Joanna Trollope's new novel takes up where she left off at the end of her last novel, Girl From the South, which raised the question of how our personality is influenced by family, and where we come from. Brother and Sister takes that issue and explores it further by dealing with adoption.

She said: "I found that at the end of writing Girl From the South, I couldn't stop thinking about this issue of identity. I'd looked a bit at how we're shaped by culture and family and so on, and then I thought, 'No, it's deeper than that, it's knowing where you come from'. It's a very short step from thinking about 'where do I come from?' to thinking about adoption."

As always with her novels, Trollope went through a rigorous research process, interviewing not only adoptees, but also their adoptive and natural families, their work colleagues and friends -- all of whom are affected by adoption in some way. What happened when people searched for their birth mother?

"Even though the reunions with birth mothers seldom led to anything warm, rapturous and idyllic, so many question were answered about 'how did you come to conceive me?', 'what were the circumstances of my birth?' -- that kind of thing -- that the journey was worthwhile."

In the novel, there is a strong sense of the stability gained through answering questions that are asked "in the very cells of their blood", questions which non-adoptees cannot conceive of.

"I think this particular sense of lostness is almost impossible to imagine unless you are adopted, you don't have the knowledge of your parentage."

Another issue was how intensely the subsequent relationships of the adoptees are affected by their initial 'abandonment' by their natural parents.

"Talking to the partners of adoptees, I got a very strong sense that somehow they were not just the partners of these people, but also the rescuers of them, particularly if they had children together."

Trollope gives the example of one husband whom she talked to, and drew heavily from, for one of the characters in Brother and Sister, who told her: "What I couldn't bear was that I had given her the first three known blood relations of her life, and that wasn't enough? What else was I supposed to do?"

Trollope said: "I could see that his role as a rather galant knight in shining armour had been tarnished by all this. She had said 'that's all lovely, but I still need to know who my mother was'."

What makes Brother and Sister such a good read is how unexpectedly and suddenly the intense need to search for identity arises, often prompted by the birth of a child.

"I don't think we know how powerfully we feel until it's challenged. What is so uncomfortable for life, but so very gratifying for fiction, is that our feelings are so very primitive, when they are aroused. 'Visceral' is the word. It has nothing to do with our rational self. It's to do with primitive feelings of possession and belonging, and terror of being alone."

Following adoption, it is often difficult to trust or truly give yourself to someone in a loving relationship. There is also a marked lack of trust, especially distrust of the feminine.

"Men and women need to find their birth mothers. Some of them need to find their fathers, but the prime person is the mother. It is the mother who still carries the moral can. Although the stigma of illegitimacy has gone, and not before time, I still feel that we're very harsh on single mothers. Millions of women have sex outside marriage, but those who don't get pregnant are regarded rather admiringly, whereas we have all this baggage about those who, possibly through no fault of their own, do get pregnant."

Trollope's own experiences as a young woman were thrown in to persepective by the experiences that were related to her in her research. She talked of the problems of reaching sexual maturity in the sixties, when new attitudes demanded open sexuality, yet old mores promised punishment if you should fall foul of the demon 'fertility'.

"The pressures were wicked. Instead of this panic now of 'what if I don't get pregnant?' the panic then was 'what if I get pregnant?'. Every month you would wait with baited breath, it was absolutely panic-inducing. It's only one generation where this sort of opprobrium has gone. That gives me hope that in another 25 years we will learn to be supportive of single mothers, instead of regarding them as parasites."

Brother and Sister is published by Bloomsbury at £16.99. Joanna Trollope will sign copies at The Bookshop, Chipping Norton, at 1pm today and at the Bodleian on March 26 at 4.30pm, as part of Oxford Literary Festival.