Ladies. Any comments?? Men. Any comments?? Being in a long, loving and secure relationship may not be such bliss after all. A warning to all men: Once women feel secure in their relationships, their sex drive plummets, reports the BBC News. And it doesn't take long. Just four years into a relationship is enough to make a woman's libido wane, according to researchers at Hamburg-Eppendorf University in Germany, who found in a study of 520 men and women that less than half of 30-year-old women wanted regular sex. Specifically, they found that at the beginning of the relationship, 60 percent of the women wanted sex "often," but four years later less than 50 percent wanted this. After 20 years in a relationship, just 20 percent wanted to have sex often. The opposite is true for men. No matter how long they are in a relationship with the same woman, 60 to 80 percent of men still want to have sex often with her. Why? Blame it on human evolution. A man has a vested interest in keeping a high sexual interest in his partner. "For men, a good reason for their sexual motivation to remain constant would be to guard against being cuckolded by another male," lead study author Dr. Dietrich Klusmann told the BBC. But it's different for women. They have evolved to have a high sex drive in the initial stages of a relationship in order to form a "pair bond" with their partner. Once that bond is formed, interest in sex wanes. In later years, women possibly limit sex just to increase their partner's interest--you know, the old supply and demand theory. If the supply is limited, the demand will be high.