Some two decades ago, a beautiful but unstable woman called Lamorna was temporarily separated from her husband, Patrick Seale. During their estrangement she had something of a fling with the young Amis, the result of which was a daughter, Delilah. But by the time the child was born, the mother was again living with her husband ... at least until her suicide, two years later.
Seale, to his eternal credit, raised the child with love and care until she was 19 - when, he felt, she was "ready" to be told the truth about her paternity. For the first time, and quite out of the blue. The question now is whether Seale - or come to that, Amis - did the right thing. The answer has to be that they did not.
Amis, knowing of the child's existence, did ponder the dilemma.His mother, Hilly, however, was firm with her advice: "I told him he should not rock the boat," she says. And even now, "I know what Martin did was right." Seale, for his part, probably pondered even more. But while it is hard to criticise a man of such blatant decency, in delaying the moment of truth he ran an indefensible risk with the emotional security of the young, and indeed the adult, Delilah.
As it happens, in this case, she appears to have survived; she is thriving in her studies at Oxford, a bright young thing with the traditional Amis success apparently ahead of her. But this can only be called a lucky escape, not a blueprint for sensible behaviour. For out of the woodwork come the witnesses to testify to the destruction that may accompany the late discovery that you are not, after all, who you thought you were.
There is the woman of 45, who sought psychiatric help when she discovered - through old papers and only after her parents' recent deaths - that she was adopted. There is the young hooligan, right off the legal rails after being told that his "aunt" is actually his mother. There is the woman who did not know that her mother died when she was an infant and that she was in fact raised by a stepmother ... at least, she did not know until a kindly local butcher saw her at 16 and gasped: "Look at you! What would your mother say if she could see you now?" Twenty years later, it is still on her mind.
The deceit can be smaller and still shock. A friend recalls her brother, at the age of 12, alighting upon birth and marriage certificates and doing the sums that told him he was really only her half-brother. "It caused a great deal of distress. It shook all his certainties, and all at once there were family subjects that became taboo."
At the Post Adoption Centre in London, where for ten years they have dealt with the trauma of ill-handled truths, the abiding rule is that it is never, ever, too early for a child to be told. One of their workers, Michael Mallows, a psychotherapist, says: "There is never a right time - but the longer you put it off, the worse it will be. People say they want to protect the child, but protect it from what? The fact is that not telling the young person is always about the needs of the person not telling, rather than the needs of the person not being told. Guilt, shame, embarrassment, the need to create the illusion of a 'normal' family, whatever."
There are two main reasons for speaking up. The first is the importance of the sense of identity, a sense that children work hard to grasp. When they ask how they got into Mummy's tummy and how they got out again, they are not teasing us with sexual prurience; it is only part of the search for knowledge about themselves. They even play early with the concept of it happening differently. Which parent has not had a child fantasise - at, say, the denial of a lollipop - that they are really a princess and the parents stole them? Irritating as it is, it is still the testing of the same waters. For if we do not know who we are, how do we make our choices about who we shall become?
The second reason is the wider one of children and truth in general. If we are found, once, to have lied we shall not, again, be believed. The 12-year-old who found himself to be a half-brother was beset, his sister says now, with one question: "What else might not be what it seems?" And if finding the words for the truth is difficult, the difficulty is ours - not the children's."Children are unbelievably resilient," said Mr Mallows. "They will deal with the world as it is presented to them. If we make problems with it, they will have problems with it. Telling them the truth may sometimes be painful, but children 'do' pain and fury just as they 'do' happiness and joy. It's not the facts that hurt children, it's that grown-ups have problems with the facts."
They also have, as any parent knows, a robust ability to filter what they are told; to take from what they are told only that which they can absorb at the time, and put the rest back for later. The well-meaners who discuss, for instance, sexual detail with a child too young to grasp it will not induce trauma. They will simply induce unbridled, disbelieving mirth.
There are ways and ways of doing it. One family with whom Mr Mallows worked had waited too long to tell a boy of ten that his father was not his natural father. He guided them through it; not, he suggested, a Monday morning - better a Friday night, when they would all be together for the weekend afterwards. Then the man told the child, "I didn't actually put you in Mummy's tummy, but in every other way I'm your Dad and I'm still going to tell you off and we're still going to argue." A few questions were asked - very few at the time; more came later - and the boy wandered off to watch television. Difficult, yes. Desirable, certainly.
Above all else, said Mr Mallows, "you must always work on the basis that eventually they will find out. And when they do, they will have to deal with the fact of discovery as well as the fact that they have been lied to, by omission, by the people they trusted most."
Many, sadly, cannot handle that at all. Perhaps as the years go by, Delilah - with her love for Seale and her face the Amis mirror - will be more fortunate.Reuse content