Girl ... You Lookin' So Fine Tonight, It Must Be 10 Days Before Your Menses

More Incognito, on our instinctual assessment of sexual attractiveness:

Women at peak fertility (~10 days prior to their period) are judged more beautiful - proved by stripper income as well as lab results.

Briefly glimpsed people excite us as though the uncertainty will all pan out in the direction of more beauty (encouraging further investigation) - this also works for briefly flashed photos.

Pale skin shows imperfections better, so allows for better judgment of health (if so, then the above “glimpse” effect would make darker skinned people initially seem more exciting). I always believed this preference to be learned, not innate.

Women with a .67-.8 waist:hip ratio are most exciting to men (given a fixed overall slenderness).

Dissimilar genes (“MHC” - “histocompatability”) are identified by scent (by their chemical effects and not directly, I presume) in mice, and lead to sexual preference (presumably because dissimilarity in those genes is important to avoid inbreeding-type recessive defects). Similar receptors exist in humans, and sweaty t-shirts are preferred with increasingly dissimilar MHC.

Women prefer the sweaty t-shirts of more physically masculine men when fertile; otherwise they don’t care as much or can’t detect it as well.

Women tolerate feminine-looking men (e.g. soft, youthful face) relatively more when they’re less fertile.

Women’s menstrual cycles may change after smelling another woman’s sweat.

Activating vasopressin receptors in the brain causes pair-bonding;polygamous species can be made monogamous and vice versa by manipulating the receptors for vasopressin, or its prevalence. For human males, there’s already a genetic test (in the area known to code for vasopressin receptors) that predicts infidelity and partners’ dissatisfaction (did they measure vasopressin response directly?)

Foxes pair-bond for a single breeding season (the male stays long enough to help raise the kits).