Far from the Tree: Parents, Children and the Search for Identity. By Andrew Solomon. Scribner; 962 pages; $37.50. To be published in Britain by Chatto & Windus in February; £30. Buy from Amazon.com, Amazon.co.uk

ANDREW SOLOMON never knew a time when he was not gay. He chose pink balloons over blue ones and described operas on the school bus rather than trade baseball cards. He was teased at school for being effete and ignored by children issuing party invitations. In his teens Mr Solomon began to suffer from depression. His parents, supportive and understanding, would have preferred their son to be straight and encouraged him to marry a woman and have a family. The recognition that he was gay came only when he understood that gayness was not a matter of behaviour, but of identity; and identity is learned by observing and being part of a subculture outside the family.

Coping with depression led Mr Solomon to his life’s work: psychiatry. He became “a historian of sadness”. His 2001 book, “The Noonday Demon: An Atlas of Depression”, was a finalist for the Pulitzer prize and has been published in 24 languages. His new book, “Far from the Tree” grew out of his experience of being gay.

Most people share at least some traits with their parents. These vertical identities include ethnicity, and often language, religion and nationality. Most Greeks bring up their children to speak Greek; most children of colour are born to parents of colour. The apple does not fall far from the tree, the saying goes. Researching a magazine article about deafness and the world of sign language led Mr Solomon to wonder how people with inherent or acquired traits might acquire identity from a peer group rather than their parents; this he calls horizontal identity.

A friend whose daughter was a dwarf asked if she should bring up her child to consider herself as everyone else, only shorter? Should she make sure her daughter had only dwarf role models? Or should she look into possibly lengthening her limbs through surgery? Mr Solomon saw a pattern. Where vertical identities are celebrated in families, horizontal ones are too often deemed flaws. Homosexuality was once believed to be an illness; now it is an identity, a way of validating the same way of being. Deaf children often stumble in adolescence on a world that celebrates deaf identity and chooses sign language over having cochlear implants, and find it immensely liberating.

Having found common ground with the deaf and a dwarf, Mr Solomon began to wonder “who else was out there waiting to join our gladsome throng”. He had always thought of himself as part of a small minority, but now he began to learn that families with children whose identities are bound up with being deaf or gay or dwarfs, or having Down’s syndrome or schizophrenia, or being a prodigy or the child of a rape, may have more in common than he realised. More important, he learned that many of them had come to feel a profound common humanity and had a lesson for the world.

Based on in-depth interviews with more than 300 families, “Far from the Tree” is brought to life by its intimate domestic voices, many of them people who ended up falling in love with children they never knew they wanted. Megan Williams hired a sign teacher who moved in with the family after her son, Jacob, was born deaf and she learnt that the biggest curse of deafness is illiteracy. Jacob became adept at communicating not just with his family, but also with other deaf people. At five, he signed to his mother, “Am I deaf”? When she said yes, he went on: “I wish you were deaf.” At his first dwarf convention, the author hears a teenager half-sobbing, half-laughing. “This is what I look like,” she said. “These people look like me.” Not all were so positive. After Julia Hollander’s second daughter was diagnosed with multiple severe disabilities, she left her in the hospital, saying, “I’m not the right mother for this child,” and then went home and threw away the clothes, rattles, bottles, steriliser and cradle.

Over the past 50 years social change in America has tended to follow the same pattern: first religious freedom, women’s suffrage and race rights, followed by gay liberation and disability rights. The disability-rights movement, Mr Solomon observes, seeks to accommodate difference rather than erase it. And it has fought for groundbreaking new laws, including the US Rehabilitation Act of 1973, which was passed by Congress over President Nixon’s veto, the 1990 Americans with Disabilities Act and the Individuals with Disabilities Education Act of the same year. The fact that some activists fight for disabilities to be regarded as an identity rather than an illness, which may lead to many of the legal safeguards enshrined in these laws being abandoned, is an unresolved paradox.

Individual disabilities, such as Down’s syndrome or dwarfism, can be terribly isolating for the individuals concerned and their families unless help is available from support groups and professional organisations. Mr Solomon reckons there are 550m people with disabilities worldwide. In the rich world, more than the poor, their lives have been transformed by identity politics and medical progress. But even in the poor world social media and the internet are helping disabled people find others who share their quiddities.

The inclusiveness Mr Solomon sees in horizontal identities seems to be strongest in families with deaf children or those with Down’s syndrome. Mothers of children born from rapes and parents of schizophrenics and criminals are more isolated. One exception in the book is Sue Klebold, the mother of one of the two Columbine high-school killers, who, despite the opprobrium heaped on her by some in the local community, now says, “When I hear about terrorism in the news, I think: ‘That’s somebody’s kid.’ Columbine made me feel more connected to mankind than anything else possibly could have.”

An American playwright, Doug Wright, once said that family inflicts the deepest wounds and then salves them the most tenderly. The recurring themes of this book are generosity, acceptance and tolerance of children who are not what their parents originally had in mind. But Mr Solomon’s most important message is that understanding how these families came to think well of their children may give others the motive and insight to do the same.