back in caveman days men sought healthy women to be the mothers of their children. This being part of the reason men generally prefer younger women and it elucidates the point SoonForgotten was making. Hair would've been a great help in assessing the health of a prospective mate: lustrous, shiny hair denotes physical wellbeing whereas sickly people's hair loses its luster and can fall out. A woman with shoulder length hair therefore provides a health status of the past few years for the discerning caveman.
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sticking back on topic, my brother always notes that he thinks girls look like lesbians if they have short hair. he's a bit shallow though, he is.