"Parenting," writes Andrew Solomon in Far from the Tree, "is no sport for perfectionists." It's an irony of the book, 10 years in the making and his first since The Noonday Demon, that by militating against perfectionism, he only leaves the reader in greater awe of the art of the achievable. The book starts out as a study of parents raising "difficult" children, and ends up as an affirmation of what it is to be human.

The project grew out of Solomon's desire to forgive his own parents, who, while they effortlessly accepted his dyslexia as he was growing up – his mother campaigned for his rights in the face of educational prejudice – flunked the same test when it came to his sexuality. (An early sign that he was gay, writes Solomon, with the dryness of tone that makes the book so enjoyable, is that "when I was 10, I became fascinated by the tiny principality of Liechtenstein".) They didn't throw him out of the house, but neither did they disguise their disappointment. Years later, he got to thinking about how parents deal generally with children whose identities fall outside of their own – what he calls the child's "horizontal" as opposed to "vertical" identity – and the result is a fascinating examination of the accommodation of difference.

Religion, race, language and nationality are the customary verticals passed down from parent to child; horizontal refers to traits in a child that are foreign to the parents, either inherent, like a physical disability, or acquired, like criminality. "Vertical identities are usually respected as identities," writes Solomon. "Horizontal ones are often treated as flaws." Chapters follow on families coping with autism, dwarfism, schizophrenia, Down's syndrome, disability, deafness, child prodigy, transgender issues, criminality and children born of rape, and the first lesson of Solomon's research was the non-transferable sympathies of each group. Participants in the book who had shown extraordinary humanity in their own difficult circumstances bridled at the prospect of being lumped in with what they saw as less deserving special interests.

"Deaf people didn't want to be compared to people with schizophrenia; some parents of schizophrenics were creeped out by dwarfs. The prodigies and their families objected to being in a book with the severely disabled. Some children of rape felt that their emotional struggle was trivialised when they were compared to gay activists."

Solomon spoke to some 300 families in the course of researching the book, a rebuke to everything shoddy and dashed off in the culture, and the density of his empirical evidence decimates casual assumption. What unites most of his interviewees is a political sense of injustice in the way they are perceived by the mainstream. "Fixing is the illness model," writes Solomon. "Acceptance is the identity model."

With delicacy, he weighs the rights of various pressure groups to self-definition against the pragmatic limits of their arguments. In almost all cases, he finds, it is a better time to be different than it ever was. In the chapter on deafness, for example, he tells heartbreaking stories of deaf children growing up 30 years ago being denied any language, when their parents prevented them from learning sign, thinking it unduly stigmatising. Now, deaf- and sign-culture is widely accepted as valuable in its own right and deaf pride one of the most successful advocacy movements around.

Autism, the subject of Solomon's most interesting chapter because of the complex nature of the condition, is trickier. He interviews animal behaviour expert Temple Grandin, who is autistic, and has worked hard to explain what the condition is like from within, she argues for "aspie and autistic" pride without denying its drawbacks. "If you got rid of all the autism genetics," she says, "you'd get rid of scientists, musicians, mathematicians and all you'd have left is dried-up bureaucrats." Solomon notes that campaigners for autistic pride suffer somewhat in their advocacy, since they are, by definition, autistic, and lack the charm that campaigns of that nature tend to run on.

It's a timely book; the internet has changed the fortunes of many millennial children who might otherwise have grown up feeling isolated, and, along with their parents, given them communities. "I was determined not to be around folks who saw us as tragic," one exasperated mother of a disabled child told Solomon. "Unfortunately, that included my family, most professionals, and just about everyone else I knew." But online, she had instant access to others in her position.

The most contentious of these advocacy groups are the "neurodiversity" campaigners, also known as Mad Pride, who argue for the rights of those with serious mental illnesses to reduce, and in some cases reject, their medication. Here, Solomon presents page after page of interviews with those tormented by psychosis, most of whom became ill in their 20s, compounding a sense in their parents of having "lost" them. Contrary to other categories in the book, it is hard to see schizophrenia as anything other than a theft of identity, and Solomon quotes E Fuller Torrey, the psychiatrist and researcher into the illness: "Freedom to be insane is an illusory freedom."

Consistent across all categories is the extraordinary tenacity of parents' love for their children. (This is not the same as straightforward acceptance.) There are moments of casual heartbreak. The father of Maisie, a severely mentally disabled child in New York, takes her to Central Park and reflects on the fact that, in his position, no one ever thinks to come over "and suggest that their child could play with your child". If it hadn't been for Maisie, he adds, he would have been one of them.

There are reminders that, however hard they try, parents can't always protect their children from bigotry, most starkly in the case of Lateisha Green, a transgender woman from Syracuse, New York, shot dead at a party with the words: "We don't want faggots here."

And there are surprises. It's a virtue of the book that it ranges across the socioeconomic scale, and Solomon finds that those parents with high socioeconomic status "tend towards perfectionism, and have a harder time living with perceived defects" in their children than those struggling at the lower end.

The most fascinating and painful interviews are with those parents who forfeit the good opinion of their peers by not doing what is "expected" of them: a woman from Oxford who, after a terrible period of indecision, gives her mentally and physically disabled child up for adoption; the mother of two severely autistic children, who, when her husband asks, "Would you marry me again?", replies, "Yeah, but not with the kids." She adds, "Do I love my kids? Yes. Will I do everything for them? Yes." But, "I wouldn't do it again. I think anybody who tells you they would is lying."

The most powerful interview of the book is with Tom and Sue Klebold, parents of Dylan, one of the two teenagers who carried out the Columbine massacre, and who killed themselves after the shooting. The Klebolds have been vilified on the assumption that they must, surely, have contributed to their son's mental state but there is, Solomon writes, no evidence for it. Of everyone he interviewed, he felt the greatest connection with them. "It would have been better for the world if Dylan had never been born," says Sue of her son. "But I believe it would not have been better for me."

Solomon is never sentimental and, with a cool eye, he acknowledges that "aggrandising the nobility of woe is a coping strategy". Nonetheless, time and again in the book, a positive outlook is shown to be helpful. "A study that looked at children with various complications at birth found, simply, 'the children of mothers who had tried harder to find meaning had a better development outcome'." The mother of a child lost to gang culture would not give up her idea of him as basically good. "In the end, his mother had believed him into becoming who he had sometimes pretended to be."

There are philosophical dividends that slowly accrue as you read story after story of ordinary families made remarkable by circumstance, and Solomon quotes Foucault's argument that if error is "at the root of what makes human thought and its history" then to prohibit error would be to "end evolution". As EM Forster wrote, "the perfect organism would be silent". At the end of the book, Solomon writes of having a son with his partner, John, with the aid of a surrogate – a family he never thought he'd have. "What did I do?" asks the mother of a child with dwarfism who would never quite be like her. "I loved him."

• Emma Brockes's She Left Me the Gun: My Mother's Life Before Me is published in April.