The question of what happens when you separate young children from their parents is not a political one. It’s a scientific one.

And the answers are tragic.

In 1937, John Bowlby began working at the Child Guidance Clinic, a mental hospital for youth in North London. One of the children under Bowlby’s care was a six-year old named Derek, who had been sent to the Clinic for “persistent thieving, truancy and staying out late.” At first glance, Derek’s childhood appeared normal and happy; his parents were loving and well-adjusted. However, Derek’s medical file contained one notable event: when he was eighteen months old, Derek was hospitalized for nine months with diptheria. He was completely isolated from his family. According to Derek’s mother, this separation changed her son. When he returned home, he called her “nurse” and refused to eat. “It seemed like [I was] looking after someone else’s baby,” she said.

Derek’s story led Bowlby to review the histories of his other thieving patients. What he discovered next would define the rest of his career. According to the case files, approximately 85 percent of “affectionless” children prone to stealing had also suffered, like Derek, from a prolonged separation in early childhood. This became their defining trauma. These kids stole candy and toys and clothes, Bowlby argued, to fill an emotional void. “Behind the mask of [affectionless] indifference,” he wrote, “is bottomless misery.”

Bowlby was haunted by this correlation between separation from loved ones and emotional damage. During World War II, Bowlby followed the reports from wartime orphanages. He spoke often with Anna Freud, Sigmund’s youngest daughter and the head of the Hampstead War Nursery, who described the “severe deterioration” of the kids in her care. In many instances, the toddlers were simply not able to cope with the sudden absence of their family. Patrick, for instance, was a three-year old boy whose mother had to work in a distant munitions factory. The boy was distraught, but he refused to cry because his parents said they wouldn’t visit if he cried. And so Patrick constructed an elaborate routine instead, telling himself over and over again that “his mother would soon come for him, she would put on his overcoat and would take him home with her again.” As the days turned into months, Patrick’s monologue became increasingly detailed and desperate: “She will put on my overcoat and leggings, she will zip up the zipper, she would put on my pixie hat.” When the nursemaids asked Patrick to stop talking, he began mouthing the words silently to himself in the corner.

These stories made Bowlby determined to conduct his own study on the “effect on personality development” of an extended separation between children and parents. His subjects were patients in the pediatric wards of hospitals. At the time, British doctors enforced a strict visitation policy, as frequent family contact was believed to cause infection and emotional neediness. Most hospitals limited parental visits to a single hour on Sundays, with no visits allowed for those under the age of three.

It didn’t take long before Bowlby realized that these separations were traumatic. What’s more, the trauma followed a predictable arc, much like the progression of a physical disease. (Bowlby would later compare the damage of separation to a vitamin deficiency, in which the lack of an “essential nutrient” causes permanent harm.) When first left alone at the hospital, the children collapsed in tears and wails; they didn’t trust these strangers in white coats. Their violent protest, however, would soon turn into an eerie detachment, especially if the separation lasted for more than a week. Instead of crying, the children appeared withdrawn, resigned, aloof. It was as if they had forgotten about their parents entirely. The hospital staff referred to this phase as “the settling down.” Bowlby called it despair.

He was right. One of the recurring themes of attachment theory—the psychological theory Bowlby pioneered with Mary Ainsworth and others—is the enduring damage of early separation from one’s parents. (A Book About Love covers attachment theory in detail.) Consider the results of a natural experiment that took place during World War II, when more than 70,000 young Finnish children were evacuated to temporary foster homes in Sweden and Denmark. For the kids who stayed behind in Finland, life was filled with moments of acute stress—there were regular air bombardments and invasions by the Russians and the Germans. But for those sent away, the stress of being separated from their parents was unceasing. They lacked what they needed most.

This early shock had lifelong consequences. A 2009 study found that Finnish adults who had been sent away from their parents between 1939 and 1944 were nearly twice as likely to die from cardiovascular illness as those who had stayed at home. Although more than 60 years had passed since the war, these temporary orphans were also significantly more likely to have high blood pressure, type 2 diabetes, elevated levels of stress hormone and severe depressive symptoms.

The pragmatist philosopher Richard Rorty argued that the ultimate goal of liberalism was the elimination of cruelty. For Rorty, cruelty wasn’t just the infliction of suffering—it meant making others suffer while ignoring their plight. It meant choosing not to care, usually because we focus on our “traditional differences (of tribe, religion, race, customs, and the like)” rather than our “similarities with respect to pain and humiliation.”

The science of attachment theory is a powerful reminder of those similarities. It doesn’t matter where a child comes from: we know what will happen when we separate them from their parents. We are causing pain that lasts, inflicting wounds that might never heal.

It is the very definition of cruelty.

Bowlby, John. "Maternal care and mental health: A report prepared on behalf of the World Health Organization as a contribution to the United Nations programme for the welfare of homeless children." (1952).