Women have an average life expectancy that's about 4 years longer than men's - regardless of culture or geography. Even among animal species, females outlive males.

It has been long believed that women outlived men because men are traditionally more prone to taking risks, obesity and habits of using alcohol, cigarettes and drugs.

However, scientists from the University of California San Francisco have found that the secret may lie in the second X chromosome present in females. The study, conducted on mice, showed that females live longer than males, and that XX largely governs this trait in ageing mice.

All mammals are born with two sex chromosomes. Females have two X chromosomes, whereas males have one X and one Y. The X chromosomes are necessary for survival and contain important genes related to the brain. The Y chromosomes, on the other hand, are found only in males and are not crucial for survival.

Scientists gave experimental mice four different combinations of chromosomes and gonads: the two found in nature - XX with ovaries and XY with testes - and two others created in the laboratory - XX testes and XY ovaries.

It turned out that mice with two X-chromosomes tended to live longer, regardless of whether they had ovaries or testes.