The fundamental difference between men and women is that the latter have two X chromosomes whereas the male has one X and one Y chromosomes. Although the rest of the chromosomal make up of men and women is the same the genetic differences caused by the differences in the sex chromosomes are responsible for all of the differences between the sexes. The Y chromosome contains the genes for development of the testes - which produce the male sex hormone testosterone. Although this hormone is also present in women the larger amounts in males accounts for the male primary and secondary sexual characteristics including body fat distribution and muscle bulk. Without testosterone all men would be women ! On the converse females develop ovaries right from the embryonic stage which produce oestrogen leading to the development of female sexual characteristics. It is these fundamental differences in hormones which lead to the differences observed between the sexes. These can be summarised as biomechanical differences physical size body fat distribution and percentage menstruation vs sperm production dietary requirements effects of training puberty and ageing psychology and behaviour