The American Psychological Association has written a letter to President Trump, warning of the lasting psychological harm of family separation and asking the administration to reconsider its immigration policy.

Decades of research have demonstrated that family separation can lead to , , and , among other conditions, the organization writes. They ask that the administration alter its immigration policy so that families can stay together throughout the legal process and obtain physical and mental health services.

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“Based on empirical evidence of the psychological harm that children and parents experience when separated, we implore you to reconsider this policy and commit to the more humane practice of housing families together pending immigration proceedings to protect them from further trauma,” reads the June 14 letter, signed by APA president Jessica Henderson Daniel and APA chief executive officer Arthur Evans Jr.

The letter comes as a response to the Trump administration’s new “zero-tolerance” immigration policy. On May 7, Attorney General Jeff Sessions declared that anyone who attempts to cross the border between the United States and Mexico will be prosecuted. As a result, children have been removed from their parents during legal proceedings—nearly 2,000 children in the six weeks between April 19 and May 31, according to the Associated Press.

The APA previously released a statement about the perils of family separation in late May. The organization wrote the new letter to take a stronger stance as separations continued, Evans says: “As we’ve seen the issue escalate, we’ve also escalated our advocacy and our voice.”

One important element to consider is the duration of the separation, he says, because the longer children are away from their parents, the more likely they are to develop harmful physical and psychological problems.

“I think the country is pretty unified in that as a matter of government policy, we should not be in the business of hurting children and families,” Evans says. “We should have policies that line up with that reality and line up with science.”

The psychological trauma of separation stems from the interconnection of neurological growth and social growth, explains Lisa Fortuna, the medical director of medicine and adolescent at Boston University School of Medicine. Children develop a sense of relational concepts such as and safety as the pathways that govern critical systems like response are still growing. When a parent disappears, the brain is flooded by stress and fear, and nascent pathways can become impaired. The initial impairment can ultimately lead to anxiety, depression, and post- stress disorder, Fortuna says.

“When the parent is not there, it’s almost as if you were trying to walk a tightrope and someone pulls the rope out from under you,” Fortuna said. “The person at the center of your being—who keeps you safe, alive, warm, fed, and loves you—is gone. It’s like taking everything away from you that holds you up. That in itself is a traumatic event.”

Furthermore, immigrant children are often escaping or violent experiences in their home country. Love and support from their parents help children cope with the situation and decrease the likelihood they go on to develop anxiety or depression. “The closeness, even being able to see one another, has biological implications for how that child responds to stress,” Fortuna says. Stripping the child from their guardian decreases the chances the child will emerge unscathed from the stressful circumstances.

The effects of the family separations currently occurring at the border have not yet been documented. But scientists can extrapolate from data on young immigrants who are unaccompanied minors or seeking asylum, Fortuna says. In these populations, around 30 to 40 percent of children have post-traumatic stress disorder and around 50 percent of children have anxiety.

The psychological toll of family separation stretches even further beyond the individuals at the border, says Joanna Dreby, a sociologist at the University of Albany who focuses on immigration. Dreby has studied immigrant children who were separated from their parents and those who were not. The separated children exhibited upsetting behaviors, such as inexplicable crying, an inability to concentrate in school, and feeling like “their heads had exploded,” she says.

But children whose parents were not detained or deported were also affected by deep fear and insecurity that their parents would be taken, she notes. “The government is using fear to police our borders, and that has an impact on children who live far from the borders, who live in families with immigrant parents all over the country,” Dreby says. “That’s the devastating fallout.”