Mainstream definitions of beauty and even the concept of "beauty" are highly subjective and malleable constructs. Historically many different body types have all held the title "most preferred" at some point. If we think back to the Medieval era, plump, pink, long-armed ladies epitomized the feminine beauty ideal. Partially "plumpness" was attractive because it signaled a person who had enough wealth to feed themselves luxuriously. Fast forward to this century, starting circa 1967, and "thin" is definitely "in." Models tended (and still tend) to have "stick" body types, and even boyish figures. Sexuality was in the process of being redefined and androgyny had a certain appeal. During most of the 1980s and 1990s, thinness was still the main beauty trait of concern, but androgyny had mostly faded away in exchange for a more "sexy" and feminine body type. This combination led to skinny ladies with large breasts, not to mention an obsession with plastic surgery (and a regression back to pre-1960s antifeminist sentiments.) Currently, for a lot of the general population, the thin/large breasted woman may still embody "beauty," however, in the last decade I have noticed an interesting trend in a lot of youth mainstream culture, and that is the glorification of the "booty." For many men and women, breasts are of little concern...appeal is all in the round "applebottom."
If one were to listen to any given hour of their local Top 40 (or pop hip hop, r&b) station, they might hear lyrics such as these:
"Shorty drop em to the ground like she ain't got manners/ too much booty for one man to handle..damn lil mama you know you fit my standards" - Akon "Buck in Here"
"She turned around and gave that big booty a smack" - Flo Rida "Low ft. T-Pain"
"You got me so hypnotized/ the way your body rollin round and round/ that booty keep bumping" - Plies "Hypnotized ft. Akon"
"I don't care 'bout your breast/ you can be an A cup/ I know what I like/ and baby that's below the waist" - Mims "Like This"
Now before everyone gets hung up on the "objectification of women issue," I would just like to acknowledge that, yes, these lyrics definitely treat women as objects, but part of the nature of Top 40 is to appeal to the lowest common denominator of listener, and an examination of these lyrics is very telling as to what traits comprise the current feminine beauty ideal. We live in a society where MTV, fashion ads, television, etc. influence what men find attractive, and what women seek to emulate.
The "new" ample-bottomed figure seemed to work its way into the mainstream initially through some key African-American and Latina actresses (J.Lo immediately comes to mind). Almost a year ago, Latoya Peterson contributed a post to the blog Racialicious entitled, "Real women (of color) have curves." http://www.racialicious.com/2007/02/21/real-women-of-color-have-curves/ Peterson criticized other media publications for lumping together J.Lo, Queen Latifah, and America Ferrera. These women may possess larger behinds, but they do not in any way share the same body types (one would benefit knowing the difference between ectomorphs, endomorphs, hourglass figures, etc)...instead they are lumped together because of their "otherness." Peterson explains, "One could argue that these women, each one quite pretty, are not considered part of the mainstream - their ethnicity is still a regularly used modifier in their professional lives. They stand just a little apart, so they are exempt from adhering to mainstream definitions of beauty."
While, I can see that argument holding true in certain circles, I would personally argue that this "alternate beauty model" is redefining the current mainstream preferences. Partially this may be due to the fact that America is becoming a ever more diverse country, or maybe even partially because of the "exotic factor" or ethnic fetishes (although it is pretty silly to think all Latinas/African-Americans have this body type, or that white girls all have flat asses). However, I think the biggest factor is the current inseparable connection of youth mainstream culture with hip hop culture. In the book, "Why White Kids Love Hip Hop," Bakari Kitwana explains how the younger black community serves as "gatekeepers" of what will become cool. Hip hop artists, slang, and I would argue fashion and therefore one aspect of "beauty" have to first be approved and legitimated by the black community before being latched onto by broader white audiences. First, the big booty, stereotypically aligned with black women and their latina counterparts (since Latinos also share a piece of hip hop culture) is seen as attractive to a lot of the black community. Hip hop lyrics, music videos, and other media depicting blacks reflect this preference. Then some time passes, and unsurprisingly many young white men begin citing the "booty" as the main physical feature in a woman that they find attractive...even if these men personally do not listen to or participate in hip hop culture, they do not escape the influence of the new beauty ideal. Since much of this segment of white America has adopted these preferences, I think it's safe to say that the "booty" is mainstream.