Hair care
Scalp hair grows, on average, at a rate of about 1.25 centimeters per month, and shampoos or vitamins have not been shown to noticeably change this rate. Hair growth rate also depends upon what phase in the cycle of hair growth one is actually in; there are three phases. The speed of hair growth varies based upon genetics, gender, age, hormones, and may be reduced by nutrient deficiency (i.e., anorexia, anemia, zinc deficiency) and hormonal fluctuations (i.e., menopause, polycystic ovaries, thyroid disease).
Long hair
In most current cultures, it is more typical for women to wear long hair than for men to do so. An American study shows significant correlation between hair length and age, which indicates that younger women tend to have longer hair than older women. A significant correlation was also found between women's hair length and hair quality. Moreover, hair quality was correlated with the women's perceived physical health. Consistent with principles of evolutionary psychology, these results indicate that hair length and quality can act as a cue to a woman's youth and health, signifying reproductive potential. The correlation between the woman's hair length and marital status, or number of children, was however not more than to be expected from the correlation between hair length and age.