In some societies/cultures it's buttocks. In the days of low cut gowns overflowing with cleavage, a glimps of a well turned ankle was a turn-on. I doubt women are interested in the IQs of the Chippendales dancers either. In other words, I don't think we should get too worked up about the social or political implications of the human libido. Fortunately, while there may be an "ideal," there's always someone for everyone in terms of body types and attraction. We do need more intelligent, rich, beautiful women with large breasts who like skinny guys though from my perspective :-)

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken