What Solomon calls a “horizontal” identity is one that separates you from your parents. Illustration by Brian Cronin

For Megan and Michael, a Los Angeles couple, the crucial turn of parenthood came not in the delivery room but eight months later, when they started to worry that something had gone wrong with their son. The baby, Jacob, didn’t respond to the surrounding world the way his older sister had; when Megan started banging on pots, one night, he did not even flinch. At the hospital, a test confirmed their fears: Jacob was deaf, and most of the assumptions that they had about his future would change. At first, Megan and Michael took the difference in stride, seeking programs that would help Jacob acquire language and find a place in the hearing world. But the offerings were, almost without exception, rather grim, and few promised a life at anything near standard speed. The instructor at one celebrated clinic boasted that Jacob would be saying “apple” by the age of two. Megan protested that her daughter, at that point, could talk in sentences. “Your expectations are too high,” the instructor said. Megan knew they’d need to take another path.

The secret history of sex is not a story of fulfilled desires; it’s a story of expectations dropped off the cliff of the unknown. Coupling reroutes lives, and delimits them, and when the stork turns up bearing a charming bundle the chances for complication grow alarmingly profuse. On the day of the twenty-week sonogram, perhaps, you learn that your child has foreshortened limbs. Amniocentesis might identify abnormal chromosomes; an obstetrician in the delivery room whisks away your newborn to run tests. Maybe, back home, a fire alarm goes off and he does not wake, or maybe, at ten months, you notice that your baby will not look you in the eye. Or perhaps none of this happens and you’re one of the lucky ones, and so you send your kid to oboe lessons and good schools and camp and proms, and then, at twenty, on a campus where he has a girlfriend and a full course load, he begins hearing voices in his head. Ordinary family life is perilous enough: healthy kids fail at school, have drug problems, get bullied, or are shattered by foul divorces. When the quirks of biology intercede, too, the foreverness of parenthood can turn into a long walk in the dark.

In the case of Megan and Michael, being the parents of a deaf child meant travelling into a foreign country. Frustrated with the clinic, they enrolled Jacob in sign-language education, and then learned the language so that they could communicate with him. Megan helped found a deaf-services hotline, called Tripod, and a Montessori school that taught deaf and hearing students together. But these efforts weren’t entirely triumphant. At one point, a Deaf activist (deafness is a condition; Deaf is the community it creates) told Megan, “The best thing would be to give your child to a Deaf family and let them raise him.”

The story of Megan, Michael, and their unexpected family life is one of many in Andrew Solomon’s “Far from the Tree: Parents, Children, and the Search for Identity” (Scribner). Solomon, an assiduous journalist with an essayistic bent, is fascinated by the paradoxes of procreation: how do you nurture a child who may be unlike anything you’ve encountered before? Most people who consider themselves black, say, or Jewish, have parents who do, too. Solomon calls this “vertical identity,” because it flows naturally down the generations. It’s a conduit through which the benefits of shared experience—empathy, hindsight, a sense of who you are—can travel. But what if, like Jacob, you are a deaf child with hearing parents? What if you’re a dwarf with parents of normal proportions? These identities are “horizontal”: there’s a rupture between the child’s life and the parents’ experiences. They seem to challenge many premises of family and interrupt the basic continuity that it presumes.

Solomon is in many ways the perfect writer for the subject—nuanced, thorough, humane, and a gifted stylist—and, trying to get to the root of this conflict, he pushes horizontal identity as far as it will go. He includes chapters not only on deafness and dwarfism but on Down syndrome, autism, schizophrenia, multiple severe disability, early genius, conception through rape, criminal behavior, and transgender life. He talks with more than three hundred families; interviews those around them; and reads extensively about the conditions they face. When bonds within families begin to fray, he seeks to understand what went wrong.

His motives, he says, are partly personal. In his previous book, “The Noonday Demon” (2001), Solomon explored the science and culture of clinical depression through interviews and the scrim of his own struggles. “Far from the Tree” began as a companion piece, for, in addition to being depressive, Solomon is gay. (“In my adulthood, being gay is an identity; the tragic narrative my parents feared for me is no longer inevitable.”) In an effort to “recast” his fraught relationship with his parents and their reaction to his sexuality, he began studying other forms of horizontal identity. The research overflowed the mold. At more than eight hundred pages, including notes, “Far from the Tree” is a cross-section of something that defies sectioning, an exploration of difference as it shapes family life.

In certain ways, this is a headlong run into a minefield. Start with that term “difference.” Today, the word often appears behind a shield of euphemism, used by polite people to refer to otherness, misfortune, or just something ineffably bizarre (Björk’s taste in clothes, your Aunt Matilda’s carrot-carob torte). For Solomon, though, the concept is more ecumenical, and distinguishing among its shades becomes his crucial task. Are blindness, autism, paralysis, depression, and other conditions “illnesses” to be avoided? Or are they simply another way of moving through the world? Parents of a sleepless autistic kid who smears her feces across the kitchen walls may be loath to see that conduct as just-gotta-be-me identity. But what about Sir Isaac Newton, whom some contend was autistic?

For most of human history, conditions of the kind Solomon explores were stigmatized and moralized, and blame was assigned to any party who could be held accountable—usually the parents. Imaginationism, the belief that abnormal children come from mothers who indulged in perverse thoughts and general bad juju, persisted into the modern age. It was suggested, for example, that Charles Sherwood Stratton, the nineteenth-century circus entertainer known as General Tom Thumb, owed his short stature to his mother’s shock at seeing the family puppy die.

A basic paradox of contemporary social thought is that in order to respect difference we elide it. Maybe being a black man in the South isn’t quite like being a gay Northerner from an observant Catholic family, but isn’t the idea more or less the same? Maybe having Down syndrome is not strictly comparable to being an academy-trained sculptor who is blind; yet they both entail living with major impairments, so don’t they require similar protections? We generally agree that everyone should be treated fairly, and that the vulnerable should be guarded. Working from this principle to specifics—who’s vulnerable, and when?—is a frequent area of disagreement, though, and one that benefits from the clarifying efforts of social work and jurisprudence.