Direct response marketing has been around long before internet marketing came on the scene. Pay-per-click marketing is just a platform shift for advertising methods that have been utilized by the direct mail industry for decades. SearchEngineLand recently came out with an interesting post pointing out that a lot of the things experienced marketers have learned about consumers from dropping catalogs in the mail year-after-year probably still hold true today for pay-per-click marketing. In fact, they are currently performing a study to see if these two principles that have proven to be accurate in the direct mail model will apply to PPC:

• Rural, low-population density areas have a significantly higher response rate to ads than urban areas. (Their early numbers show this is true: those in rural areas click on PPC ads around 60% more than city dwellers.)
• Ads shown to consumers by companies who have a brick & mortar store in or near the consumer's zip code are much more effective at driving sales.

I'll be very interested to see how this study turns out. I'm predicting that both of these principles hold true when applied to pay-per-click marketing. As further data is gathered around what factors heavily influence PPC click-through-rates and sales, SearchEngineLand points out that Google would be smart to provide tools to Adwords advertisers that allow them to target their ads to smaller, thinly sliced demographics. Maybe in the near future, "Urban" will be an Adwords targeting option. I'd personally settle for "Frequently honks when driving."