- Sex differences in humans
A sex difference is a distinction of biological and/or physiological characteristics associated with either males or females of a species. These can be of several types, including direct and indirect. Direct being the direct result of differences prescribed by the Y-chromosome, and indirect being a characteristic influenced indirectly (e.g. hormonally) by the Y-chromosome. Sexual dimorphism is a term for the phenotypic difference between males and females of the same species.
Direct sex differences follow a binary distribution. Through the process of meiosis and fertilization (with rare exceptions), each individual is created with zero or one Y-chromosome. The complimentary result for the X-chromosome follows, either a double or a single X. Therefore, direct sex differences are usually binary in expression (although the deviations in complex biological processes produce a menagerie of exceptions). These include, most conspicuously, male (vs female) gonads.
Indirect sex differences are general differences in class, as quantified by empirical data and statistical analysis. Most differing characteristics will conform to a bell-curve (i.e. normal) distribution which can be broadly described by the mean (peak distribution) and standard deviation (indicator of size of range). Often only the mean or mean difference between sexes is given. This may or may not preclude overlap in distributions. For example, males are taller than females on average, but an individual female may be taller than an individual male.
The most obvious differences between males and females include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioural effects, including gonadal differentiation, internal and external genital and breast differentiation, and differentiation of muscle mass, height, and hair distribution.
Sex determination and differentiation
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 almost 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.
- For information about how males and females develop differences throughout the lifespan, see sexual differentiation.
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. Although sex is a binary dichotomy, with "male" and "female" representing opposite and complementary sex categories for the purpose of reproduction, a small number of individuals have an anatomy that does not conform to either male or female standards, or contains features closely associated with both. Such individuals, described as intersexuals, are sometimes infertile but are often capable of reproducing.
Some biologists[who?] 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.
Size, weight and body shape
- Externally, the most sexually dimorphic portions of the human body are the chest, the lower half of the face, and the area between the waist and the knees.
- Males weigh about 15 % more than females, on average. For those older than 20 years of age, males in the US have an average weight of 86.1 kg (190 lbs), whereas females have an average weight of 74 kg (163 lbs).
- On average, men are taller than women, by about 15 cm (6 inches). American males who are 20 years old or older have an average height of 175.8 cm (5 ft 9 in). The average height of corresponding females is 162 cm (5 ft 4in).
- On average, men have a larger waist in comparison to their hips (see waist-hip ratio) than women.
- Women have a larger hip section than men, an adaptation for giving birth to infants with large skulls.
Skeleton and muscular system
Strength, power and muscle mass
On average, males are physically stronger than females. The difference is due to females, on average, having less total muscle mass than males, and also having lower muscle mass in comparison to total body mass. While individual muscle fibers have similar strength, males have more fibers due to their greater total muscle mass. The greater muscle mass of males is in turn due to a greater capacity for muscular hypertrophy as a result of men's higher levels of testosterone. Males remain stronger than females, when adjusting for differences in total body mass. This is due to the higher male muscle-mass to body-mass ratio.
As a result, gross measures of body strength suggest an average 40-50% difference in upper body strength between the sexes as a result of this difference, and a 20-30% difference in lower body strength. This is supported by another study that found females are about 52-66 percent as strong as males in the upper body (34-48% difference), and about 70-80 percent as strong in the lower body (20-30% difference). One study of muscle strength in the elbows and knees—in 45 and older males and females—found the strength of females to range from 42 to 63% of male strength.
Comparison between a male (left) and a female pelvis (right).
- Males, on average, have denser, stronger bones, tendons, and ligaments.
- 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).
- Men have a more pronounced 'Adam's Apple' or thyroid cartilage (and deeper voices) due to larger vocal cords.
- On average, men have longer canine teeth than women.
- Male skulls and head bones have a different shape than female skulls. One difference is in the roundness of the eye cavities, another is the male's bony brow, and a third difference is the shape of the jaw.
- Male and female pelvises are shaped differently. The female pelvis features a wider pelvic cavity, which is necessary when giving birth. The female pelvis has evolved to its maximum width for childbirth — an even wider pelvis would make women unable to walk. In contrast, human male pelves did not evolve to give birth and are therefore slightly more optimized for walking. The female pelvis is larger and broader than the male pelvis which is taller, narrower, and more compact. The female inlet is larger and oval in shape, while the male inlet is more heart-shaped.
