Physical punishment of children – reporting

There’s a new research paper out from the Christchurch Health and Development Study, which recruited a group of people when they were born, in mid-1977, and has been following them ever since.  Those of the participants who were parents have been asked about physical discipline of their children on four occasions: when they were 25, 30, 35 and 40.  Obviously, over time, the number who are parents has increased (from about 150 to over 600), and the children have (on average) gotten older — when the parents were 25, most of the kids would have been pre-school; the group now includes a few very young children but many who are teenagers.

The good feature of birth cohorts like the Christchurch study is that you get to see the same people throughout the course of their lives; the bad feature is that at any given time everyone is exactly the same age.  In statistician jargon, age is completely confounded with period: you are completely unable to distinguish effects of ageing from time trends. When you see that the proportion of parents reporting hitting their kids has gone down from 77% to 42% over the 15 years, you can’t tell, at all, whether this is an effect of these specific parents getting older and more experienced or an effect of parents in general being less likely to hit their kids.  It’s hard (though not impossible) even to tell if it’s an effect of the kids being older.

As you’d hope, the research paper, in the NZ Medical Journal (paywalled) is very clear on this

…explanations include: increasing maturity of the parenting sample over time (less reactive, more experienced, older parents); a cultural shift towards the unacceptability of violence towards children over the period of the study; and the law change in 2007, which prohibited physical punishment and violence towards children. Given the nature of its design, it is not possible for the current study to distinguish between these explanations.However, it does not seem unreasonable to conjecture that all three processes are likely to have played a role.

And indeed it doesn’t seem unreasonable, as long as you recognise that the not seeming unreasonable isn’t a conclusion from the data and relies entirely on external plausibility.  The researchers do conclude that there’s still a lot of physical punishment going on, and that efforts are needed to stop it; the former is well-supported by the data and the latter is a policy response, not a scientific conclusion. That’s all good.

So let’s look at the reporting (some of this may have changed before or after I read it, of course)

• Radio NZ: Number of parents smacking children drops by half in 15 years. No caveat about the study design meaning this conclusion is basically unsupported. Gets the journal name wrong.
• 1 News: More than 40% of parents still use physical discipline years after law change, latest data shows. The story is better than the headline, and the Children’s Commissioner is quoted as saying “It’s representative of one cohort born in 1977, one group in one year in one generation, but there has been a discernible drop over the years.” I’d be happier if it was clearer from the beginning that this doesn’t claim to be representative of NZ in general over time.
• NZ Herald. Parents’ physical punishment of children decreasing, but still common – report. Slightly better headline; much clearer in the story. “…the rate of physical punishment against children was higher when parents were younger, and then decreased with age… because of the way the study was designed, it couldn’t pinpoint how much the rates reduced because of the law change.”
• Otago Daily Times. Parents still smacking, study finds. Good. “The authors warned that their method of studying a cohort of people over time meant they could not gauge what the attitude of new parents in 2021 might be to physical punishment. However, the research did suggest rates of smacking or hitting children were high enough to be a public health concern.”
• Stuff. Physical punishment of children still ‘fairly common’, despite anti-smacking law change – study.  There’s no caveat about the study design, and the story says “New research, published in the New Zealand Medical Journal on Friday, examined how the prevalence of child physical punishment changed in the 15-year period between 2002 and 2017 – before and after the legislation came into force.”, which isn’t true. And that’s not a link to the research paper.
• Newshub. Who’s most likely to use physical discipline against their kids revealed. The headline’s a bit dodgy given the non-representative group of parents, but the caveats are good “Because the study followed a cohort of parents who aged 15 years over the course of the study, “it is unclear what rates of physical punishment of children would be in studies of contemporary young parents”.