Well, conventions of beauty pretty much rely on symmetry. More symmetrical partners are probably more successful at reproduction and passing on more symmetrical phenotypes. I suspect this applies to non humans too...and it;s interesting to consider whether symmetry provides more robust functioning.

Even Jane Austen noticed asymmetry as a detriment to beauty in Pride and Prejudice.

As for the data...well individual manifestations are summed up across samples, populations, although you may mean something else.