The most immediately striking thing about your new book, Far from the Tree: A Dozen Kinds of Love, is what a doorstopper it is [976 pages]. It's massive! Did it really take you a decade to write? And what did it feel like when you'd finished?

It took 11 years and it was both a source of enormous relief and enormous anxiety when I finished it. Though the fact that I actually did still feels like a daily miracle because there was a long time when I thought I'd never reach the end.

You explore how families cope with having children who are in some way different from them – whether they're autistic, schizophrenic, incredibly gifted or conceived in rape – by interviewing them, 300 in total. Did you not get to 150 and think, "Oh, that'll do."

I think the detail is necessary. The tension of the book is what all these experiences have in common but I also wanted to describe what each of them individually is like. I felt that I couldn't write about the experience of families dealing with deafness if I only interviewed four families. I felt that you would only be telling the stories of four families. Whereas if you have 15 families and have also done a great deal of other research, you can make a claim that it's a representation of the deaf experience.

The very first line of the book states that reproduction is "a euphemism to comfort prospective parents". This is ruptured very abruptly for parents of children with disabilities but do you think that all parents are forced to face this eventually?

Yes, that's exactly what I think. In the same way that we test flame-retardant pyjamas by putting them in an inferno, so I think these cases are extreme versions of parenting as a way of illuminating a universal reality. At some point everyone who's a parent looks at their child and thinks, "Where did you come from?" And the more extreme circumstances that these families have gone through and their ability to rise above them seems to me to be a role model for how we all need to love our children, rather than trying to turn them into our own image.

You say that writing the book addressed a profound sadness in yourself but that somehow the act of writing it cured it. How?

The framing story of the book was what it was like for me to be the gay child of straight parents and what it was like for me to become a parent myself. I had managed to find some sort of peace with being gay but I still felt some sort of legacy of disapprobation that had been attached to my coming out, and it had lingered really throughout my adult life. It was a relief to discover that many families struggle with having children who are different. And the fact that a family takes a while to get to the point of acceptance does not mean that the family does not also experience love.

People who are different are constantly dealing with families who don't understand them. I suddenly felt that it's almost everybody's experience at one level or another, and that was really amazing.

Did it also inform your decision to have a child yourself?

People kept saying to me: it's so extraordinary that you would decide to have a child when you are writing a book about everything that can go wrong. And my response was to say it's actually not a book about everything that can go wrong, it's a book about how well things can be pulled together, even when everything goes wrong. How much love parents can experience, even when facing these difficulties. I thought, "Gee, I see, even in these terrible cases, how profoundly rewarding parenting is for all these people." And it drew me toward that experience.

You've spoken about how you've created a new sort of "composite" family, and how it's a bit like being Christopher Columbus discovering the wilder shores of love – but that sometimes it would have been nice to get there when the luxury hotels had already been built.

My now husband is the biological father of two children with some lesbian friends in Minneapolis, and he had already had one of them at the time I was working on the book. I have a daughter with a friend from university and they live in Texas. And then my husband and I decided that we wanted to have a child together, and we now have a son, George, who is here with us now at this very moment, running in and out of the room. He'll be four in April. It's five parents and four children in three states.

Does that present some challenging logistics?

Practically, yes. But not emotionally. There were some challenges in the beginning but not now. We really do all get on incredibly well and it's very nice having this extended family around us, to be part of this real web of people all connected to one another. When I was growing up I kept hoping that I wasn't really gay because I wanted to have children. I went through a long, tortured period so the fact that I have been able to be true to myself and have a family has been the nicest surprise of my adulthood.

One of the most striking interviews is with the parents of Dylan Klebold, the teenager killer in the Columbine massacre. You actually say that of all the families you interviewed, it's probably the one you'd have most liked to be part of.

I thought that if I really got to know them I could understand why things had gone so horribly wrong, and how they could have had a child who committed such a heinous crime. And the more I got to know them, the more bewildered I felt. They were just really lovely people: kind, intelligent, generous. And I thought, you know what? Criminality is just like any of these other things. It's an affliction. It reflects on you but it isn't something that you've necessarily caused. The Klebolds showed me that this thing of having a child who is broken in some way and is wantonly destructive could happen to any of us.

You live in Britain part-time – what's the great attraction?

I did my graduate studies here and then lived here full-time for a bit afterwards, and I have a lot of friends here, including seven godchildren. It's a big piece of my life. I like the relative literacy of at least some of England. I mean, I didn't come for the food or the weather!