I am simply withholding judgment on the efficacy of direct marketing until I see some solid evidence. Same with the rest of the examples you mention. So far a

I am simply withholding judgment on the efficacy of direct marketing until I see some solid evidence. Same with the rest of the examples you mention. So far all I've seen are claims (mostly by direct marketers themselves), not any kind of reliable proof. As for how the studies would be done, I have no doubt that scientists (who are very clever at coming up with innovative research protocols) could figure something out. But here is a modest proposal: track the spending habits of a reasonably large, randomized group of people (should be easy enough to do these days), along with the ads they receive. See how many of the products are bought by people who receive and read direct mail ads vs those that don't. That should be a good start.