Download Discovering Biological Psychology: Chapter 10

Chapter Ten
Sexual Behavior
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Following an accident
during a routine
circumcision, David
Reimer was raised as a
girl.

Money reported David was
developing as a “normal”
girl

At 14, he refused to
continue living as a girl,
and his parents told him
about his medical history.
© Royalty-free/CORBIS

© Royalty-free/CORBIS
The Nature and Nurture of Sex
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The Genetics of Sex
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Stages of Prenatal Development
Stages of Differentiation
 Development of gonads.
 Development of internal organs.
 Development of external genitalia.
 (Secondary characteristics)

Normally, first three processes proceed in a specific
order to produce an unambiguous male or female.

In the case of intersexes, elements of both male and
female development occur in the same fetus.
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1. The Development of Gonads

Up to the 6th week following conception, both males and
females have undifferentiated primordial gonads.
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At the 6th week, the sex-determining region of the Y
chromosome (SRY) gene is expressed in males.
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The SRY gene encodes testis-determining factor, which
turns the primordial gonads into testes.
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Alternate genes guide the development of the primordial
gonads into ovaries in females.
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2. The Differentiation of Internal
Fetuses contain both
Organs

a male Wolffian system and
a female Müllerian system.

Secretion of testosterone and
anti-Müllerian hormone by the
testes develops male internal
organs.
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In the absence of testosterone
and anti-Müllerian hormone,
female internal organs develop.
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Adult females retain
nonfunctional remnants of the
Wolffian system.
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Normal development of the accessory
sex organs
Testosterone
Mullerian inhibitory hormone
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3. Development of External
Genitalia
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No hormonal influences are necessary for the
development of female external genitalia.
Masculinization of external genitalia requires
stimulation by 5-alpha-dihydrotestosterone from
the testes.
5-alpha-dihydrotestosterone results when
testosterone is acted on by the enzyme 5-alphareductase.
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Androgen-Insensitivity
Syndrome (AIS)

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
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A defective gene produces
abnormal androgen receptors in an
XY fetus.
Tissues are “blind” to androgens, so
the Wolffian system does not
develop.
Anti-Müllerian hormone prevents
development of normal female
internal organs.
Outward appearance and gender
identity is typically female.
From Money, J., and Ehrhardt, A.A.
Man & woman, boy & girl. Baltimore:
Johns Hopkins University Press, 1972.
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Congenital Adrenal Hyperplasia
(CAH)

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
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Heritable condition in which the fetus’s adrenal
glands release excess androgens.
Males may mature early.
Females may have ambiguous external genitalia.
Females with CAH often
describe selves as
“tomboys,” and engage in
more lesbian and bisexual
behavior.
From Kalat, James, and T. Norton, eds.,
Brains to behavior: some recent and
classical contributions. New York: MSS
Information Corp., 1973.
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Five genders?

Male
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Male Pseudohermaphrodites (merms)
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True hermaphrodite (herms)
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Female Pseudohermaphrodites (ferms)
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Female
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Development at Puberty
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Genitals and secondary
sex characteristics mature.
Mean age at puberty is
dropping.
Puberty may be triggered
by accumulated fat.
The hypothalamus
releases gonadotropinreleasing hormone
(GnRH).
The anterior pituitary
releases follicle-stimulating
hormone (FSH) and
luteinizing hormone (LH).
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Puberty in Males


FSH and LH promote increased release of
testosterone by the testes.
Testosterone promotes development of male
secondary sex characteristics:
 Muscular
development
 Maturity of external genitalia
 Facial hair
 Enlargement of the larynx (deepening the voice)
 Regulation of sperm production (along with FSH and
LH)
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Puberty in Females
FSH and LH stimulate estradiol production
by the ovaries.
 Estradiol leads to female secondary sex
characteristics:

 Breast
development
 Maturity of external genitalia
 Maturity of the uterus
 Changes in quantity and distribution of fat
 In both males and females, estradiol slows
skeletal growth
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Sex Hormones
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Synthesized from cholesterol.
Classified as steroids.
Males and females both
produce androgens and
estrogens.
Females produce about 10% of
the amount of androgens
produced by males.
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The
Human
Menstrual
Cycle
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Some Structures Are Sexually
Dimorphic
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Rats
 SDN-POA
and
the SNB
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Humans
 INAH-3,
hypothalamus,
anterior
commissure and
thalamus
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Example of a sexually dimorphic
anatomical difference:
Females: more synapses on
spines
Males: more synapses on shaft
Axon
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Masculinization of the Nervous
System
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Aromatization
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Changing of testosterone to estradiol
Responsible for masculinizing the developing brain.
In humans and many other mammals, the placenta
normally blocks the mother’s estrogens, so females do not
get masculinized; males make their own testosterone.
Prenatal exposure to synthetic estrogens may masculinize
the female brain (ex. CAH).
The hyena placenta does not
prevent masculinization.
Courtesy Dr. Stephen Glickman
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Hormones and Sexual
Orientation
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Early exposure to androgens may affect
adult behavior in rats.
 Low-testosterone
rats are more likely to
engage in sexual activity with males.
 Unusual exposure to testosterone in females
may promote sexual activity with females.
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Exposure to testosterone necessary for
sexual dimorphism of SDN-POA in rats.
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Hormones and Human Sexual
Orientation
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Women exposed to high levels of prenatal
androgens are more likely to engage in bisexual
and lesbian behavior.
But…most women exposed to high prenatal
androgens are heterosexual.
The vast majority of lesbians and bisexual
women have not experienced exposure to high
prenatal androgens.
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Brain Structure and Sexual
Orientation
Simon LeVay compared INAH-3 in
heterosexual and homosexual males.
Courtesy Simon LeVay,
Salk Institute
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Other Correlations between Brain
Structure and Sexual Orientation
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SDN-POA in rams that mate with other rams is
about the same size of SDN-POA in ewes, and
is different from rams that mate with ewes.
The anterior commissure varies in size between
homosexual and heterosexual male humans.
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Genes and Sexual Orientation

The likelihood of a gay man having a gay
brother is:
 25%
for fraternal twins.
 50% for identical twins.
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Data for lesbians are less clear, leading
LeVay and others to question a genetic
role in female homosexuality.
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Hormones and Female
Cognition
Female performance on spatial tasks is
best when testosterone levels are high
and worst when estrogen levels are high.
 Verbal fluency and manual dexterity are
correlated with high estrogen levels in
women.
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Androgens and Male Cognitive
Behavior
Males have a slight advantage over females
in spatial tasks.
 Older men receiving testosterone
supplements improved their performance on
spatial tasks.
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Seasonal Testosterone Levels
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Spatial Ability
at Different
Times of the
Year
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Average sex differences in behavior often reflect
significant overlap between the sexes
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Parental Behavior

Males
 promiscuity
(being less selective) is balanced
by the need to protect offspring.

Females
 likely
to be selective in their sexual partners
because of high cost of reproduction
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Hormonal Influences on Mating
and Parental Behavior

Monogamy and nurturing in voles may
result from levels of vasopressin and
oxytocin.
 Prairie
voles are monogamous and nurturant.
 Montane voles are promiscuous and not
nurturant.

Oxytocin is released during human
orgasm and when a mother is nursing her
infant.
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More Biological Influences on
Parenting
Inhibiting progesterone activity in male
mice increases nurturing behaviors.
 Mice lacking PEG3 genes do not nurture
offspring.
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