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This article is about biological sex
differences. For gender differences in humans, see Gender
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
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.
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
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
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).)
- For information about how males and females develop
differences throughout the lifespan, see sexual
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
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
Such undisputed sexual dimorphism include gonadal
differentiation, internal genital differentiation, external genital
differentiation, breast differentiation and body/facial hair
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).[Full citation needed] 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
Humans show some sexual dimorphism, but are less dimorphic than
most other primates.
- On average, men are taller than women, by about half a foot
(See sexual dimorphism).
- On average, men have a larger waist in comparison to their hips
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, 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) and 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).
- 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).
- 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,
cancer  and
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
- 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.
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
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
- 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
- Women are more likely to suffer from osteoarthritis and osteoporosis.
Anterior cruciate ligament
injuries, especially in basketball, occur more often in women than
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
- Females have a more sensitive sense of smell than males, both
in the differentiation of odors, and in the detection of slight or
- 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.
- Males have a more developed sense of conscious direction, while
females have a greater sense of subconscious direction.
- Males have a more developed sense of spatial awareness.
- 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
- 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).
- 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
- 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
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