Over the years, I've read all sorts of speculations about search engine ads suggesting that people click on ads: Because they don't know that they're ads. Because they are perceived to be of greater quality than the actual search results (for example, in searches for travel). When they're searching for something that they want to purchase (intent to buy = desire to get to merchants quickly). When they're bored. When they think that they might win something or get something for free.

Ninety-nine percent of Web users do not click on ads on a monthly basis. Of the 1% that do, most only click once a month.

Who are these "heavy clickers"? They are predominantly female, indexing at a rate almost double the male population.