There's little doubt that many conceptions of attractiveness are faddish - the size zero female model being an obvious example. However, other notions of beauty are more hard-wired, perhaps reflecting an evolutionary adaptation. These aspects of appearance have come to be associated with fertility, signifying 'reproductive fitness' to potential mates. Male facial symmetry is one example. Another is the hour-glass female form. Men in cultures across the world report a preference for women with a lower waist-to-hip ratio. And women with this body shape tend to be more fecund.
Now Steven Platek and Devendra Singh have provided brain imaging evidence to complete the picture. They've shown that the reward centres of men's brains fired up in response to the sight of naked women who'd chosen to have cosmetic surgery to accentuate the curviness of their figures. By contrast, changes to the women's body mass index - including increased slimness - had no such effect. Platek and Devendra said the finding could explain 'some men's proclivity to develop preoccupation with stimuli depicting optimally designed women' - i.e. porn. The weaker neural response to slimness, by contrast, suggests 'BMIs role in [attractiveness] evaluations is less the product of evolved psychological mechanisms and more the part of culturally driven, or societal based norms and perceptions.'
Platek and Singh made their observations after asking men to look at photographs of women taken before and after they'd undergone surgery in pursuit of an hour-glass figure. The post-op pictures triggered more brain activity in reward-evaluation areas such as the orbitofrontal cortex. The surgery had the effect of lowering the women's waist-to-hip ratio and there were also slight changes to their BMI scores. The former change was associated with more reward-related activity in the men's brains whereas changes to BMI was only associated with activity adjustments in lower-level visual brain areas. Finally, increases in the attractiveness ratings given by the men to the post-op pictures were associated with activation in neural reward areas, such as the nucleus accumbens, which are also involved in drug-based reward and craving.
Platek, S., & Singh, D. (2010). Optimal Waist-to-Hip Ratios in Women Activate Neural Reward Centers in Men. PLoS ONE, 5 (2) DOI: 10.1371/journal.pone.0009042