I think the real secret of many advice books, baby or otherwise, is that nobody knows what they're doing. It's especially hard for baby and other parenting advice, though, because the goals are so varied. What is the aim? A quiet baby? A happy baby? A hard working child, a smart child, a well-behaved child, a happy child? A well-adjusted (whatever that means) adult? A driven and successful adult? A good relationship with the kid? Any of thousands of others?
It's impossible for any scientific study to address all possible outcomes and the relationships between them.
It's impossible for any scientific study to address all possible outcomes and the relationships between them.