Feminine beauty has been celebrated across the ages, but an enduring belief is that what constitutes attractiveness in a woman cannot be pinned down.
It depends on the prevailing fashion, culture or ethnicity, and on the eye of the beholder.
For instance, in Victorian England a tiny, puckered mouth was the zenith of physical beauty.
Today, the rosebud look has been replaced by what has been called the trout look, as women in Western cultures strive to make their mouths look as wide and full-lipped as possible.
In many societies, the focus of secondary erogenous zones has been on ankles, necks and knees and make-up and hairstyles change accordingly.
The desired female form has shifted too, driven in part by prosperity and the social advancement of women.
In the 1950s, Marilyn Monroe was the template of feminine beauty; today she would be urged to sign up at Weightwatchers.
It would seem that the "beauty standard" does not exist and that there is no eternal benchmark .
Not so for evolutionary psychologists.
For them, fashion is a fluffy cover for a force that is unchanging and deeper, as old and enduring as our genes: the Darwinian drive of survival and genetic fitness.
In a test of these rival hypotheses, scientists at the University of Texas and at Harvard University trawled through three centuries of English-language literature and through three Asian literary classics dating back nearly 2000 years.
Their goal: which parts of the woman's body were praised as beautiful by writers across the ages? Their sources were English literature from the 16th, 17th and 18th centuries, Chinese palace poetry from the fourth to the sixth century AD and two ancient Indian epics, the Mahabharata and the Ramayana, from the first to the third century AD.
Breasts, buttocks and thighs, the primary erogenous zones, predictably featured large in these descriptions. But a slim waist trumped them all.
In English literature, a glowing description of a narrow waist showed up 65 times.
That compared to 16 references for romantic descriptions of breasts, 12 for thighs and a mere two apiece for hips and buttocks.
Before anyone cries fattism, the literature was studded with romantic tributes to plumpness, but relatively few to slimness.
But what counted, plump woman or slim, was the relative narrowness of the waist. There was not a single evocation of beauty that said the object of veneration had a bulging tummy.
In the Asian works, the slim waist was even more adored, though there was no flattering reference to plump beauty. It scored a massive 35 references in the two Indian epics. The other body parts garnered a total of 26.
In the Chinese poetry, the narrow waist was evoked 17 times, while breasts, buttocks, hips and thighs got zero, and there was a solitary romantic reference to a woman's legs.
The study said this shows that a slim waist is an object of desire that spans time and cultures.
The authors say that a narrow waist is a sign of strong health and fertility. Men instinctively assess a woman's waist for its potential for successful reproduction and to further their own genes.
Modern research has established a link between abdominal obesity and decreased oestrogen, reduced fertility and increased risk of diseases.
Authors Devendra Singh, Peter Renn and Adrian Singh said: "In spite of variation in the description of beauty, the marker of health and fertility - a small waist - has always been an invariant symbol of feminine beauty." - Sapa-AFP