- Contrary to popular belief, however, males and females do not differ in the number of ribs; both have twelve pairs.
Males typically have larger tracheae and branching bronchi, with about 56 percent greater lung volume per body mass. They also have larger hearts, 10 percent higher red blood cell count, higher haemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance. 
Skin and hair
The skin of females is warmer on average than that of males.
On average, males have more body hair than females. Males have relatively more of the type of hair called terminal hair, especially on the face, chest, abdomen and back. In contrast, females have more vellus hair. Vellus hairs are smaller and therefore less visible.
Baldness is much more common in males than in females. The main cause for this is male pattern baldness or androgenic alopecia. Male pattern baldness is a condition where hair starts to get lost in a typical pattern of receding hairline and hair thinning on the crown, and is caused by hormones and genetic predisposition.
On average and after the end of puberty, males have darker hair than females and according to most studies they also have darker skin (male skin is also redder, but this is due to greater blood volume rather than melanin). Male eyes are also more likely to be one of the darker eye colors. Conversely, women are lighter-skinned than men in all human populations.[non-primary source needed] The differences in color are mainly caused by higher levels of melanin in the skin, hair and eyes in males.[non-primary source needed] In one study, almost twice as many females as males had red or auburn hair. A higher proportion of females were also found to have blond hair, whereas males were more likely to have black or dark brown hair.[non-primary source needed] Another study found green eyes, which are a result of lower melanin levels, to be much more common in women than in men, at least by a factor of two.[non-primary source needed] However, a more recent study found that while women indeed tend to have a lower frequency of black hair, men on the other hand had a higher frequency of red-blond hair, blue eyes and lighter skin. According to one theory the cause for this is a higher frequency of genetic recombination in women than in men, possibly due to sex-linked genes, and as a result women tend to show less phenotypical variation in any given population.[non-primary source needed] Also, women tend to bleach or color their hair while men tend not to, which would make the proportion of blond or red-haired women seem higher than what it is naturally.
Sexual organs and reproductive systems
Men and women have different sex organs. Women have two ovaries that stores the eggs, and uterus which is connected to a vagina. Men have testicles that produce sperm. The testicles are placed in the scrotum behind the penis. The male penis and scrotum are external extremities, whereas the female sex organs are placed "inside" the body.
Men's orgasm is nearly essential ("nearly" as small groups of sperm can escape the penis before orgasm is reached) for reproduction, whereas female orgasm is not. The female orgasm was believed to have no obvious function other than to be pleasurable although some evidence suggests that it may have evolved as a discriminatory advantage in regards to mate selection.
Reproductive capacity and cost
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.
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.
Brain and nervous system
Human males, on average, have larger brains than females.
In 1861, Paul Broca examined 432 human brains and found that the brains of males had an average weight of 1325 grams, while the brains of females had an average weight of 1144 grams. More recently, a 1992 study of 6,325 Army personnel found that men's brains had an average volume of 1442 cm³, while the women averaged 1332 cm³. These differences were shown to be smaller but to persist even when adjusted for body size measured as body height or body surface, such that women averaged 100 g less brain mass than men of equal size.
According to another estimate, on average, male brains have approximately 4 % more cells and weigh 100 grams more than female brains do. However, both sexes have a similar brain weight to body weight ratio. Female brains are more compact than male brains in that, though smaller, they are more densely packed with neurons, particularly in the region responsible for language.
In studies concerning intelligence, it has been suggested that the ratio of brain weight to body weight (rather than actual brain weight) is more predictive of IQ levels. While men's brains are an average of 10-15% larger and heavier than women's brains, some researchers propose that the ratio of brain to body size does not differ between the sexes. However, some argue that since brain-to-body-size ratios tend to decrease as body size increases, a sex difference in brain-weight ratios still exists between men and women of the same size.
