This is the question technologically pondered by a couple of New Zealand babes with tight butts. They fitted a rear- facing hidden video camera to their butts and then walked around L.A., capturing the looks those butts got.
http://www.huffingtonpost.com/2011/02/16/rear-view-girls-roam-around-los-angeles_n_824311.html Funny, and it confirms many of our biggest fears about both men and women. On the whole, the men gaze at the asses in question approvingly or longingly, while women are clearly scornful, resentful, and jealous. My fave clip is towards the end, where Jesus is checkin' out... uh...the end. :-) It makes me wonder whether anyone checks out *my* old, saggy butt any more. Maybe I'll walk around today with my iPhone in my back pocket, Bette Davis style, and find out. :-)