to glucose availability, blood pressure and immune function.

Alterations in cortisol function are found in people with depression and post-traumatic stress disorder (PTSD), but are also a risk factor for these disorders. A recent study found soldiers with lower cortisol output before going to war in Afghanistan were more likely to develop PTSD symptoms after traumatic events experienced during their deployment.

Cortisol has traditionally been measured in saliva, blood or urine, reflecting output over minutes or days. These studies have found either unusually high or low levels of cortisol in children who have experienced maltreatment. This defective regulation suggests a system initially pushed into overdrive, and then overwhelmed, becomes fatigued.

But this changing physiological picture, and our constantly varying levels of cortisol, has led to many inconsistent findings.

What did our research find?

Our new research studied the cortisol levels of 70 nine-year-old children living in and around Melbourne. We recorded these levels from scalp hair to determine their total cortisol output over months.

We found that the number and types of traumatic events experienced earlier in childhood correlated with hair cortisol levels. These events included illness and deaths in the family, and being sick or hurt in an accident. More extreme events, such as fires, floods or being threatened or attacked, were relatively uncommon.

Traumatic experiences in childhood alter the development of brain structures during adolescence. These structures include those directly linked to cortisol production, such as the pituitary gland, as well as others linked with emotion processing and memory, such as the amygdala and hippocampus. Alterations in these structures have been identified in mental health disorders.

We have shown that adversity increases the risk of mental health disorders in adolescence. These disorders in turn further alter the way the brain develops.

Childhood adversity, particularly maltreatment, is associated with increased risk for numerous behavioural problems. These include drug use, suicide attempts, risky sexual behaviour and sexually transmitted infections. The earlier the experience of adversity in childhood, the greater the risk of poor mental health outcomes.

What does this mean for detained children?

Clearly there are differences between the environments of our Melbourne families and those of detained children. But these differences are largely matters of degree.

Children in detention are at very high risk of exposure to physical and sexual assault, family separation, environmental deprivation and forced relocation. They also commonly witness traumatic events affecting loved ones. These experiences roughly double their risk of developing mental health problems later in life.

The more traumatic events a child experiences, the more likely lasting problems will emerge. A recent German assessment of families seeking refugee status found that 86.5% had already experienced multiple traumas involving war, torture and involuntary displacement.

Children entering detention in Australia already have elevated rates of psychological problems. The trauma of detention is likely to compound these harmful effects, which may persist long after the resolution of the immigration process.

Childhood is a sensitive and vulnerable period; experiences of trauma and adversity can produce harm that endures into adulthood. This inescapable knowledge should inform policy on the release of children from detention and remind us of the care and support they will need when they are finally released.