I just read a very Gruntled Center argument. Bryan Caplan, based on twin studies and his own parenting, came to this conclusion: parenting matters less to how kids turn out than most anxious "concerted cultivation" parents think. So lighten up and enjoy your kids. He calls this "Serenity Parenting."
The corollary is more interesting: if you don't really need to run yourself ragged raising each kid, you can have more of them.
I am looking forward to reading (and blogging) his book, Selfish Reasons to Have More Kids: Why Being a Great Parent Is Less Work and More Fun Than You Think.