Again, scientists are all over this. Yes, the features of physical attraction varies from person to person and across generations and cultures, but T&A has held up as attractive to men (possibly because wide hips denote lower potential for mortality in childbirth) and masculine features have held up as attractive to women (because most women don’t want to mate with a douche). A narrow waist magnifies the appearance of the T&A (doy), but since women tend to become larger as they age, “thinness” (as defined by being able to shop for normal-sized clothes, not by whether someone has curves) suggests youth, which men are also generally attracted to. The paintings and sculptures of eastern and western cultures usually prize young, curvy women somewhere closer to Y than X. See classical Italian sculpture and pretty much any depiction of a woman from the Indian subcontinent.