Over the next five years, Paul's behaviour - at one extreme a regression to babyhood, at the other violent or sexual - grew worse before it got better. He made sexual advances to Catherine, wrote a letter to her mother asking her to have sex, threatened to stab her best friend, used her cashcard to steal money and drenched the cat with cleaning fluid. The strain on the family was immense; three years after they adopted Paul, Catherine and her husband, Neil, separated.
Yet for Catherine it is a success story. Unlike many parents in her position, she has never given up the struggle and returned her son to care.
Catherine and Neil, both social workers, were better prepared than most for the problems of adopting an abused child. They were not typical adopters. Catherine had a teenage son, and the couple decided to adopt an older child rather than have another baby.
The picture that social workers painted of Paul's past was painfully accurate. He had been neglected and assaulted, locked in cupboards and sexually abused by his mother from the age of two; sometimes he had initiated sex with her. When he was seven he went to live with foster parents, who requested that he be moved after a year; he spent the next two years in a children's home.
Catherine was unprepared for the desperation she would feel. To some extent she blames the training she and Neil received before they adopted Paul, which she says was all theory and ignored feelings: 'They said a child might swear or whatever, but no one asked you to imagine how it would feel if somebody did that in front of your mother or in the middle of the town you live in.
'Nobody said: 'You're going to feel really, really bad, there will be days when you'll be in absolute despair, days when, if he discloses things about his past, you might want to be sick.' So when all those things started happening it made me feel I was the only one who felt like this.'
It is one thing, Catherine says, to understand why a child is having a tantrum; quite another to know what to do about it. A few pointers on how to react when Paul made sexual advances, or advice on answering the inevitable unanswerable questions, would have given the couple more control.
Catherine's account of her first year with Paul, published this month by the Children's Society, was written to help other parents who have adopted abused children. She also welcomes a new scheme, being discussed by the British Agencies for Adoption and Fostering, which would enable adoptive families with similar children to contact each other and give mutual support.
Because Catherine was Paul's only confidante, as well as the target of his worst behaviour, her sense of isolation, particularly from her husband, escalated in the first year. 'Neil was never around when the bad behaviour was happening, or when the disclosures occurred. I described things to him, but in an almost mechanical way - I think you have to build up a barrier to your own feelings to cope. So I did isolate myself from Neil.'
Neil often reacted angrily to the way Paul was treating Catherine; but she always sought explanations for Paul's behaviour in his past. As time and the tantrums went on, their positions polarised.
Paul's sexual advances towards his mother, which became disturbingly aggressive in his early teens, did not help. 'It made me feel as though I was being abused myself,' she remembers. 'It challenged my sexual identity and affected my relationship with Neil. How could I go from Paul touching me to going to bed with my husband? It was impossible.'
She soon learnt that she must constantly reinforce the message that sexual behaviour was inappropriate. If Paul was going to give Catherine a goodnight kiss she held his hands down by his sides so that he could not grab her. But the sexual advances stopped only when she started telling her husband about the incidents in front of Paul.
Her experience has convinced her that anyone who adopts a sexually abused child must feel comfortable discussing sex. 'I had to say: 'You can't kiss me in this way; you can hold my hand but you can't touch my breasts'. I had to make him unlearn things he had learnt for seven years, and then replace them with behaviour that was acceptable.'
It took Catherine a long time to come to terms with the need to be a much tougher parent. After one particularly bad weekend, when Paul wrecked the central heating system and kicked a dent in the car, she was forced to lay down the law. Social workers offered to take Paul into care, but Catherine, although frantic, refused. Instead she suggested that they both stayed in a local authority assessment centre until he agreed to her conditions of behaviour. It took two days.
She now thinks she was too soft for too long: 'I didn't take control. I was trying to be too understanding, too forgiving, saying: 'I'll accept you no matter what.' '
The crisis between Catherine and her husband came to a head with another tantrum that ended with Neil pushing his son against a wall. Paul wanted to report Neil to the police and, because of his history of physical abuse, Catherine backed him up. She drove Paul to the police station and when they got back Neil was leaving. They have lived apart since the incident, 18 months ago, and Neil now sees Paul once a month. Although Catherine feared the worst, Paul's behaviour has improved since the separation.
The sexual advances have stopped and Paul's tantrums are shorter and less frequent. But now that he is much bigger than Catherine, he is harder to control. After a long struggle to keep him in mainstream education, he is now a weekly boarder at a special school, which has eased the relationship's intensity.
It is difficult to understand how Catherine could have continued to love a child who pushed her so far. She puts it down to bloody-mindedness. At first she did not want to be proved wrong. 'But as time went on, I'd been through so much, I wasn't going to let all that heartache go for nothing. I couldn't have given Paul up. Living with him was painful, but it would have been much more painful to say goodbye.'
The final justification must be Catherine's conviction that the Munroe family, warts and all, is better than a children's home. 'If Paul had stayed in care I don't think there would have been any hope for him. He would have been on drugs, I'm sure, or joyriding. I know that I've made a lot of mistakes in being Paul's parent, but I also know that he would have been worse if he hadn't been with me. That helps me to carry on. I feel as though I've done some good. I've given him the best chance he's ever going to get.'
'The Child Within' is published by the Children's Society at pounds 4.95.Reuse content