This is what happens inside children when they are forcibly separated from their parents.

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.

“The effect is catastrophic,” said Charles Nelson, a pediatrics professor at Harvard Medical School. “There’s so much research on this that if people paid attention at all to the science, they would never do this.  To pretend that separated children do not grow up with the shrapnel of this traumatic experience embedded in their minds is to disregard everything we know about child development, the brain, and trauma.”

Nelson has studied the neurological damage from child-parent separation — work that he said has often reduced him to tears.  As the children grew older, Nelson and his colleagues began finding unsettling differences in their brains.

Those separated from their parents at a young age had much less white matter, which is largely made up of fibers that transmit information throughout the brain, as well as much less gray matter, which contains the brain-cell bodies that process information and solve problems.

The activity in the children’s brains was much lower than expected. “If you think of the brain as a lightbulb,” Nelson said, “it’s as though there was a dimmer that had reduced them from a 100-watt bulb to 30 watts.”

The children, who had been separated from their parents in their first two years of life, scored significantly lower on IQ tests later in life. Their fight-or-flight response system 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.

What alarmed the researchers most was the duration of the damage. Unlike other parts of the body, most cells in the brain cannot renew or repair themselves.  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 mother.  If separated from her, a caregiver can mitigate some of that damage but not all of it.

“Our bodies secrete hormones like oxytocin on contact that reinforces the bond, to help us attach and connect,” Lisa Fortuna, medical director for child and adolescent psychiatry at Boston Medical Center 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.

The reason such children often develop PTSD later in life is that 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.