How did they know?

In a conversation with a doctor friend over a meal the other day, she marveled at the specificity with which marketers and list brokers were able to target a person's individual interests.

She'd received a catalog she hadn't ordered, junk mail, IOW, but said on her way to throw it away she realized she was actually interested in some of the things they were selling. She was both a little embarrassed that her interests were so transparent, and a little creeped out by it.

I mentioned that I'd experienced the same thing many times. I'll receive an email from someone I've never heard of asking, "Do you want to have sex tonight?" And I always wonder, "How did they know?"