Transcript Document

Sexual Behaviors
PSYC 370
Sex and Hormones
• Sexual reproduction between two individuals
increases variation in the gene pool.
• Variation in the gene pool of a species
enables quick evolutionary adaptations to
change in the environment.
• Also corrects genetic errors and
disadvantageous mutations.
Sex and Hormones
• Widespread communication throughout the
body is accomplished through the release of
hormones.
• Two kinds of hormones include:
– Steroid hormones
– Sex hormones
Sex and Hormones
•
Steroid hormones are derived from
cholesterol, contain four carbon rings and
exert their effects in three ways.
1. Binding to membrane receptors like
neurotransmitters.
2. Entering cells and activate certain kinds
of proteins in the cytoplasm.
3. Binding to chromosomes where they
activate or inactivate certain genes.
Sex and Hormones
•
The sex hormones are a special kind of
steroids, released mostly by the gonads and
to a lesser degree by the adrenal glands.
– affect the brain, genital and other organs
• Two types of sex hormones include:
1. Androgens
2. Estrogens
• Both sexes have these categories in varying
quantities .
Sex and Hormones
• Androgens are a groups of sex hormones
that include testosterone and others.
• Generally referred to as “male hormones”
because men have higher levels than
women.
Sex and Hormones
• Estrogens include estradiol and others and
are referred to as “female hormones”
because women have higher levels.
• Progesterone is a type of hormone that
prepares the uterus for the implantation of a
fertilized ovum and promotes the
maintenance of pregnancy.
Sex and Hormones
• Sex limited genes are those activated by
androgens or estrogens and control most of
the differences between male and female.
– Example: estrogens activate the gene for
breast growth; androgens activate the
gene for the growth of facial hair in men.
• Sex hormones increase or decrease the rate
of apoptosis in various regions of the brain.
– Certain areas are slightly larger in males or
females
Sex and Hormones
•
Sex hormones can have the following
effects:
1. Organizing effects- occur mostly at sensitive
stages of development.
-Determine whether the brain and body
will develop male or female
characteristics
2. Activating effects- occur at any time of life
and temporarily activate a particular
response.
Sex and Hormones
• The distinction between the activating and
organizing effects of hormone is not absolute.
– Example: hormones early in life can exert
temporary effects; during puberty
hormones can also induce long-lasting
structural changes
Sex and Hormones
• Obvious differences can exist between the
reproductive organs and the gonads of males
and female.
• Sexual differentiation begins with the
chromosomes.
• Female mammal has two X chromosomes
and a male has an X and a Y.
• During an early stage of prenatal
development, both male and female have a
set of Mullerian ducts and a set of Wolffian
ducts as well as primitive gonads.
Sex and Hormones
• Wolffian ducts are the precursors to other
male reproductive organs.
– Develop into the vas deferens and seminal
vesicles.
• Mullerian ducts are precursors to the female’s
oviducts, uterus, and upper vagina.
Sex and Hormones
• The male Y chromosome includes the SRY
gene which causes the primitive gonads to
develop into testes, the sperm-producing
organ.
• The developed testes produce the hormone
testosterone.
• Testosterone induces the development of the
penis and scrotum.
• Females are not exposed to high
testosterone levels and their gonads develop
into ovaries, the egg-producing organs.
Sex and Hormones
• Sensitive periods are early periods when
hormones have long-lasting effects.
• Sexual differentiation depends mostly on the
level of testosterone during a sensitive
period.
• The human sensitive period for genital
formation is about the third and fourth month
of pregnancy.
Sex and Hormones
• Female rats exposed to testosterone shortly
before or after birth are partly masculinized in
anatomy and behavior.
– Clitoris grows larger than normal
– At maturity, pituitary and ovaries produce
steady levels of hormones instead of
cycles
– Parts of the hypothalamus appear more
male
– Sexual behavior becomes masculinized
Sex and Hormones
• Extra estradiol does not determine whether
the individual looks female or male.
• Estradiol and other estrogens do modify
various aspects of the development of the
brain and the internal sexual organs.
• The absence of sex hormones generally
leads to female-looking external genitalia
• If a male rat lacks androgen receptors or is
castrated, it develops female-like anatomy
and behavior.
Sex and Hormones
• Sex hormones early in life bind to receptors in
specific areas of the hypothalamus,
amygdala, and other brain areas and produce
anatomical and physiological differences.
• The sexually dimorphic nucleus is an area in
the anterior hypothalamus that is larger in the
male and contributes to control of male
sexual behavior.
• Parts of the female hypothalamus generate a
cyclical pattern of hormone release; the
hypothalamus of a male cannot.
Sex and Hormones
• In adulthood, sex hormones exert activating
effects to temporarily modify behavior.
