In biological evolution, sexual selection is one of the processes of natural selection by which an organism increases its ability to attract and copulate with a mate (a famous example is the tail the peacock has evolved). In humans, a modern form of sexual selection occurs in things like desire for clean-shaven-ness, eating well and not smoking - as these are seen to be attractive qualities. Facial hair is associated with high testosterone, which is why in the old westerns the baddies were nearly always bearded and the good guy was usually clean-shaven (maybe that’s why the sheriff usually had a moustache – he had to represent the conflation of those two personalities). It’s easy, though, to see why men shaving caught on – I should imagine the number of women who prefer clean-shaven men far outweighs the number who prefer men with facial hair. But also, the number of women who would prefer not to go out with a hairy man is much greater than the women who wouldn’t go out with a man because he is clean shaven.
The same is true when it comes to eating fatty foods and smoking – in terms of partnership costs and benefits, if you smoke and eat fatty foods you get all the benefits of indulgence (we're good at discounting the future - the costs come later) but your partner only gets the smoky breath and the flabby stomach and double chin. If you don’t shave then potential mates incur precisely the number of costs commensurate with number of women who don’t like facial hair. Therefore a man who eats well, doesn’t smoke and is clean-shaven is sexually selectable in two ways – not just because those things are more attractive, but also because it gives a potential female partner indication that you have the mindfulness for looking after your body. This makes a man good husband and father material.
