After dropping to a historic low in 2012, the number of children in foster care has steadily increased in recent years. The number entering foster care because of parental drug use, in particular, has more than doubled since 2000 as the opioid crisis has consumed the United States. Lamb and psychologists like her are the ones tasked with helping to decide what will happen to these children. They find themselves in a difficult situation: Based on just a few, stilted interactions, they must describe the true nature of the bond between a child and a mother—she’s usually a single mother, Lamb says—and predict how it will play out into the future. If the mother’s care seems unsafe, they must recommend separating her from her child, an outcome most Americans find tragic.

Lamb recognizes the importance of preventing abuse and neglect, but she nevertheless feels some unease about helping to make distinctions between good and bad mothers. Does running into the water without your child make you a bad mom? Does packing cheese sticks make you good enough? If not, what does?

Lamb is a psychology professor at the University of Massachusetts in Boston. Separately, she also takes two or three of these child-custody cases each year. “They’re so stressful,” she told me in an interview. At one point in her book, Lamb mentions she has considered giving up these types of evaluations altogether. She began doing them shortly after she moved to Vermont in 1996, when a local attorney called her to evaluate the case of a boy in foster care. That led to her name being bounced around listservs of family-court lawyers.

Typically, Lamb says, the Department of Children and Families calls her in to evaluate cases involving younger kids who have been in foster care for more than a year and whose parents still aren’t improving. In more than half the cases she works on, the judge permanently places the child with a new family. Lamb told me she sometimes does evaluations for parents’ attorneys, too, who want her to show the courts that their clients are fit.

To be clear, parents who are in danger of losing their children have done more than simply hurt their kids’ feelings at the beach. Typically, Lamb and other experts told me, those parents’ cases involve multiple reports of abuse or neglect, including heavy drug use at home and a failure to take advantage of the help offered by social workers. A neighbor might call because a 2-year-old is left unattended; a teacher might call because a child is not coming to school. Some parents are isolated and friendless, and child protective services is the final net that catches them. It is the point at which the state says that adults might live that way, but children can’t.

Lamb understands all this, but in a deeply personal way, she also knows how even the best-intentioned parenting can’t prevent every calamity. Lamb’s own son, she writes, struggled with addiction. At one point in the book, she weighs what she calls “motherlove”—the special connection between a birth mother and her child—against “the stability of a foster home.” She asks herself, What kind of stability did I provide? and wonders whether her mothering contributed to her son’s addiction. One section is dedicated to the slog of finding a rehab facility for her son, and several more to the lies and worries that come with having an addicted family member.