• Behavior can also influence hormone
secretion.
• Hormones do not cause behavior but rather
alter the activity in various brain areas to
change the way the brain responds to certain
stimuli.
• Hormones also change sensitivity in the
penis, vagina and cervix.
Sex and Hormones
• Sex hormones facilitate sexual behavior.
• Arousal also depends on previous sexual
experience.
• Sex hormones activate sexual behavior by
enhancing sensations.
• Estrogens increase the sensitivity of the
pudendal nerve, which transmits tactile
stimulation from the pubic area to the brain.
Sex and Hormones
• Testosterone and estradiol trigger the release
of dopamine by the MPOA and other areas.
• Dopamine stimulation of D1 and D5 receptors
is associated with sexual arousal.
– Facilitates erection of the penis and
sexually receptive postures in females
• Higher concentrations of dopamine stimulate
D2 receptors and lead to orgasm.
• Serotonin activity decreases sexual activity by
blocking dopamine release.
Sex and Hormones
• Humans are less dependent on current sex
hormones than other species but changes
can increase or decrease sexual arousal.
• For males, levels of testosterone correlate
positively with sexual arousal and the drive to
seek out partners.
Sex and Hormones
• Decreases in testosterone levels generally
decrease male sexual activity and interest.
– Example: castration
• Impotence is the inability to maintain an
erection.
– usually caused by impaired blood
circulation, not low testosterone.
• Erection partially depends on testosterone
increasing the release of nitric oxide.
– facilitates the hypothalamic neurons and
increases blood flow to the penis.
Sex and Hormones
• Most sex offenders have normal testosterone
levels.
• Hormone levels do not completely explain the
behaviors.
• Testosterone reduction has been tried as a
means of controlling sex offenders as it
reduces sexual activities.
• Results favorable for those who continue
taking medication, but dropout rate is high.
Sex and Hormones
• In women, the hypothalamus and pituitary
interact with the ovaries to produce the
menstrual cycle.
• The menstrual cycle is the periodic variation
in hormones and fertility over the course of
about 28 days.
Sex and Hormones
• After the end of a menstrual period:
– the anterior pituitary releases folliclestimulating hormone (FSH)
– FSH promotes the growth of a follicle in the
ovary.
– The follicle nurtures the ovum and
produces estrogen.
• Towards the middle of the menstrual cycle,
the follicle builds up receptors to FSH.
• As a result, the follicle produces increasing
amounts of estradiol, a type of estrogen.
Sex and Hormones
• Increased estradiol causes the anterior
pituitary to increase release of FSH and
luetinizing hormone (LH).
• FSH an LH cause the follicle to release an
ovum.
• The remnants of the follicle release the
hormone progesterone.
– prepares the uterus for implantation of a
fertilized ovum
– inhibits the further release of LH
Sex and Hormones
• If the ovum is not fertilized, the lining of the
uterus is cast off and menstruation occurs.
• If the ovum is fertilized, the levels of estradiol
and progesterone increase gradually
throughout pregnancy.
Sex and Hormones
• Birth control pills prevent pregnancy by
interfering with the usual feedback cycle
between the ovaries and pituitary.
• The “combination-pill” contains both estrogen
and progesterone and prevents the surge of
FSH and LH that would release an ovum.
– also thickens the mucus of the cervix
making it harder for the sperm to reach the
egg.
Sex and Hormones
• The periovulatory period is the time of
maximum fertility and high estrogen levels
when ovulation occurs.
• Studies suggest that women become more
sexually responsive during this time when
estrogen levels are high.
– Show increased attention to sex-related
stimuli.
– show increased mate preference towards
men who act and look more masculine.
Sex and Hormones
• Oxytocin is a pituitary hormone also important
for reproductive behavior.
• Stimulates contraction of the uterus during
delivery of a baby
• Stimulates the mammary gland to release
milk.
• Released during orgasm and triggers a state
of complete relaxation.
• Facilitates formation of pair bonds between
mating partners and mother and infant
Sex and Hormones
• Hormones released around the time of giving
birth facilitate maternal behavior in females.
• Late in pregnancy, the female secretes large
amounts of estradiol, prolactin, and oxytocin.
• Prolactin is necessary for milk production.
• Oxytocin is associated with maternal behavior
and social attachment.
Sex and Hormones
• Females also change patterns of hormone
receptors.
• Late in pregnancy, the brain increases its
sensitivity to estradiol in areas responsible for
maternal behavior, but not for sexual
behavior.
• The hormonal changes increase the attention
of the mother to the young after birth.
• Hormones also increase activity in the medial
preoptic area and the anterior hypothalamus.
Sex and Hormones
• Vasopressin is a hormone synthesized by the
hypothalamus and secreted by the posterior
pituitary gland.
