[Please note that some of the varnish has been removed from the following opinion, and I think I got down into the wood grain.]
Since people are just complicated evolutionary machines, their behaviors can be altered by the laws of cause and effect. Thanks to neurological science, advertisers can now guarantee sales of fruit juice to preconditioned monkeys.
Give any primate (or suitable rodent) the proper cues, rewards, and routines, and its habits will change.
People can be reprogrammed because they are not fundamentally different from computers or other machines.
Even god is a tool for change: spirituality seems to be one of the keys to AA’s success, at least.
The possibility that someone might desire the good and not be able to attain it (Rom. 7:15), even given the secret key of the power of habit, doesn’t seem to occur to Mr. Duhigg. If you don’t have the willpower to change your habits, you just need to do some more willpower exercises. (And what if you don’t have enough willpower to do the willpower exercises…?)
The most profound portion of the book came at the end, where he compared a mom’s compulsive gambling to a murder someone committed while sleepwalking. He probed the questions of moral responsibility inherent in these two tragic stories. But the appendix confirmed what the book was: a well-written (I definitely give him that), anecdote-laden, pop-psychology sermon for the more intellectually rigorous among the self-help crowd. Skip it.