Images of children lined up in tent cities, living in deserted Walmarts, or crammed together into cages are horrifying child development experts, who say the kids are suffering severe trauma.

They are a reminder of why the United States no longer has orphanages, several experts told NBC News.

And, they say, there is no excuse for it.

“This is one of the most significant traumas that a child can experience,” said Nim Tottenham, who studies child brain development at Columbia University.

Tottenham should know. She’s researched children adopted into U.S. families from orphanages in other countries and said the trauma can be very difficult to overcome.

“Once a country has the means, they abandon institutional care because it is known that this is not a great way for children to grow up,” Tottenham told NBC News.

“In the U.S. we don’t have formal orphanages any more for that reason.”

The Trump administration has given several reasons for the new policy, but critics from Republicans in Congress to governors from both parties have said it’s not a legal requirement and denounce it as immoral.

More than 2,300 children have been taken from their parents at the U.S. border with Mexico since early May, and put into facilities including "tender age" shelters for the littlest children.

Pediatricians, internal medicine specialists, family practitioners, the American Medical Association and public health organizations have all said taking children away from their parents does permanent damage to them.

Nathan Fox, a child development specialist at the University of Maryland, sees similarities to the notorious orphanages found in Romania after the 1989 revolution there and the downfall of dictator Nicolae Ceaușescu. At least 100,000 children were found in the network of orphanages, the product of strict anti-abortion, anti-birth control policies.

“The way to ameliorate these affects is to reunite the kids with their parents."

Fox has followed some of these children and tracked their development, and says there are parallels with the border detention centers.

The conditions in Romania were much worse than the conditions of the large warehouses where the border children are, but there is a common element, he said.

"There is no one-on-one care," said Fox. "There is not an adult who is the child’s caregiver who is buffering that child’s stress and to whom that child looks to for safety, protection and security.”

American Academy of Pediatrics president Dr. Colleen Kraft has described how she saw toddlers crying hopelessly as workers in one center insisted they were not allowed to touch or comfort the children.

Massing the children together in the centers is an extremely harmful situation, Fox said.

“They’re imprisoned and separated from their parents, who they look to for safety, security, love and protection," Fox said. "That will have a significant impact upon these children’s behavior and upon their mental health.”

Sudden, often violent separation

Some supporters of the policy argue that this treatment is no different from separating the children of people convicted and imprisoned for crimes.

Children in an orphanage in Bucharest, Romania, circa 1995. Romano Cagnoni / Getty Images file

But it’s not the same, said Dr. Julie Kaplow, director of the Trauma and Grief Center at Texas Children’s Hospital.

For one thing, the separations at the border are sudden, unexpected and often violent.

“These children are being brought to this country by a caring caregiver who wants the best for them and wants them to be safe,” Kaplow said. “Then they are put in a situation where a caring caregiver is taken away from them.”

When a parent is convicted of a crime, there is time for a child to adjust to the separation, Kaplow said.

Plus, the children of U.S. citizens convicted of crimes are not put into institutions, which were eliminated for young children in the U.S. long ago, Fox said. If a child is abused or neglected and if child protective services gets involved, the child is placed in a foster home.