What is beauty in human beings? Poets have written a lot about it. I will take a different approach. Beauty is the traits that humans have evolved consider attractive. The characteristics of a good mate, in other words. A good mate is someone with whom one would expect to have numerous fit offspring. A healthy and fertile person. Beauty consists of the outward signs of health and fertility.
Health signals are similar for men and women – good posture, energetic movement, strength and speed, neither excessive fat nor skeletal thinness, smooth skin without patches of different colour, clear voice, bright eyes, no strange smells, thick lustrous hair etc. Posture, movement and strength signal that there is no internal disease that causes weakness or fatigue. Excessive fat or thinness suggest metabolic problems or malnutrition. Skin diseases manifest as roughness and redness. Clear voice signals absence of throat or lung diseases, bright eyes show no eye infection etc.
Fertility signals differ somewhat across genders. Youth and health are associated with fertility in both genders, but a masculine or feminine figure is a fertility signal only for the appropriate gender. Broad shoulders, big muscles, wide angular face and hairyness are all caused by high testosterone levels. These suggest a fertile male, but if observed in a female, are signs of some endocrine disease. Smooth curves, large breasts, narrow waist and wide hips are signs of high oestrogen levels and a fertile female. The narrow waist further suggests the woman is not pregnant already (pregnancy means current sex is unlikely to lead to offspring, which can be interpreted as temporary infertility as far as the current partner is concerned).