Ask people why they chose and you get the reasons they can say, often rationalisations; watch what they do to find the driver.

The claim

Asking people why they chose something returns the reasons they can put into words, which are often after-the-fact explanations of a choice made on other grounds. Research that only asks customers what they want therefore measures their explanations, and watching what they do can reveal a different driver. The case offered: detergent makers competed for years on whiter whites because customers said so, until observers saw that the first thing people did with clean laundry was smell it. From our own knowledge, not from the corpus: the observation has independent support in psychology (Nisbett and Wilson, Telling more than we can know, 1977), so it does not need the limbic-brain mechanism the book gives for it.