The ratio of males to females at birth for humans is approximately 105:100 – meaning there are 105 males born for every 100 females. This is known as the “sex ratio.” It is thought that it is evolution‘s way of compensating for males’ shorter life expectancy. The longer life expectancy of females evens out this disparity and actually results in an excess of females among the elderly. In the U.S. the ratio for those over 65 is .72 (72 males for every 100 females).
What about hermaphrodites? Approximately 1 in 4500 births results in an infant whose external sexual organs don’t exactly match what the chromosomes say they should be.
The amount by which females live longer than males on average is referred to by researchers as the “Female Advantage.” Currently in the U.S. the life expectancy at birth is about 80.1 years for females and 73.4 years for males. There are a number of factors that are thought to contribute to the Female Advantage and they can be roughly classified into two buckets: Behavioral and Biological.
Behavioral: Male life expectancy is negatively impacted by the following social and behavioral factors:
- Wars – the vast majority of soldiers have been and continue to be males
- Jobs – the majority of jobs at which you can become disabled (which often reduces life expectancy) or die are held by men. For example, police, firemen, tree trimmers, power line workers, underwater welders, miners, oil wildcatters, etc. are predominantly held by males.
- Effects of Testosterone on Behavior: Males do stupid stuff as adolescents and young adults. Between the ages of 15 and 24 years, men are four to five times more likely to die than women. Researchers call this time period “the testosterone storm.” Causes of death of males in those years in order: motor vehicle accidents (far outweighing the others), homicide, suicide, cancer and drownings. From 25 to about age 60 mortality rates for men and women are about the same. Past age 60 men once again begin to die at higher rates.
- Other dumb stuff: traditionally, males have been nine times more likely to be smokers than women. And make smokers typically smoke more cigarettes than female smokers. Men are more likely to be big drinkers (leads to health problems and accidents). Men do more types of chores around the house (or village) that are not part of their profession that could lead to death. Like falling off ladders, getting electrocuted, falling out of trees, chopping wood, etc.
Biological: Some researchers think that from an evolutionary perspective women live longer because they are more useful for longer. Females have two main evolutionary drives: (1) to pass on their genes and (2) to stay healthy enough to rear as many children as possible. With males it is believed that their main evolutionary purpose is to simply carry genes that ensure longevity and pass them on to their daughters. The thought here is that female longevity is the force that determines the natural life span of both males and females. This is carried out by:
- Two X Choromosomes: Male infants have a 25-30% higher mortality rate in the first year of life. From Scientific American: “When a mutation of one of the genes of the X chromosome occurs, females have a second X to compensate, whereas all genes of the unique X chromosome of males express themselves, even if they are deleterious.”
- Menopause – it protects older women from the risks of bearing children late in life and then works to adjust hormone levels to help ensure that women live long enough to raise those children.
- Hormones – the male hormone of testosterone not only increases aggressive and competitive behavior – it also increases levels of cholesterol. Estrogen actually lowers bad cholesterol and raises good cholesterol.
- Natural Selection – the longer a women lives and the more slowly she ages the more offspring she can have and rear to adulthood, thus selecting these relatively healthy women over less healthy women. With men it has been shown that a male’s reproductive capacity is more linked to his access to females and the lifespan of those females than by his own lifespan (obviously a man can impregnate a different female each day of the year while a female can only give birth every nine months). Thus, a male is more likely to pass on more of his genes by reproducing with a long-living healthy female than he is to live long himself.