‘Kirsty was slower to develop empathy than most children. She didn’t respond well to punishment. But I know now that punishment’s not a good way to change behaviour. She forced me to realise that. To find a better way.’

‘For God’s sake, Dad, I’m a dog trainer. I know punishment’s not a good way to change behaviour.’

‘Yes. Well, Kirsty is why I got interested in that kind of behaviour modification in the first place.’ He wiped his forehead. ‘If you treat these children the right way, they turn out fine. That’s what happened with Kirsty.’

‘These children?’

‘Children with …’ He touched his mouth as if he didn’t like saying the words. ‘Callous and unemotional traits. They find it challenging to think long-term, and they don’t respond well to punishment, but they can learn how to behave well in order to get what they want. If you reward good behaviour, it becomes a habit. Like flushing the lavatory. Even using the lavatory – children don’t toilet-train themselves. You know all this from your dog training. A dog trained with positive reinforcement becomes a good citizen. The good habits become ingrained. The dog doesn’t avoid chasing the cat out of any moral imperative or because it’s suddenly developed empathy for the cat. It does it because that behaviour has been rewarded so many times it becomes a habit. It’s the same with these children. When they’re well brought up, they often end up very successful in politics or business. As your sister is, in fact. It’s only if they’re brought up in a toxic environment that they become …’

‘Become what, Dad?’

I knew very well what these children become if they’re brought up in a toxic environment.

‘Nothing. Your sister was well brought up. She’s fine now. All this is ancient history.’

‘You clicker-trained my sister?’

‘In a way. But isn’t that what we try to do with all children?

Reward good behaviour? Callous and unemotional children force you to be better at it, like the more challenging dogs do.’

Bex swallowed. ‘I thought I was the problem. You kept her and sent me away.’

‘I’m sorry, Bex. That wasn’t it at all. I sent you away for your own good.’

‘For my own good?’ Bex scrambled to her feet, puffing with the effort. ‘What did you think she might do?’

‘She’s fine now. She’s had no problems for years. This is twenty years ago we’re talking about.’

Bex stared at her father. The air felt heavy. ‘Are you sure, Dad?’ she said. ‘Are you absolutely sure?’

He fiddled with a Royal Crown Derby paperweight on the table beside him. ‘She was just a little rebellious, Bex. And a thrill-seeker. Don’t start thinking all sorts of terrible things about your sister.’