– associated with establishing long-term
bonds in some species.
• Mothers are also stimulated by the odors of
their babies.
– Infant rats release chemicals that stimulate
the mother’s vomeronasal organ.
Sex and Hormones
•
Mammals have two mechanisms for
stimulating maternal behavior:
1. Hormones in the early phase compensate
for the lack of familiarity with the young.
2. Later experience maintains the maternal
behavior as hormones decline.
• Although hormonal changes are necessary
to nurse a baby, they are not necessary to
elicit care for a baby by humans.
Variations in Sexual Behavior
• A wide degree of variation exists between
people in terms of frequency of sexual
behavior, preferred types of sexual activity,
and sexual orientation.
• Evolutionary explanations are controversial in
explaining mating behavior.
Variations in Sexual Behavior
• Gender differences in sexual behavior include
the following:
– Men are more likely to seek multiple sex
partners, especially for short-term
encounters.
– Women are more likely to be concerned
about a mates earning potential: men are
more likely to be concerned about a mate’s
youth.
– Men usually show greater jealousy at
indications of sexual infidelity.
Variations in Sexual Behavior
• Buss (2000) argues that gender differences
reflect past evolutionary pressures.
• Men are interested in brief sexual
relationships with multiple partners because
such a strategy increases the likelihood of his
genes being passed along to the next
generation.
Variations in Sexual Behavior
• Research has also suggested that women
can gain from having multiple sexual partners
as well.
• Multiple mates increases resources available
to her child and herself.
• No direct evidence suggests that specific
genes influence whether people prefer single
or multiple mates.
Variations in Sexual Behavior
• Both men and women prefer a mate that is
healthy, intelligent, honest, and physically
attractive.
• In almost all cultures, women prefer mates
who are likely to be good providers.
• Evolutionary explanations suggest that
choosing a father who is likely to be a good
provider aids the women while she is
pregnant or caring for a small child.
Variations in Sexual Behavior
• Men tend to prefer a young partner.
• Evolutionary explanations suggest that this
preference exists because younger women
are more likely to be fertile than older women.
• Men remain fertile well into old age so
preference for a young mate for women is not
as pronounced.
Variations in Sexual Behavior
• Evolutionary explanations of gender
differences in jealousy suggest that men
need to be sure that the children he supports
are his own.
– Unfaithful wives threaten this certainty.
• Women are always sure that the child is their
own.
• Although cultures vary in attitudes towards
infidelity, no culture exists where infidelity is
more acceptable for women.
Variations in Sexual Behavior
• Although mating habits of people can be
explained in terms of increasing the
probability of passing on one’s genes, we can
not assume a genetic basis.
– Behaviors and preferences may be a
product of learning.
• More data, especially about the effects of
particular genes, is needed to draw
conclusions.
Variations in Sexual Behavior
• Gender identity refers to how we identify
sexually and what we call ourselves.
• Biological differences are generally referred
to as “sex differences”.
• Differences that result from people’s thoughts
about themselves as male or female are
referred to as “gender differences”.
Variations in Sexual Behavior
• Most people have a gender identity that
matches their external appearance.
• Some people have a gender identity that is
opposite their biological sex.
• Psychologists and researchers once believed
that gender identity was learned and more a
product of rearing and experience.
• Current evidence strongly suggests that
biological factors, especially prenatal
hormones, play a large role in gender identity.
Variations in Sexual Behavior
• Intersexes are people that have anatomies
intermediate between male or female.
• Poorly developed or ambiguous genitalia can
be caused by:
• XY males have a mutation of the SRY gene
• Born with an XX chromosome pattern but an
SRY gene translocates from the father’s Y
chromosome onto another chromosome.
• An atypical hormone pattern occurs or a
mutation of testosterone receptors occurs
before birth.
Variations in Sexual Behavior
• Congenital adrenal hyperplasia (CAH) is an
overdevelopment of the adrenal glands from
birth
• Most common cause of the intersex condition.
• Caused by a genetic defect in which cortisol
production leads to overstimulation of the
adrenal gland.
• Overstimulation of the adrenal gland leads to
extra testosterone production.
– The female fetus becomes partly
masculinized.
Variations in Sexual Behavior
• Hermaphrodites are individuals whose
genitals do not match the usual development
for their genetic sex.
• An estimated 1 out of 100 children is born
with some degree of genital ambiguity.
• 1 in 2000 has enough genital ambiguity to
make the sex uncertain.
Variations in Sexual Behavior
• The brains of genetic females with CAH are
exposed to higher than normal levels of
testosterone during prenatal and early
postnatal life.
• Research indicates that CAH girls show a
greater preference for boy-typical toys than
do other girls.
• During adolescence and early adulthood,
they also show partly masculinized interests.
