[araneae]

That actually doesn't surprise me, given the work of Dan Ariely. He's done similar experiments (but not with sex; with people doing some silly task on the computer) and found that people would do better on the task if their reward was a gift, like chocolate, then if there was a comparative dollar amount. The difference disappeared if he referred to it as "$5 worth of chocolate" versus "$5."

We think about getting paid with money differently than we think about getting paid with gifts, for some reason.