When should family secrets be left to lie, and when should they be unearthed? asks Hester Lacey
When Gwen Richardson was three or four years old, her grandmother came close to letting a family secret slip. Gwen was talking to her grandmother about her favourite cousin, and her grandmother said, "She's not your cousin: she's your sister." Gwen ran home to ask her mother if this was true: her mother flew into a rage and the subject was never discussed again. Gwen forgot the incident; but years later, in 1976, when her mother had been dead for four years, she began to wonder. "I was looking back on all the atmosphere, little odd remarks that people had made, and suddenly it all came together. It was as though a thunderbolt had hit me. I thought: 'I've got a brother or sister somewhere'."

By this time, there were no close family members left alive to ask; but she managed to contact a woman who had known her mother as a young girl, and found that her mother had indeed had another daughter, who had been brought up by one of her sisters. Gwen's cousin was in fact her sister.

The shock to both of them was immense. "At first I wanted to see my mother, put my arms around her, say 'Tell me all about it'. Then I felt terribly angry: why hadn't she trusted me with her secret? My cousin also realised she was not who she thought she was - it resolved a lot of questions she'd had about her own life."

Since she found her sister 10 years ago, Gwen has been helping others get to the bottom of family secrets; she runs Searchline, an organisation which helps adopted children find their birth parents. Sometimes, she says, when a birth mother is contacted, she has since married and kept the truth about the baby she gave up even from her husband, for years and years. "Imagine what it is like keeping a secret like that for so long. The longer you keep a secret, the worse it becomes," warns Gwen. "And none of us has the right to get other people to keep our secrets. It puts too much of a burden on them."

When is it right to keep a secret and when should the facts be disclosed? A new book, The Secret Life of Families, by Evan Imber-Black, explores the way that secrets affect the balance of family life. Dr Imber-Black is a professor of psychiatry and director of the Ackerman Institute for Family Therapy in New York. A family therapist for 25 years, she has found that the keeping or revelation of secrets is one of the most difficult dilemmas at the heart of family life, when dealing with problems as diverse as adoption, divorce, suicide and addiction. "A great many issues touch on secrets in some way," says Dr Imber-Black. "There are taboo subjects, or there may be a central family secret, or it may be because of the family's style of communication." She says that even though in the Nineties we seem able to discuss intimate topics with greater ease, for individual families there can still be a great deal of difficulty in bringing up certain subjects. And there is as yet no framework for dealing with modern issues like birth technology (do you tell your child they were conceived using donor sperm?) and Aids (do you tell the neighbours you are HIV positive?)

It is, in fact, very difficult to keep a secret water-tight. "We communicate on a non-verbal level as well as a verbal level," says Imber-Black. "Even though others may not know what is being hidden, they know that something is not being told." The key, she says, is distinguishing unhealthy secrets from a quite healthy desire for privacy. "Secrets have shame attached; private things do not. And there is an ethical guideline: do other people have the right to know, are they making decisions without access to information that would affect their lives?"

Opening up secrets, she says, takes time: it is a process, not an event. "Don't try to do it on holiday or on a special occasion; do it in regular time, not ritual time. And don't hit and run, be able to spend some time, not necessarily sitting over it for hours and hours, but so you can come back to it when necessary." And don't, she says, drop a bombshell in anger or revenge or in an attempt to shift guilt. "Good motivation is getting a relationship back on track. The question to ask is 'Am I doing this to regain my own integrity and re-open a relationship that has shut down?''' And don't, she says, assume that telling will automatically make everything all right. "The notion that everything is knowable and resolvable is very Nineties, and simply not true."

Even close and happy families can harbour surprises. Lisa, now 36, had always sensed that her mother's background was a no-go area. "We learnt very early on that my mother's childhood was something you didn't ask about," she recalls. Just a few hours after her mother's death, her father announced that in fact his wife had been adopted. "Half of us - the daughters I think - said, 'Oh, that's what it was', knowing there had been something but also that it was out of bounds to ask about it, and the others - my father included - looked amazed at our reactions."

Her father told them that their mother had once, as an adult, got as far as tracking down her natural father (then still alive) and found out his name, where he lived and his occupation, but that she had never wanted to approach any of her natural family and if the subject of adoption came up she was always vehemently opposed to anyone doing so.

"One interesting aspect of all this was that her natural father had been Jewish," says Lisa. "This really changed the way we thought about ourselves. We're all blond and blue-eyed, but when you know there's a Jewish line, you can see its traces. Then I started to remember things my mother had said - when I remarked, for example, that I seemed to be pursued by Jewish men a lot. She seemed rather pleased, as if she relished a little trace of her real heritage. I know how she felt, because with her dead, it now feels like a part of her that is an innate part of me."

Lisa wishes that she knew more. "I got quite obsessive after she died and spent a lot of time rummaging in those family records offices, looking for information. I'd like to know what happened." But she doesn't feel resentful that her mother wanted to keep this particular secret a secret. "I don't ever have the feeling that I wish I could have asked my mother about it. She didn't want to dwell on it, and I respect that."

'The Secret Life of Families' is published by Thorsons pounds 8.99. Searchline, tel: 01202 693102