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The Biology of Sex and Gender
In the 1960s William Masters and Virginia Johnson conducted
groundbreaking research on the human sexual response.
They identified four phases of sexual response:
The excitement phase is a period of arousal and preparation
for intercourse.
During the plateau phase, the increase in sexual arousal levels
off.
During orgasm, rhythmic contractions occur in the vagina and
those in the penis are accompanied by ejaculation.
Resolution follows as arousal decreases and the body returns
to its previous state.
Males have a refractory phase, during which they are unable
to become aroused or have another orgasm for minutes, hours, or even days, depending on the individual and the circumstances.
The Coolidge effect is a quicker return to sexual arousal when
a new partner is introduced.
Castration, or the removal of the gonads (testes or ovaries)
is one technique used to study hormonal effects because it removes the major source of sex hormones.
Castration results in a loss of sexual motivation in nonhuman
mammals of both sexes.
Androgens are the class of hormones responsible for a number
of male characteristics and functions.
Testosterone is the major sex hormone in males.
Estrus is a period when the female is ovulating, sex hormone
level are high, and the animal is said to be in heat.
Estrogen is a class of hormones responsible for a number of
female characteristics and functions.
The MPOA (medial preoptic area) of the hypothalamus is one of
the more significant brain structures involved in male and female sexual behavior.
Stimulation of the MPOA increases copulation in rats of both
sexes.
The MPOA is active when rats copulate spontaneously.
The MPOA appears to be more responsible for performance than
for sexual motivation.
When it was destroyed in male monkeys they no longer tried to
copulate, but instead they would often masturbate in the presence of a female.
A part of the amygdala known as the medial amygdala also contributes
to sexual behavior in rats of both sexes.
The medial amygdala’s role apparently is to respond to
sexually exciting stimuli, such as the presence of a potential sex partner.
More significant for males is the sexually dimorphic nucleus
(SDN), located in the MPOA.
The name comes from the fact that the SDN is five times larger
in male rats than in females.
The level of sexual activity in males is related to the size
of their SDN, which in turn depends on prenatal (before birth) exposure to testosterone.
The ventromedial nucleus of the hypothalamus is important for
sexual behavior in female rats.
Activity increases there during copulation and destruction reduces
the female’s responsiveness to a male’s advances.
Injection and microdialysis studies show that dopamine activity
in the MPOA is involved in sexual motivation in males and females of several species and is critical for sexual performance
in males.
Ejaculation is also accompanied by serotonin increases in the
lateral hypothalamus, which apparently contributes further to the refractory period.
Oxytocin
is a hormone that causes smooth muscle contractions, for example during milk ejection in lactating
(breast feeding) females.
is released as a neurotransmitter by hypothalamic neurons.
in non-human animals contributes to male sexual behavior and female receptivity and appears to
account for monogamous pair bonding in prairie voles.
Pheromones are airborne chemicals released by an animal that
have physiological or behavioral effects on another animal of the same species.
Pheromones are detected by the VNO (vomeronasal organ), a cluster
of receptors located in the nasal cavity.
The Biological Determination of
Sex
Sex is the term for the biological characteristics that divide
humans and other animals into the categories of male and female.
Gender refers to the behavioral characteristics associated with
being male or female.
Gender role is the set of behaviors society considers appropriate
for people of a given biological sex.
Gender identity is the person’s subjective feeling of
being male or female.
For the first month, XX and XY fetuses are identical.
Later, the primitive gonads (testes and ovaries, the primary
reproductive organs) in the XX individual develop into ovaries, where the ova (eggs) develop.
The Müllerian ducts develop into the uterus, fallopian tubes,
and the inner vagina.
The Wolffian ducts that would become the male organs wither
and are absorbed.
The undifferentiated external genitals become a clitoris, the
outer segment of the vagina, and the labia, which partially enclose the entrance to the vagina.
If the fetus receives a Y chromosome from the father, the SRY
gene on that chromosome causes the primitive gonads to develop into testes, the organs that will produce sperm.
The testes begin secreting two types of hormones:
Müllerian inhibiting hormone defeminizes the fetus by causing
the Müllerian ducts to degenerate.
Testosterone, the most prominent of the androgens, masculinizes
the internal organs.
The Wolffian ducts develop into the seminal vesicles, which
store semen, and the vas deferens, which carry semen from the testes to the penis.
A derivative of testosterone, dihydrotestosterone, masculinizes
the external genitals.
Organizing effects mostly occur prenatally and shortly after
birth.
They affect structure and are lifelong in nature.
