The way that a young child's brain hums along at rest could predict their active intelligence later in life.

Researchers in the United States and Germany have followed up on a past study in Romania to show, for the first time, how a kid's upbringing might influence their brain power years down the road.

The famous study, called the Bucharest Early Intervention Project (BEIP), began in the early 2000s and tracked the cognitive development of abandoned children in Romania.

Comparing the cognitive abilities of fostered and institutionalized children with those who grew up in home care, researchers found institutionalized children had relatively lower IQs at 18 years of age.

In this latest research, scientists found a relationship between brain wave patterns and those IQ scores in the same data.

"These findings demonstrate that experientially-induced changes in brain activity early in life have a profound impact on long-term cognitive development, highlighting the importance of early intervention for promoting healthy development among children living in disadvantaged environments," the researchers write.

Today, the way that a person's brain behaves at rest is thought to remain relatively stable across adulthood, even as active brain power fluctuates with age.

How this stable, resting activity is developed in early life, however, is poorly understood.

As the typical human child grows from a toddler to a 10-year-old, their resting brain activity is usually marked by fewer low-frequency, or slow, brain waves and greater high-frequency, or fast, brain waves.

The slow waves during rest tend to be associated with the pruning of unnecessary neural connections, which makes the brain more efficient when actively tackling mental tasks.

This is a key step in refining a child's cognitive development, but if it occurs too much or for too long, it can become detrimental.

If a young child doesn't receive enough emotional support or cognitive stimulation, evidence suggests it can affect their neurocognitive development, possibly by impacting neural pruning.

The current study, led by researchers at the University of Maryland, supports that idea.

An IQ assessment of 202 18-year-olds who had been enrolled in the Bucharest study revealed those with lower scores tended to have more slow wave activity as toddlers.

The results suggest that slow-wave activity in a child's resting brain can somehow mediate the effects of institutional rearing and the time of foster care placement.

The authors say the "significant correlation" they've identified is "particularly striking" given the long time span between measurements and the number of personal and environmental factors that can influence cognitive development in life's early stages.

Past studies have also found that slower brain waves are especially sensitive to environmental factors, like poverty or sociocultural disadvantages. But this research is the first to connect slow brain waves in childhood to long-term cognitive impacts in young adulthood.

Further studies among larger cohorts will be needed to confirm this correlation and scientists will need to investigate how slow brain waves might drive long-term cognitive changes in practice.

While there is still much work to be done, neuroscientists hope that one day, brain waves in infancy can help us "rapidly identify children at heightened risk for poor cognitive development and also help predict where early intervention may help children with learning difficulties so that they have better outcomes in later life."

The study was published in the Developmental Cognitive Neuroscience.