In "Selfish Reasons to Have More Kids", Bryan Caplan argues that raising children is easier than we think. That twin and adoption studies show that parenting, unless is spectacularly bad, doesn’t have that much impact. That our children are less like clay that we mould and more like plastic that snaps back into it’s natural shape when we relax our grip. He goes from this claim to the conclusion that most of us should probably have more kids. The conclusion I agree with, at least for the demographic his book targets, The premise I’m skeptical of.
1: Metrics for a good life
If you look at how he measures life quality, it’s the standard indicators. Class. Wealth. Profession. Education. Health. That’s not the measure of a life. The measure of a life is character and ethics.
If you disagree with his metric of child success, his argument is less persuasive. He presents no evidence that parenting can’t improve character or ethics.
The problem with Bryans book is that the metrics he uses reflect the people he seeks to convince. Most people are deeply amoral in their thinking and so by extension is his conception of "good" parenting.
2: This may not apply to outlier parenting
Most parents parent in certain ways. Twin studies will reflect this. It seems plausible that outlier parenting methods could be effective at drastically altering outcomes. If that was the case, the data probably wouldn’t show it due to how rare those methods are.
On the low end this seems obviously true. Serious abuse and neglect can and do drastically impact life outcomes. (Then again, this may be genetic)
On the high end it could be true. We don’t know. Maybe there are certain high impact parenting methods which can have lasting effects.