Never shake a baby. A simple sentence that for decades has conveyed a simple concept. Shake an adult and you might make them dizzy: shake a baby and you'll cause a massive permanent brain injury. Every year about 20 babies are admitted to hospital after being shaken. Four will die. And that's after that same simple mantra - just four words - have been repeated over and over: never shake a baby.
But what about never kick a baby? Never stomp on a toddler? Never wrench the arm of a child so hard that they will be thrown against a wall?
All of that seems obvious, but it seems we still don't get it - we still don't understand how fragile children are. The evidence is not only the stories of harm and abuse, but also the convictions handed down.
Murder may appear black and white, and in our law books it essentially is. The Crimes Act sets out that if you mean to cause the death, or if you simply mean to cause an injury you know is likely to cause death, that is murder.
The rest is manslaughter, which is precisely the conviction in many of the cases involving children.
They have been etched into our memories: Cherishsiliala Tahuri-Wright, or Cherish, was 3 years old when she received serious head injuries at the hand of her grandmother, who herself was the victim of shocking abuse. Jayrhis Lock-Tata was shaken to death when he was 5 weeks old by his father, who had also been assaulting his mother. Or Sahara, who was 6 when she was killed by her mother's boyfriend. He told police that he "just lost the plot ... I just wanted her to be quiet". In 40 of the cases over the past decade that I could find details on, half of the offenders were convicted of manslaughter.
I understand why our anger takes us straight to pointing at the statute books. I did exactly the same thing recently. No case highlights why quite so acutely as that of Moko Rangitoheriri. Here was a case of sustained and brutal abuse, and it resulted in a manslaughter charge.
Calling a death what it is may bring a greater sense of justice, but it won't save one child's life.
But as much as I have argued that Moko's case should have gone before a jury as murder, even I have to acknowledge that this is the cold end of the justice system. Calling a death what it is may bring a greater sense of justice, but it won't save one child's life. Addressing ignorance born out of a cycle of violence will.
For most of us it's unfathomable that someone could not only stomp on a child's torso, but also do so without knowing it could cause internal bleeding that kills them.
Who does that? Someone who never learned restraint and control; someone who had it done to them; someone who witnessed it done to others; someone who was never told or experienced anything else. After all, the research tells us that if you were the victim of violence, you are more likely to parent like that.
I am in no way excusing anyone who harms a child. There is no excuse. But if we are serious about changing anything, we have to get our heads around how the unfathomable has become commonplace.
Every conviction places a challenge before us. It tells us to listen to the warning signs of domestic violence that are so often entangled with child harm and address them together. It tells us that if we want to raise a new generation of parents, we won't do it by crossing our fingers, we have to do things differently. And ultimately, it tells us one simple message - the violence has to stop.
Jacinda Ardern is a Labour Party MP.
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