Activating effects can occur at any time in the individual’s
life
They may come and go with hormonal fluctuations or be long lasting,
but they are reversible.
The presence of testosterone masculinizes certain brain structures.
That statement is somewhat misleading, though, because it is
estradiol, the principle estrogen hormone, that carries out the final step of masculinization.
When testosterone enters the neurons it is converted to estradiol
by a chemical process called aromatization.
A the critical time when brain masculinization occurs, aromatase
increases in the areas that are to be masculinized.
Gender-Related Behavioral and Cognitive
Differences
Back in 1974 Eleanor Maccoby and Carol Jacklin reviewed over
2,000 studies that included measures of sex differences.
They concluded that the evidence firmly supported three differences
in cognitive performance and one difference in social behavior:
Girls have greater verbal ability than boys.
Boys excel in visual-spatial ability.
Boys excel in mathematical ability.
Boys are more aggressive than girls.
However, the cognitive differences have turned out to be rather
limited:
Females excel in verbal fluency and writing, but not in reading
comprehension or vocabulary.
Males’ scores exceed females’ most on tasks requiring
mental rotation of a three-dimensional object and less on other spatial tasks.
Although females are better at computation, males do better
on tests of broad mathematical ability like those on the Scholastic Assessment Test (SAT).
Sexual Anomalies
Pseudohermaphrodites have ambiguous internal and external organs,
but their gonads are consistent with their chromosomes.
Androgen insensitivity syndrome, a form of male pseudohermaphroditism,
is caused by a genetic absence of androgen receptors which results in insensitivity to androgen.
Ovary development is suppressed by Müllerian inhibiting hormone
but, because the individual is unaffected by androgens, the testes do not descend and the external genitals develop as more
or less feminine.
In the absence of testosterone’s influence, androgen insensitive
individuals tend to have well-developed breasts and a flawless complexion.
One cause of female pseudohermaphroditism is congenital adrenal
hyperplasia (CAH), which results form an enzyme defect that causes the individual’s adrenal glands to produce large
amounts of androgen during fetal development and after birth until the problem is treated.
Parents often choose reconstructive surgery to reduce the size
of the clitoris and eliminate labial fusion, giving the genitals a more feminine appearance.
If masculination is more pronounced, the parents may decide
to rear the child as a boy.
In that case, the surgeons usually finish closing the labia
and insert artificial testes in the scrotum to enhance the masculine appearance.
Reversing the sex hormone balance during prenatal development
changes the brain and later behavior in nonhuman animals.
Women born with CAH have been described as tomboyish in childhood.
Androgen-insensitive males are typically feminine in behavior,
have a strong childbearing urge, and are decidedly female in their sexual orientation.
Androgen insensitive males are like females in that their verbal
ability is higher than their spatial performance, and their spatial performance is lower than that of other males.
CAH women, like men, show higher spatial ability than other
women, and CAH girls even draw pictures more typical of boys, using darker colors and including mechanical objects and excluding
people.
The “neutral-at-birth” theorists claim that individuals
reared in opposition to their chromosomal sex generally accept their sex of rearing, and that this demonstrates that rearing
has more effect of gender role behavior than chromosomes or hormones.
Milton Diamond, who advocates a “sexuality-at-birth”
hypothesis, argues that the reason individuals with ambiguous genitals accept their assigned gender is that sex of rearing
is usually decided by whether the genital appearance is predominantly masculine or feminine, which in turn reflects the prenatal
hormone environment.
According to Diamond, there is no case in the literature where
an unambiguously male or female individual was successfully reared in opposition to the biological sex.
Sexual Orientation
Homosexual men are often referred to as gay, and homosexual
women are often called lesbians.
The term for those who are not exclusively homosexual or heterosexual
is bisexual.
During development, homosexuals do how a high rate of gender
nonconformity – a tendency to engage in activities usually preferred by the other sex, and an atypical preference for
other-sex playmates and companions while growing up.
Genetic studies provide the most documented and most consistent
evidence for a biological basis for sexual orientation.
Homosexuality is two to seven times higher among the siblings
of homosexuals than it is in the population.
Identical twins are more concordant for homosexuality than fraternal
twins or non-twin siblings.
Simon LeVay (1991) found the INAH3 (for third interstitial nucleus
of the anterior hypothalamus) to be half the size in gay men and heterosexual women as in heterosexual men.
In other research, the suprachiasmatic nucleus (SCN) was larger
in gay men than in heterosexual men and contained almost twice as many cells that secrete the hormone vasopressin.
The anterior commissure was larger in gay men and heterosexual
women than in heterosexual men.
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