It’s not too hard to find cultural exceptions; in early to mid-18th Century France, for example, men’s hair was typically longer than women’s. But it remains the case that through history and across the world, long hair has tended to be seen as a feminine asset.
One explanation comes from evolutionary psychology. It states that men are generally attracted to women with a youthful and healthy appearance (indicative of greater fertility), whereas women are more attentive to signs of physical strength and ‘formidability’. Hair length might have emerged in this context as a visual signal of a woman’s fecundity.
Certainly, hair tends to grow faster and thicker when people are younger and healthier. Consistent with this account, research including a 2017 study by US psychologists has found that men, on average, perceive women with longer hair as more attractive, healthy and youthful. Of course, even if this evolutionary account is true, cultural forces and individual differences also play a strong role in whether we think of long hair as ‘feminine’.