I have a peculiar perspective on the nature/nurture debate. I have five children by four mothers, all born within six years, all raised separately but in similar environments (same city, same social class).
The key here is that there is only one child who is biologically mine (the others were adopted at birth), and I had little to do with her upbringing before the age of 10. So you might say she’s the control group.
I was close to most of the mothers most of the time, and was able to observe first-hand their successes and problems. I know varying amounts about the biological mothers of the adopted children.
My conclusions: most people debating nature/nurture have no idea what they’re talking about. Both are very important, but at different times and in different ways.
I started out think everything was nurture, but my biological child has many of my characteristic behaviors, and I refer only to those not shared by her mother.
Then there are many things which are plainly not nature. The adoptees of Mother #1, for example, share her deep-rooted ability to remain calm under pressure. The panicky Mother #2 has a panicky child.
One thing that seems innate: some children are more easily damaged than others. I’ve seen the same kind of mistakes made by each mother, but what one child rebounds from easily may cause another lasting difficulties. The worst mother (even in her own estimation) did not produce the most neurotic child. The mother of the most problematic child was not significantly worse than the others, though the problems seem clearly connected to her mistakes.
About the book: I found some of the suggestions very helpful, others less so, with results often contrary to my expectations. (That’s a compliment.) I have no problem recommending it.
I haven’t looked at twin-studies in many years, so I can’t comment on them.