Sex differences in human physiology

This article is about biological sex differences. For gender differences in humans, see Gender differences.

A sex difference is a distinction of biological and/or physiological characteristics typically associated with either males or females of a species in general. This article focuses on quantitative differences which are based on a gradient and involve different averages. For example, males are taller than females on average, but an individual female may be taller than an individual male.

This article describes differences which clearly represent a binary male/female split, such as human reproduction. Though some sex differences are controversial, they are not to be confused with sexist stereotypes.

Sex Determination

The human genome consists of two copies of each of 23 chromosomes (a total of 46). One set of 23 comes from the mother and one set comes from the father. Of these 23 pairs of chromosomes, 22 are autosomes, and one is a sex chromosome. There are two kinds of sex chromosomes–"X" and "Y". In humans and in all most all other mammals, females carry two X chromosomes, designated XX, and males carry one X and one Y, designated XY.

A human egg contains only one set of chromosomes (23) and is said to be haploid. Sperm also have only one set of 23 chromosomes and are therefore haploid. When an egg and sperm fuse at fertilization, the two sets of chromosomes come together to form a unique "diploid" individual with 46 chromosomes.

The sex chromosome in a human egg is always an X chromosome, since a female only has X sex chromosomes. In sperm, about half the sperm have an X chromosome and half have a Y chromosome. If an egg fuses with a sperm with a Y chromosome, the resulting individual is usually male. If an egg fuses with a sperm with an X chromosome, the resulting individual is usually female. An egg's sex chromosome is always an X, so it is the sperm's sex chromosome that determines an individual's sex. There are rare exceptions to this rule in which, for example, XX individuals develop as males or XY individuals develop as females (See Sex determination and differentiation (human).)

Physical differences

For information about how males and females develop differences throughout the lifespan, see sexual differentiation.

Sexual dimorphism

Pioneer plaque
Male pelvis	Female pelvis
Top: Stylised illustration of humans on the Pioneer plaque, showing both male and female. Above: Comparison between a male (left) and a female pelvis (right).

Sexual dimorphism (two forms) refers to the general phenomenon in which male and female forms of an organism display distinct morphological characteristics or features.

Sexual dimorphism in humans is the subject of much controversy, especially relating to mental ability and psychological gender. (For a discussion, see biology of gender, sex and intelligence, gender, and transgender.) Obvious differences between men and women include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physical, psychological and behavioral effects.

Such undisputed sexual dimorphism include gonadal differentiation, internal genital differentiation, external genital differentiation, breast differentiation and body/facial hair differentiation.

Some biologists theorise that a species' degree of sexual dimorphism is inversely related to the degree of paternal investment in parenting. Species with the highest sexual dimorphism, such as the pheasant, tend to be those species in which the care and raising of offspring is done only by the mother, with no involvement of the father (low degree of paternal investment). This would also explain the moderate degree of sexual dimorphism in humans, who have a moderate degree of paternal investment compared to most other mammals.

Comparative and social psychologists have observed that males and females, in general, differ in the way they carry books while walking. Upon using a classification system of the five common methods of carrying books, a high percentage of females will partially cover their body with the books they are carrying, such as by holding them in front of the chest. Most males carry their books at the side of body, leaving the front uncovered (Jenni, M.A. 1976). The most common explanation of this observation is that women typically have less strength than men, making it difficult to balance, and resulting in the need to rest the objects they are carrying on their bodies. Some psychologists hypothesize that it is a maternal instinct in many women causing them to carry inanimate objects in a protective manner.

Appearances

See also: Secondary sex characteristics

Humans show some sexual dimorphism, but are less dimorphic than most other primates.

Man and woman androgenic hairs. Not shown in the woman here is the common (but usually lighter) distribution of hair on the thighs, up the belly, and underarms. Many women also have sparse facial hair and even hair on the breasts, which they commonly remove.

• On average, men are taller than women (See sexual dimorphism).
• On average, men have a larger waist in comparison to their hips (see waist-hip ratio) than women.
• On average, men have longer canine teeth than women.
• On average, men have a greater capacity for cardiovascular endurance. This is due to the enlargement of the lungs of boys during puberty, characterized by a more prominent chest.
• On average, women have more endurance than men.
• On average, men are stronger than women. This is due to a greater capacity for muscular hypertrophy as a result of men's higher levels of testosterone.
• On average, men have more body hair than women.
• Men’s skin is thicker (more collagen), oilier (more sebum) than women’s skin.
• Women's skin is warmer on average than men's.
• In men, the second digit (index finger) tends to be shorter than the fourth digit (ring finger), while in women the second digit tends to be longer than the fourth (see digit ratio).
• On average, women tend to have skin that is 3-4% lighter than men. One scientist (Rebato, 1999) hypothesized this is an adaptation required for increased production of Vitamin D during pregnancy. Vitamin D is necessary to help the body absorb calcium and deposit it in the bones of fast growing embryos. Lighter pigmentation allows more of the sun's UV radiation to penetrate the skin, and increases a woman's ability to produce vitamin D.
• Women have a larger hip section than men, an adaptation for giving birth to infants with large skulls.
• Men have a more pronounced 'Adam's Apple' or thyroid cartilage due to larger vocal cords (and deeper voices).

• Studies examining the leg/trunk ratio between men and women have had conflicting results. In adolescents, there may be no significant gender difference.