• Sexual interest and activity also differs for
CAH girls as well.
Variations in Sexual Behavior
• Androgen insensitivity or testicular
feminization is a condition in which individuals
with an XY chromosome pattern have the
genital appearance of a female.
• Production of androgens remains normal but
they lack the androgen receptor that enables
it to activate genes in a cell’s nucleus.
• Condition occurs in various degrees from a
smaller than average penis to genitals that
develop a female appearance.
Variations in Sexual Behavior
• Physicians have traditionally recommended
that intersex people be reared as girls.
– surgery was often conducted to make them
look more feminine.
– Assumed that children consistently raised
as female would accept that identity.
• Many intersex people protest against such
surgery and suggest that an informed
consent would have been preferred.
Variations in Sexual Behavior
• Some genetic males fail to produce an
enzyme that converts testosterone to
dihydrotestosterone.
• Most look female at birth but a penis
develops during adolescence and puberty.
• Most then accept a male gender identity.
– Brain is exposed to testosterone during
early development.
Variations in Sexual Behavior
• Many genetic males born without a penis or
who had the penis accidentally removed and
who were raised as a girl ask to be
reassigned as males.
• Many who remain female feel discontent or
conflict with being female.
• Such cases indicate that although hormones
do not determine gender identity, they do play
an important role.
Variations in Sexual Behavior
• Research has identified several predisposing
factors in genetics and prenatal environment
for male homosexuality.
• Genetic and biological predisposition appears
weaker for women.
• Gender-typed behaviors in childhood are a
stronger predictor of sexuality for men than
compared to women.
• Higher percentage of women acknowledge
physical attraction to both sexes and switch
between sexual orientations.
Variations in Sexual Behavior
• Subtle average behavioral and anatomical
differences between heterosexual and
homosexual include:
– Length of bones in the arms, legs and
hands
– Percentage of hair whorl direction
– Use of landmarks versus distances to give
directions
• Implies sexual orientation is an integral part
of the individual.
• But shifting of behavior to the opposite sex
does not equally apply to both sexes.
Variations in Sexual Behavior
• Studies of twins suggest sexual orientation is
influenced by genetic factors.
• Probability of homosexuality is highest in
monozygotic twins and lower in dizygotic
twins, and even lower in siblings and adopted
brothers or sisters.
• No single particular gene has been identified.
• Each of several genes may have an
influence.
Variations in Sexual Behavior
• Studies also suggest a higher incidence of
homosexuality among the maternal relatives
of homosexual men.
• These results suggest a gene on the X
chromosome that a man receives from his
mother may play a role.
• Other studies have not replicated this result
and thus the findings are inconclusive.
Variations in Sexual Behavior
• Sexual orientation may be influenced by
testosterone levels during sensitive periods of
brain development.
• Studies of male animals deprived of
testosterone early in life show sexual interest
in other males as adults.
• Studies of female animals exposed to
testosterone during early development show
an increased likelihood of mounting behavior.
Variations in Sexual Behavior
• Evolutionary explanations for homosexuality
suggest:
– Genes are maintained by kin selection
– Genes that produce homosexuality
increase probability of female relatives
reproducing
– Several genes produce advantages for
reproduction, but certain combinations lead
to homosexuality.
– Homosexuality relates to activation or
inactivation of genes.
Variations in Sexual Behavior
• Sexual orientation is not related to adult
hormone levels.
• May be influenced by exposure to
testosterone during sensitive periods.
Variations in Sexual Behavior
• The probability of homosexual orientation is
also higher among men with older brothers.
• Number of previous sisters has no effect nor
do these effects apply to females.
• Results suggest that a mother’s immune
system may react against a protein in a son
and attacks subsequent sons to alter
development.
Variations in Sexual Behavior
• Prenatal exposure to stress and alcohol may
also play a role.
• Stress releases endorphins
– antagonizes the effects of testosterone on
the hypothalamus.
• Stress elevates adrenal hormone
corticosterone
– decreases testosterone release.
• Both result in changes in the structure of the
nervous system.
– makes the male anatomy closer to female
Variations in Sexual Behavior
• Laboratory research has also shown that
prenatal stress can alter sexual development.
• Male subjects subjected to either prenatal
stress or alcohol developed male sexual
behavior in addition to female sexual
behaviors.
• Male subjects exposed to both stress and
alcohol during prenatal development had
decreased sexual behavior.
Variations in Sexual Behavior
• Research also suggests that certain brain
structures differ in size between heterosexual
and homosexual men and women.
• On average, the homosexual brain is shifted
towards the opposite sex in some (but not all)
ways.
• Studies emphasize the role of testosterone at
certain times of development as certain areas
of the brain have altered sensitivities to
testosterone.