There are also differences in the structure of and in specific areas of the brain. For instance, two studies found that men have larger parietal lobes, though another study failed to find any statistically significant difference. At the same time, females have larger Wernicke's and Broca's areas, areas responsible for language processing. Studies using MRI scanning have shown that the auditory and language-related regions in the left hemisphere are proportionally expanded in females versus in males. Conversely, the primary visual, and visuo-spatial association areas of the parietal lobes are proportionally larger in males. Evidence of a sex difference in the relative size of the corpus callosum was discussed during the 1980s and 90s. However, a 1997 meta-study concluded that there is no relative size difference, and that the larger corpus callosum in males is due to generally larger brains in males on average.
In total and on average, females have a higher percentage of grey matter in comparison to males, and males a higher percentage of white matter. However, some researchers maintain that as males have larger brains on average than females, when adjusted for total brain volume, the grey matter differences between sexes is small or nonexistent. Thus, the percentage of grey matter appears to be more related to brain size than it is to gender.
A proposed alternative way of measuring intelligence is by using grey matter or white matter volume in the brain as an indicator. The former is used for information processing, whereas the latter makes up the connections between processing centers. In 2005, Haier et al. reported that, compared with men, women show more white matter and fewer grey matter areas as related to intelligence. However, he concluded that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance."  Using brain mapping, it was shown that men have more than six times the amount of gray matter related to general intelligence than women, and women have nearly ten times the amount of white matter related to intelligence than men. They also report that the brain areas correlated with IQ differ between the sexes. In short, men and women apparently achieve similar IQ results with different brain regions.
Other differences that have been established include greater length in males of myelinated axons in their white matter (176,000 km compared to 146,000 km); and 33% more synapses per mm3 of cerebral cortex. Another difference is that females generally have faster blood flow to their brains and lose less brain tissue as they age than males do. Additionally, depression and chronic anxiety are much more common in women than in men, and it has been speculated, by some, that this is due to differences in the brain's serotonin system).
Genetic and hormonal causes
Both genes and hormones affect the formation of human brains before birth, as well as the behavior of adult individuals. Several genes that code for differences between male and female brains have been identified. In the human brain, a difference between sexes was observed in the transcription of the PCDH11X/Y gene pair, a pair unique to Homo sapiens. It has been argued[by whom?] that the Y chromosome is primarily responsible for males being more susceptible to mental illnesses.
Hormones significantly affect human brain formation, as well as brain development at puberty. A 2004 review in Nature Reviews Neuroscience observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease."
- 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 more pain receptors in the skin. That may contribute to the lower pain tolerance of women.
Tissues and hormones
- Women generally have a higher body fat percentage than men.
- 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. The main male-associated hormone is testosterone.
- Adult men have approximately 5.2 million red blood cells per cubic millimeter of blood, whereas women have approximately 4.6 million.
- Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and succumb for shorter periods.
Females live longer than males in most countries around the world. One possible explanation is the generally more risky behavior engaged in by males. More males than females die young because of war, criminal activity, and accidents. However, 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. In Russia, however, the sex-associated gap has been increasing as male life expectancy declines.
Illness and injury
Sex chromosome disorders
Certain diseases and conditions are clearly sex related in that they are caused by the same chromosomes that regulate sex differentiation. Some 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.
No vital genes reside only on the Y chromosome, since roughly half of humans (females) do not have Y chromosomes. Still, there are diseases that are caused by a defective Y chromosome or of a defective number of them. One human disease linked to a defect on the Y chromosome is defective testicular development. Other conditions include Klinefelter's syndrome and XX male syndrome.
- 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, as well as developmental psychiatric disorders such as autism spectrum disorders and Tourette syndrome.
- 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.
Infectious disease prevalence varies - this is largely due to cultural and exposure factors. In particular the WHO notes that:
- 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.
Some other sex-related health differences include:
- Anterior cruciate ligament injuries, especially in basketball, occur more often in women than in men.
- From conception to death, but particularly before adulthood, females are generally less vulnerable than males to developmental difficulties and chronic illnesses. This could be due to females having two x chromosomes instead of just one, or in the reduced exposure to testosterone.
- Gender-based medicine
- Genetics of gender
- Gender differences in coping
- Sexual dimorphism
- Sex differentiation
- Sex and intelligence
- List of homologues of the human reproductive system
- Man flu
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