A woman's sex drive decreases over a period of time. This decline is slow but steady- month by month.
Sex researchers Sarah Murray and Robin Milhausen monitored and rated 170 men's and women's sexual desire levels on the Female Sexual Function Index. With each passing month the female participants who were in a relationship with the same partner reported their sexual desire decreased by 0.02 on the scale, which ranges from 1.2 to 6.0. Where as the sexual desire of the male participants remained steady.
Researchers believe this difference is driven by an evolutionary need for men to produce offspring while women focus on child-rearing.
The findings are published in the Journal of Sex and Marital Therapy.