The expected picture of the female figure in its post-puberty condition is an hourglass shape. There are those who find repugnant the idea that women's wasp-waist character has been produced (evolutionarily speaking) from their differences in attractiveness to men. That man's whims have influenced their appearance is to them degrading; but, indeed, this is a general biological principle which in human beings has worked both ways.
A man's first clue to female sexual readiness is the figure outline. A bean-pole means prepuberty, a 30-24-30 is a signal of age eligibility and there is an immediate spark of attraction to a man as his eyes scan for more information. However, a super signal of 40-24-40 gets a super response. All else being equal, someone with those measurements would attract more glances than someone who was merely eligible.
What does the wasp-waist have to do with it? Nothing directly. It is like the stripe down a burro's back: it emphasizes contour. A tighter waist dramatizes the chest and thighs, which is what makes a 33-17-33 as attractive as a 40-24-40 - probably more so, because of the decreased intimidation of smaller size.
The muskox probably illustrates best the principle of contour modification as seen in women's wasp-waists. Though in muskoxen it has to do with an exaggeration of threat paraphernalia instead of sex lures, the same principle still holds. One of the main threat display zones of the muskox is the big shoulder hump, but the top of the rump is also displayed to some degree. Both are elements of a size-height display. To enhance the dark shoulder and butt humps, muskoxen have a white saddle which visually scallops out the middle or lumbar area of the back. And since most of their social conflicts occur in fall when snow covers the ground, the white "cosmetic" shading gives humps a more awesome height and massiveness.
It is revealing to compare the differences in men and women in the fat depot patterns immediately beneath the skin - the subcutaneous depots, as they are called. These especially include the upper thigh and particularly its lateral or side portions. Women talk about being "hippy" meaning "fat" - a derogatory connotation in a society that puts emphasis on being youthful, mobile, trim, and athletic. Men, on the other hand, tend to accumulate excess fat in a roll running immediately over the crest of the ilium. Pants won’t stay up on heavy-set men - belts are made to cinch over the hipbone, the ilium.
I believe that women's fat depot patterns have been selected to accent natural sex-attractant contours. Maybe that's why many men think skinny women never look very sexy. There is, of course, a lot of cultural specificity about the degree of obesity which is considered attractive. Cultures where food is accessible and plentiful generally tend to admire hard leanness, which indicates one has leisure for sporting activity. When food is scarce, however, obesity carries with it a signal of privilege. Right now obesity is out of fashion, but a little fat helps to round out bosoms and buttocks to accent curviness.
Girdles firm the bulges in the buttock-hip areas and make them look young, yet gently, naturally, rounded. Girdles give a signal of a young lass, not a flabby matron. The same is true for bras. Breasts begin rather high on the chest and are moderately firm, protruding slightly sideways, but with age, especially after being suckled, they begin to sag and droop. Bras tighten them up and pull them back high on the chest, and at the same time round them out into the conical shape of the youthful breast.
Both Bushmen and Hottentot maidens have breasts which are unusually high on the chest, actually up near the collarbone or clavicle. This general group is especially child-like in many features; some of the reasons for this will be discussed later.
There is a shift away from the posterior bustle "shelf" type of buttocks to the broad, flatter look, which has apparently taken place from the Paleolithic to the present. Interestingly enough, the Orientals generally have wide hips but flat bottoms; this is especially true of some American Indian groups. Athabascan girls are especially broad-hipped, but have virtually no buttocks at all. Many African groups have individuals with well-developed buttocks and thighs, resembling steatopygia. These probably represent some past interracial mixture, as the archaeological record reveals that the Bushmen-Hottentot (Capoid) group once dominated most of Africa and have been pinched into a relic area, replaced by more dominant groups. The shift from the posterior shelf to broad hips has probably been subtle, for both have been functionally responsive to the same phenomena.
Movements of the hip and butt area have had a considerable role in human erotica - because they are stylized versions of the copulatory thrust. In the mid-50's Elvis Presley got away with what would earlier have been regarded as obscene gestures of back-and-forth hip wiggling, and in more recent years rock dances concentrated on the hips and their movement. The Polynesians must be the epitome of hip-wigglers, for they have developed it into a fine art in dances and in lovemaking. Many societies around the world use it in some form or other in erotic dances. High heels, as Desmond Morris pointed out, exaggerate movements of the buttocks, because women dramatize their sensuality with their gait.
Gestures also play an important part in copulatory attractants in other species. Pussycats in heat have the habit of crouching on the floor with their genitals swung up in a "lordosis" back bend, tail up and aside, ready for a phantom entry. At the same time they softly tread in place, giving a soft purr-rrowl - it gets to be kind of disquieting when performed in the middle of the living room floor for several days straight. My point here is that bumps and grinds call attention visually to the main zone of attraction, thereby strengthening the signal.
Why else at a beauty contest do they parade the girls around the judges for their evaluation? Note in those instances how often the contours are outlined as much as possible with a bathing suit (they aren't going swimming) and high heels, which are in no other social context worn with bathing gear.