Why physical discipline reduced in the Christchurch study

There were numerous stories about how a study has shown physical discipline has reduced from 77% to 42% of parents in the last 15 years. But those stories missed a key aspect, which Thomas Lumley points out:

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 impact of children getting older is likely to be hugely influential. A US study found physical discipline was used on only 35% of infants but 94% of three and four year olds and then declines from age five onwards.

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.

What would be useful would be to break the data down by age of child. I suspect that would account for much if not most of the reported decrease.