Reproduction

See also: Male reproductive system and Female reproductive system

• Female fertility declines after age 30 and ends with the menopause. Pregnancy in the 40s or later has been correlated with increased chance of Down's Syndrome in the children. Men are capable of fathering children into old age. Paternal age effects in the children include multiple sclerosis, autism, breast cancer and schizophrenia, as well as reduced intelligence. Adriana Iliescu was reported as the world's oldest woman to give birth, at age 66. Her record stood until Maria del Carmen Bousada de Lara gave birth to twin sons at Sant Pau Hospital in Barcelona, Spain on December 29, 2006, at the age of 67. In both cases IVF was used. The oldest known father was former Australian miner Les Colley, who fathered a child at age 93.
• Men typically produce billions of sperm each month, many of which are capable of fertilization. Women typically produce one egg a month that can be fertilized into an embryo. Thus during a lifetime men are able to father a significantly greater number of children than women can give birth to. The most fertile woman, according to the Guinness Book of World Records, was the wife of Feodor Vassilyev of Russia (1707-1782) who had 67 surviving children. The most prolific father of all time is believed to be the last Sharifian Emperor of Morocco, Mulai Ismail (1646-1727) who reportedly fathered more than 800 children from a harem of 500 women.

Health

Women live longer than men in most countries. One possible explanation is that more men die young because of war, criminal activity, and accidents. The gap between males and females is decreasing in many developed countries as more women take up unhealthy practices that were once considered masculine like smoking and drinking alcohol, and more men practice healthier living. In Russia, however, the sex-associated gap has been increasing as male life expectancy declines.

The World Health Organization (WHO) has produced a number of reports on gender and health. The following trends are shown:

• Overall rates of mental illness are similar for men and women. There is no significant gender difference in rates of schizophrenia and bipolar depression. Women are more likely to suffer from unipolar depression, anxiety, eating disorders, and post-traumatic stress disorder. Men are more likely to suffer from alcoholism and antisocial personality disorder.
• Worldwide, more men than women are infected with HIV. The exception is sub-Saharan Africa, where more women than men are infected.
• Adult males are more likely to be diagnosed with tuberculosis.
• Before menopause, women are less likely to suffer from cardiovascular disease. However, after age 60, the risk for both men and women is the same.
• Overall, men are more likely to suffer from cancer, with much of this driven by lung cancer. In most countries, more men than women smoke, although this gap is narrowing especially among young women.
• Women are twice as likely to be blind as men. In developed countries, this may be linked to higher life expectancy and age-related conditions. In developing countries, women are less likely to get timely treatments for conditions that lead to blindness such as cataracts and trachoma.
• Women are more likely to suffer from osteoarthritis and osteoporosis.

Anterior cruciate ligament injuries, especially in basketball, occur more often in women than in men.

Certain conditions are X-linked recessive, in that the gene is carried on the X chromosome. Genetic females (XX) will show symptoms of the disease only if both their X chromosomes are defective with a similar deficiency, whereas genetic males (XY) will show symptoms of the disease if their only X chromosome is defective. (A woman may carry such a disease on one X chromosome but not show symptoms if the other X chromosome works sufficiently.) For this reason, such conditions are far more common in males than in females. Examples of X-linked recessive conditions are color blindness, hemophilia, and Duchenne muscular dystrophy.

Sensory

• Females have a more sensitive sense of smell than males, both in the differentiation of odors, and in the detection of slight or faint odors.
• There is also indication that females are better at discerning differences in colours, while males are more aware of, and capable of discerning movement.
• Females have a higher sensitivity to pain than males, due to females having a larger number of nerve receptors.
• Males have a more developed sense of direction.
• Males have a more developed sense of spatial awareness.

Neurobiological

• On average, male brains have approximately 4% more cells and 100 grams more brain tissue than females do. However, both sexes have similar brain weight to body weight ratios. Men have larger left inferior parietal lobes, while women have larger Wernicke's and Broca's areas. Evidence of gender differences in the size of the corpus callosum is ambiguous.
• Women generally have faster blood flow to their brains and lose less brain tissue as they age than men do.
• Depression and chronic anxiety are much more common in women than in men, (it has been speculated, by some, that this is due to difference in the brain's serotonin system).

Other health differences

• Women generally have a higher body fat percentage than men. ^[5]
• Women usually have lower blood pressure than men, and women's hearts beat faster, even when they are asleep.
• Men generally have more muscle tissue mass, particularly in the upper body.
• Men and women have different levels of certain hormones. Men have a higher concentration of androgens while women have a higher concentration of estrogens.
• Adult men have approximately 5.2 million red blood cells per cubic millimeter of blood, whereas women have approximately 4.6 million.

See also

• Gender-based medicine
• Sexual dimorphism
• Sex differentiation
• Virilization
• List of homologues of the human reproductive system

Further reading

• Geary, D. C. (2006). Sex differences in social behavior and cognition: The utility of sexual selection for hypothesis generation. Hormones and Behavior, 49, 273-275. Full text

External links

• Brin, David. 'Neoteny and Two-Way Sexual Selection in Human Evolution: A Paleo-Anthropological Speculation on the Origins of Secondary-Sexual Traits, Male Nurturing and the Child as a Sexual Image.' Journal of Social and Evolutionary Systems 18 (1996): 257-276.