It is believed that those studies may lead to the discovery of new factors explaining the discrepancy in lifespan between women and men. One such is the area of foetal microchimerism, which is the persistence of certain numbers of foetal cells in the mother after pregnancy. Another may be found in further studies about the system involved in enhancing a mother's immune system, during and after pregnancy.

Further studies may find that women who give birth live longer than those who do not. In addition, those women who live to be over 100 may have special genes that slow down the ageing process and reduce the probability of heart disease, stroke, cancer and Alzheimer's disease.

The X chromosome may be another influential factor in female lifespan. Because women have two X chromosomes, if there is an abnormality on one of them, the normal one can be used rather than the faulty one. In this case the woman becomes a carrier of the disease but not its victim.

Serious studies point toward the menopause as a major determinant of lifespans. Very few species menstruate apart from women and certain whales.

The evolutionary force needed to pass on genes and the need to stay alive and bear the maximum number of children may be some of the natural forces that enable women to live longer than men.

In the USA and other developed countries, average life expectancy is about eighty years for women and about seventy years for men. In Sweden, however, during the eighteenth century it was about thirty-seven years for women and thirty-four for men.

The probability of women outliving men during the first twenty-five years is more than four times higher, mainly because of men's testosterone. This factor decreases with age, but the gap still remains in favour of women. The sex hormones are a clear factor to consider; young males are aggressive, increased levels of harmful cholesterol leading to heart disease or stroke.

The female hormone oestrogen lowers harmful cholesterol and raises 'good' cholesterol, however. Recent studies suggest that oestrogen treatment after menopause might reduce the risk of death, especially from heart disease or stroke.

Females have hearts that last longer and better blood vessels, possibly owing to the oestrogen hormone. Even diseases such as heart disease, stroke, cancer and diabetes ultimately kill more men than women.

The advantage women have over men does not apply, however, to women who smoke, drink, are overweight or under excessive stress, typical of women who work in domains previously dominated by men. Actually, there is no gap at all among male and female smokers.

There are however certain countries where there is almost no such gap, such as India and Pakistan, where sexual discrimination and practices such as female infanticide and bride-burning are rife.

Mortality rates may also be affected by chromosomal differences between men and women. They both carry genetic mutations that may cause certain life-threatening diseases.

Females have two X chromosomes, so if there is an abnormality in one of them the normal can be used, making the woman a carrier only of the disease. Men have one X and one Y chromosome, so they cannot use an alternative chromosome, if a gene is defective. The above-described differences in longevity have been observed in most animals.

Another fact is that men are much more likely than women to engage in risky and violent behaviour, consequently increasing the male death rate. More men than women also die in car accidents, homicides, and even suicides.

Foetal microchimerism is the persistence of certain numbers of foetal cells in a mother after pregnancy. A number of recent studies suggest that it may be the cause of some autoimmune diseases. Long-term persistence of foetal cells in healthy women is a contraindication, however. The long-term persistence of foetal cells may also have significance in developing the tolerance of the foetus.

If microchimerism is owed to the transfer of cells between mother and foetus, further studies might indicate the effect of those foetal cells in enhancing the mother's immune system during and after pregnancy. We know that during pregnancy cells can migrate between mother and foetus in both directions.

As pregnancy advances, the cell transfer rate from foetus to mother increases. In most cases, the foetal cells are compatible with the mother's immune system, so the mother's body does not reject them.

It is believed that such women, especially those who live to over ninety or a hundred, may have special genes that slow down ageing and reduce the probability of heart, stroke, cancer or Alzheimer's disease.

Further studies might show that women who give birth will live longer than those who do not. It might also be that those who give birth at least to one male will live even longer.

The detection of Y chromosomes or male DNA in women after pregnancy, even in a woman who had her last son many years prior to blood sampling, may support this theory.

In general, stem cells can be replicated and they are capable of long-term self-renewal. As they do not have any specific structures, they can evolve to specialized cells such as heart muscle, nerves or blood cells.

The sources of blood stem cell transplants are bone marrow, peripheral blood and the umbilical cord of newborn babies. A new technology that has generated new businesses is the collection of umbilical cord blood stem cells from babies, which are stored for future use.

The advantages of stem cells from the umbilical cord in conjunction with the fact that there is a transfer of cells between mother and foetus may be another factor explaining and supporting the theory that women who give birth will live longer than those who do not.

Finally, it should be emphasized that the reason that women live longer is based on many factors, only some of them mentioned above.

The Bible (English version) says (Genesis 16): 'Unto the woman He said: I will greatly multiply thy sorrow and thy conception; in sorrow thou shall bring forth children; and thy desire shall be to thy husband, and he shall rule over thee... '.

The Hebrew version has: '... in sorrow thou shall bring forth Boys'.

Although some may claim that 'Boys' or Banim is the generic name for children, one may wonder if the original was indeed 'Boys'...

The longer lifespan of females might be part of some grand Darwinian scheme whereby mother nature is rewarding motherhood, and especially those women who give birth to at least one male; mothers may gain another five years of life for their effort... and they well deserve it...

This article is part of the book entitled: "Sex and Scientific Philosophy" published at Amazon: 
http://www.amazon.com/Sex-Scientific-Philosophy-Dr-Giora/dp/9659162316