A new study of abandoned Romanian children shows that placement in a foster home before their second birthday affected how well they responded physically to stress a decade later.

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Some of the most powerful evidence for the importance of parental love in the first years of life comes from the Bucharest Early Intervention Project ­– a 15-year study of abandoned Romanian babies who either were raised in orphanages or placed with foster parents.

The babies who remained in institutional care for their first four-and-a-half years had profound deficits and delays in brain development that affected their thinking and emotions years later, according to a growing body of research stemming from the project.

But some of those problems were improved or even reversed when children were randomly placed in foster homes in the community, especially if they were removed from institutions before their second birthday.

Now a new University of Washington study associated with the same project shows that the complex system that orchestrates the body’s response to stress could be particularly vulnerable to the absence of intimate parenting during the first two years.

Kids who remained in institutions were less able to muster a healthy stress response at age 12 than kids who were placed in foster care before their second birthday, according to the study published this week in Proceedings of the National Academy of Sciences.

The Romanian babies had their basic physical needs met in the state-run institutions, but they didn’t get the kind of individual, consistent attention that typically developing kids get from their parents.

“There’s something about those first two years of life where these systems in the body are paying careful attention to what’s going on in the social environment,” said Katie McLaughlin, director of the University of Washington’s Stress & Development Laboratory and the study’s lead author.

The stress response system in a nutshell describes how the brain sends marching orders to the heart, lungs and muscles to respond to threats, which can be physical or social, like getting evaluated by a boss.

A healthy system is flexible and ramps up quickly, giving us sharpened senses and a boost of energy, then dissipates quickly when the threat has passed. Though the response system evolved for life or death situations, it also helps us adapt to everyday challenges in school or on the job.

But early adversity that repeatedly activates the system can throw off its sensitivity, leading to extreme responses to stress that can cause significant physical and mental health problems later in life.

Researchers are still figuring out how it works, but it appears that parents help tune their babies’ stress response systems in the first 24 months to cope with life’s everyday challenges and that the calibration lasts through childhood.

Children in McLaughlin’s study who were placed in foster homes within 24 months showed responses that were nearly identical to those of typically developing children in the community who had never been institutionalized.

Although the Romanian example is extreme, the research may shed light on the effects of child neglect and poverty in the U.S., McLaughlin said.

“By studying these kids who had a very extreme early environment, we learn what happens when the system is missing an input it certainly expects, which is the presence of a stable, responsive caregiver,” McLaughlin said.