I understand that this question cannot entirely be answered by biology as there may be equal parts neurology and social-psychology, among other considerations, necessitated to reach a larger conclusion; however, I am curious as to whether or not human facial symmetry is increasing over time and if so is there an equation that describes that correlation?
For that matter, if subconscious recognition of facial symmetry as a indicator of a potential mate's health is common in humans, is the same true for other mammalian species? If so, can we expect facial symmetry to increase over time in most, if not all, mammalian species?
How much of facial symmetry is do to genetics versus "hickups" in embryonic / fetal development? If this is the case then we would not see an increase in facial symmetry because the decreased likelihood of offspring for individuals with less facial symmetry would not affect the gene pool for that trait?