When children are forcibly separated from their parents, “the effect is catastrophic,” said Charles Nelson, a pediatrics professor at Harvard Medical School.

Their heart rate goes up. Their body releases a flood of stress hormones such as cortisol and adrenaline. Those stress hormones can start killing off dendrites — the little branches in brain cells that transmit mes­sages. In time, the stress can start killing off neurons and — especially in young children — wreaking dramatic and long-term damage, both psychologically and to the physical structure of the brain.

“There’s so much research on this that if people paid attention at all to the science, they would never do this,” Nelson said.

That research on child-parent separation is driving pediatricians, psychologists and other health experts to vehemently oppose the Trump administration’s new border crossing policy, which has separated more than 2,000 immigrant children from their parents in recent weeks.

The American Academy of Pediatrics, the American College of Physicians and the American Psychiatric Association have all issued statements against it — representing more than 250,000 doctors in the United States. Nearly 7,700 mental-health professionals and 142 organizations have also signed a petition urging President Trump to end the policy.

“If you take the moral, spiritual, even political aspect out of it, from a strictly medical and scientific point of view, what we as a country are doing to these children at the border is unconscionable,” said Luis H. Zayas, a psychiatry professor at the University of Texas at Austin.

“The harm our government is now causing will take a lifetime to undo,” Zayas said.

The effects of child-parent separation

Child-parent separation can have long-lasting effects:

• While conducting research in Romania’s state-run orphanages, Nelson found that children separated from their parents at a young age had much less white matter in their brains. White matter is largely made up of fibers that transmit information throughout the brain. The children also had less gray matter, which contains the brain-cell bodies that process information and solve problems.

The children Nelson observed had been separated from their parents in their first two years of life. He found that, as they grew older, they scored significantly lower on IQ tests. Their fight-or-flight response system also appeared permanently broken. Stressful situations that would usually prompt physiological responses in other people — increased heart rate, sweaty palms — would provoke nothing in the children.

Aboriginal children in Australia who were removed from their families were nearly twice as likely to be arrested or criminally charged as adults, 60 percent more likely to have alcohol-abuse problems and more than twice as likely to struggle with gambling.

• In China — where 1 in 5 children live in villages without their parents, who migrate for work — studies have shown that those “left-behind” children have markedly higher rates of anxiety and depression later in life.

• Other studies have shown separation leading to increased aggression, withdrawal and cognitive difficulties.

The reason child-parent separation has such devastating effects is because it attacks one of the most fundamental and critical bonds in human biology.

From the time they are born, children emotionally attach to their caregiver and vice versa, said Lisa Fortuna, medical director for child and adolescent psychiatry at Boston Medical Center. Skin-to-skin contact for newborns, for example, is critical to their development, research shows. “Our bodies secrete hormones like oxytocin on contact that reinforces the bond, to help us attach and connect,” Fortuna said.

A child’s sense of what safety means depends on that relationship. And without it, the parts of the brain that deal with attachment and fear — the amygdala and hippocampus — develop differently. Children separated from their parents can develop PTSD later in life because those neurons start firing irregularly, Fortuna said. “The part of their brain that sorts things into safe or dangerous does not work like it’s supposed to. Things that are not threatening seem threatening,” she said.

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