By “parenting”, let’s say we mean “everything parents decide to do in relationship to their children”. Let’s take “reading” as an example. Does the fact that parents read more have an effect on their children, say, level of academic achievement?
If you are doing a study on this topic, you want to disentangle this from the parents’ own level of academic achievement, as you don't consider one's own level of academic achievement to be a parenting decision. So in your study you control for that. Also, you might find that amongst people with a similar level of academic achievement, people with a higher IQ tend to read more, but since you don't take the parents’ IQ to be a “parenting decision”, you decide to control for that as well. Next, you find that some cultural aspect influence the amount of reading: say you find that parents born in the Jewish faith read more, even controlling for level of academic achievement and IQ. So again, you control for that again, as you don't take being born in a particular faith to be part of a parenting decision.
In the end, if you’re doing this enough, you’ll find no observable effect to parenting. But this is to be expected, as any of our behaviors, parenting decisions included, comes from a combination of our genes and our environment. So it is no surprise that trying to control for anything that could come from our genes or our environment, you find no effect to parenting. By wanting to disentangle parenting decisions from genes and environment, you're wiping parenting decisions out of existence. To me, such studies that concludes that "parenting decision don't matter" aren't showing anything about parenting, and only show the authors' confusion about the meaning of "decision".
