The term sex refers to biological differences between women and men. Biological differences are fixed and (mostly) unchangeable and vary little across cultures and over time (the age of menarche for instance). Gender does not necessarily refer to differences or concerns linked to biological characteristics of women and men, although gender-based differences and sex-based differences are often interrelated. While sex-based differences are unchangeable, gender-based differences and gender relations are affected by policies, regulations, and legislations, and can be changed.