The conclusion of the book is: provide a healthy environment, but don't agonize over violin lessons and other 'enriching' activities.
That said, I went and bought another book with advice on how to talk to your child to stimulate language development ('look who's talking') and we read to him a lot. So, I believe the twin studies in theory, but not in practice.
Finally, on this general topic, 'how children succeed' points out that character traits like grit (willingness to keep trying something hard and unpleasant) and conscientiousness (desire to do the right thing) are more important than intellect in success, beyond some modest intellect baseline. I suspect my son is plenty smart, but what really interests me is trying to foster grit and conscientiousness.
If you want to talk about or discuss some ideas, you can reach me at my email (on profile)
It seems what is really being shown is the rational basis in children to discount promises made by adults for future rewards, when in their experience adults had been unable or unwilling to deliver on promises made.
http://www.thedailybeast.com/articles/2010/02/19/just-let-th...
http://priceonomics.com/what-marshmallows-tell-us-about-sili...
From The Daily Beast article: "Eigsti does not feel her data disproves Mischel—he was actually a co-author on her paper, and she thinks her work is consistent."
The Priconomics article says: "The original marshmallow experiment concluded that the children's ability to wait for a second treat indicated an innate ability to exhibit self control". But nothing I've read indicates that Mischel believed the ability to delay gratification was innate. In fact, most of his previous research indicated that such an ability was strongly influenced by environmental factors.
Have you heard of GWAS? They've had some major breakthroughs over the past few years, and have recently had some interesting results with regard to educational attainment. From Science:
https://www.sciencemag.org/content/340/6139/1467.abstract
If you want some more information, here's a good article from early 2012 summarizing the previous five years of findings: