In almost all parts of the world women live from six months to four years longer than men and in some places the difference increases to 10 years. A study investigating trends over the last three hundred years reveals that the different longevity is particularly marked in periods of famine or prolonged epidemics. In early infancy females seem more likely than boys to overcome difficulties related to health when all other factors are the same. At this age behavioral differences between boys and girls are not yet that pronounced (fighting in wars, committing acts of violence, running risks in general on average involve men more than women) and so the observed difference in longevity should be due to some biological reason. Data examined refer to 7 populations that are very diverse and cover historical periods that are also very different with longevity in some cases being little over 20 years. For example, the girls born in Ucraine at the time of the great famine (1933), lived on average to the age of 10.85 years whereas boys lived to 7.3 years, that is 50% less. The difference was mainly due to a higher mortality among boys with respect to girls in the newborn age. Researchers are trying to work out if there is a hormonal or genetic explanation for what some call the strength of females and others call the weakness of males. There is a logic in all this. In periods of increased mortality for various reasons the death of many people also means the loss of their procreational potential. This means that if the population falls so will the number of births. Since a man can potentially have many more children than a woman, from the procreation point of view, to avoid the risk of extinction it is important to have relatively more women.