On occasion it is useful to outline definitions and frameworks. One thing that I often hear (i.e., I am constantly told) is that beauty is a subjective, and culturally defined, construct. In particular it is common for me to listen to explanations of "Eurocentric Western" beauty standards, as if they are sui generis. These views do not emerge in a vacuum. Rather, they grasp upon a real phenomenon: that beauty standards are malleable and vary across time and place. But like Classical Greeks who may have promoted a converse view, that beauty is an objective aesthetic reflection of innate characteristics of human value, modern subjectivists ignore the empirical reality in favor of a clean and simple narrative.
From where I stand it strikes me that Western intellectuals who engage in a discourse which engages the construction of the non-Western Other sometimes forget that the non-Western Other is itself a social construct with only constrained utility. To unpack it in more plain language, non-Western societies are diverse across themselves, and can not be bracketed as singular non-Western Other in a deep sense. And, they exhibit strong similarities to each other, and Western culture. This is all common sense, and I can attest to it personally, since my parents were raised in a non-Western society, and reflect a combination of banal and comprehensible attitudes, as well as shocking and outrageous ones. People are people. Just somewhat different.
When it comes to physical beauty this framework expresses itself in the assumption that Western standards of beauty are peculiar artificialities, with no grounding in human nature. This is an argument taken to too far of an extreme, and leads people astray. So let me outline a model which I think verbally captures the complexities of beauty, without pushing any particular interpretation in a maximal direction (and as a personal matter of fact I think a maximal argument fails).
First, you have to reconceptualize variation in beauty not as a spectrum, but as a multi-dimensional space. Some of the dimensions are deeply biological and "hardwired," but others are environmental and malleable. There are two primary biological dimensions: symmetry and secondary sexual characteristics. The first is just species typicality, and I suspect this is the primal trait upon which a 'beauty instinct' is constructed. Individuals who are not symmetrical or exhibit bodily deformities are generally not considered attractive, though there are deviations from the norm (e.g., those who cultivate peculiar fetishes, and so may seek out maimed individuals to sexual encounters). The second biological dimension has to do with exaggerated sexual characteristics. There are many beautiful children who are highly symmetrical, but these children are not sexually attractive in the least, because they have not manifested this dimension of beauty. Though one would presume the two biological characters would be correlated, the correlation is imperfect. There are individuals who have striking secondary sexual characteristics, who nevertheless are not attractive in their facial features due to sub-average symmetry. Conversely, there are individuals who exhibit attenuated sexual sexual characteristics, but have symmetrical and highly species typical faces. Another issue to consider is that secondary sexual characteristics considered attractive in one sex may not be attractive in the other. This is qualitatively different from the case with symmetry, where both sexes stand to gain.
Now we move to the environmental dimensions. Here you have a distinction between the dimension which spans cultures, and the dimension which does not. Good hygiene for example is a cultural universe. But what constitutes good hygiene is not. In some ways attractive traits which are amenable to environmental intervention and are universal across cultures are innate at a remove, in that there are strong biological functional reasons why people across all societies tend to rate those with sweet breath more attractive than those with foul breath.
Finally, you have the dimension of temporally and culturally variant standards of beauty. This is the dimension which gets a great deal of attention, to the point where some assert that all standards of beauty are culturally contingent. There is famously variation in preferences as to the ideal figure of women, but this is not the really interesting case. Foot-binding, neck-elongation, and other sorts of body modifications which exhibit no rhyme or reason are much stranger illustrations of the fact that signalling driven by cultural aesthetics can move in radically strange directions.
In sum, one can conceive of beauty as a weighted function like so:
Attractiveness = Aw + B x + Cy + Dz, where w, y, x, and z are the dimensions outlined above, and A, B, C, and D are weights.
If an alien only understood human standards of beauty through pornography, they might presume that only secondary sexual characteristics mattered. In contrast, if their understanding of beauty was obtained via reading some feminist scholarship, they might presume cultural variables reign supreme. Reading through catalogs of beauty product supplies might suggest the universal malleability of beauty, and cross-cultural preferences driving personal behavior change. As for symmetry, it is such a fundamental aspect of beauty that I have a difficult time imagining a situation where it is separated from the other variables, but some of the illustrations of the "uncanny valley" actually come close.
Addendum: Modern cosmetic surgery allows us to modify secondary sexual characteristics quite a bit, so it is more under environmental control than in the past.
Image credit: Wikipedia