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.”

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.