‘Would one of you like to come back with me while we set her shoulder?’ she asks. ‘I’m afraid it’s rather painful, even with medication.’

‘I will,’ Tom says instantly.

It should be me: I’m the doctor, and far better equipped than Tom to cope with the trauma of watching our child suffer.

But we have another child sitting at the end of the hall.

A ten-year-old boy who just pushed his sister out of a second-floor window.

Tom can’t even bear to look at him. He follows the nurse through the double doors, and I walk back down the corridor towards our son.

I want to run from this, too, but we’re Peter’s only hope, Tom and me. The difference between a CEO and a serial killer can sometimes hinge on the single question of the family into which they’re born. How we respond to this crisis could be the turning point in our son’s life.

Peter is sitting peaceably on a hard blue plastic chair in the waiting area, gently swinging his legs. He smiles as I approach. The summer sun has brought out a smattering of freckles across his nose: he looks wholesome and oddly old-fashioned, like he’s stepped from the pages of an Enid Blyton novel.

‘Is she dead?’ he asks, matter-of-factly.

‘No,’ I say, sitting down next to him. ‘She has a number of rib fractures, lung contusions, and she’s severely dislocated her shoulder. Your dad’s with her now while they set it. She’s in a lot of pain, but she’s not dead. And she’s going to be fine.’

My son nods, as if making a mental note for future reference.

I don’t waste time asking why. For people like my son, there’s only ever one answer to that question: because I wanted to.

I wanted to see what would happen.

I wanted to know if I could.

I wanted to hurt.

The signs have been there since Peter was a toddler, but stupidly I convinced myself Tom was right and I was wrong: that accidents do happen and rabbits can crawl into dishwashers and toddlers’ fingers sometimes get stuck in garden gates. I told myself that even if I was right about him, Peter would eventually learn what was allowed and what was not, and curb his natural impulses out of self-preservation, if nothing else.

But then he pushed his sister out of a second-floor window.

He’s ten. He knew Meddie could’ve been killed. He must have known, too, he couldn’t explain this one away: the trouble he’d be in.

And he did it anyway.