More Spanking Research Still Desperately Needed

TGGP pointed out this spanking study by Lynch et. al. where the authors analyze the relationship between bad outcomes (drug abuse, externalizing behaviors, and internalizing behaviors) and corporal punishment in the children of a fairly large sample of Australian twins. By applying a hierarchical linear model (HLM) to this natural experiment they try to control for genotype while looking at the association of punishment and bad outcomes. Their conclusion is that severe (e.g., beating) but not moderate (e.g., spanking) corporal punishment leads to increased drug and alcohol use and bad externalizing behavior regardless of genetics.

An HLM is type of random effects model that is used to analyze data where the experimental units can be organized into different levels. In this case there are three levels: the family the twins are raised in, the family of the twins, and the individual children of the twins. This model allows for testing the effect of punishment at the child level while at least partially controlling for genotype at the twin parent level.

Unfortunately, the authors spoil the analysis by making some poor analysis choices. The twins are a subset of larger Australian twin study including subjects who reported having problems with alcohol, being divorced, conduct disorder and chosen controls. To control for this selection bias the authors use the residuals from a simple regression at the twin level model as the dependent variable in the HLM. I find this an odd choice. They could easily have added these factors to the twin level of the HLM where these effects could be examined directly.

More suspicious is the fact that their conclusion that mild physical is equivalent to mild non-physical punishment is not based on the HLM results. Rather, after inspection of mean outcomes at the child level the authors collapse the mild physical and the mild non-physical punishment groups in the HLM and compare this pooled group to the harsh punishment group. They could instead have tested the mild non-physical vs. mild physical punishment effect in the HLM and determined if after controlling for genotype these groups are similar and then compared each group to the severely punished. Could it be that the dutifully spanked children have better behavior than the non-spanked? Did they dare not offend the powerful anti-spanking lobby? More likely, the clear difference in behavior found between the severe and the mildly punished children is muddied once you fit a more complicated model. Maybe I am cynical, but I doubt you could publish a paper that did not find some kind of association between severe punishment and bad outcomes. Despite the failure of the editors to demanded a more thorough analysis, the study does indicate that a little swat may be ok. Claeralry, more spanking research is still desprately needed.