In A Billion Wicked Thoughts: What the World’s Largest Experiment Reveals About Human Desire, neuroscientists Ogi Ogas and Sai Gaddam—systematically examining what people do on the Internet—conclude that

“breasts, no matter what size, are the most popular body part in sexual searches in every country we looked at, including the U.S., Russia, India, Germany, Japan, and Saudi Arabia”.

Still, these authors add, “Butts are almost as popular as breasts in sexual searches, and there are almost as many porn sites dedicated to butts as breasts, hinting that men may also have an innate sensitivity to buttocks cues”.

Female butts may actually be more enticing to males than breasts.

Examining the results of 184 males, the authors found that 59 percent of them revealed greater attraction to female buttocks, whereas only 38.5 percent showed more of a preference for their breasts. 

Emphasis on the upper female torso has recently given way to the lower area of the body, specifically the buttocks. Such a change happened quite recently [but now over 30 years ago!] when denim jeans became fashionable. In order to emphasize fit, jeans manufacturers accentuated hips.

And after brand name jeans became so popular with the designer’s name on the hip pocket, even more accentuation was given to the posterior. The more jeans sales increased, the more ads were used which emphasized the derriere, to such an extent, in fact, that this particular area may eventually surpass breasts as the number one sexual image of the female body