Shaun King, the writer and Black Lives Matter activist, had a surprise visitor last night. Someone, his doorman called to say, had come to watch the kids.

King replied that he was not expecting any kind of babysitter. But the visitor was insistent. She had been sent by child protective services, she explained.

As King explained in a series of tweets about the encounter, the woman, "demanded to see three of my children. She called them by name. And said that she had to see them immediately."

King told her to get lost, but she wouldn't take no for an answer. She explained that she worked for New York Child Services, and was responding to a formal complaint contending that King and his wife had abandoned the children and allowed them to do drugs.

This might sound crazy. It's actually dismayingly common. Over the course of their childhood, about 37 percent of American kids will receive a visit from CPS, according to a study published in the American Journal of Public Health. And if they're African American, the rate is 53 percent.

That's right: More than half of all African American children will be investigated.

King insisted the investigator connect him to her supervisor, and then explained to the supervisor that somebody—an anonymous critic of King's writings, probably—had played a "cruel prank" on him and his family. Someone was wasting everyone's time. But the supervisor didn't care.

Anonymous reporting is allowed in many states. But just how reliable are these anonymous calls?

Diane Redleaf, legal director of the National Center for Housing and Child Welfare and author of the upcoming book, They Took the Kids Last Night, says that about one in four calls to child protective services are substantiated, but when it comes to anonymous reports, it is only one in 20.

What kind of person would deliberately traumatize a family by calling in a false accusation? Martin Guggenheim, an NYU Law professor specializing in child welfare, could not say conclusively. But he has heard of angry exes who call CPS repeatedly, knowing that each call triggers another investigation. "I once complained to the agency for being utterly insensitive to this problem and asked them to figure out a way to get some sense of whether a caller has made multiple reports that have proven to be unfounded, so that you not only save your own resources but save the parents from the horrible experience of being investigated countless times," he says. "And the agency said we have to do it this way. We have no choice."

But parents put in King's situation have more of a choice than they think. By law, Redleaf said, they do not have to let the investigator in or let them talk to the kids. (This is something the investigators are supposed to inform the parents of.)

On Twitter, King explained that he had spoken with a lawyer, which is absolutely the right thing to do. But it's still a terrifying position for a parents to find himself in, and an indictment of the entire CPS system.

"An anonymous troll has weaponized New York's Department of Child Services against my whole family by filing false reports of neglect and drug use by our children," he wrote.