Organizing Effects of Sex Hormones
Defined-determine whether the brain and body will develop as male or female
Sex Differences in the Gonads
Chromosomal pattern is XX for females and XY for males
Both fetuses have primordial gonads for both sexes
Males SRY gene stimulates the release of Mullerian inhibiting substance and androgens
In the absence of androgens, females develop only the
Mullerian system into ovaries and uterus

Figure 11.1 Differentiation of human genitals
The male’s SRY gene causes the gonad to become a testis, and the testis produces testosterone, which masculinizes development. In the absence of testosterone, development follows the female pattern.

Organizing Effects of Hormones
Sex Differences in the Hypothalamus
Sexually dimorphic nucleus-larger in males than females and is linked to male sexual behavior
These changes are testosterone dependent
Testosterone must be aromatized into estradiol to have effects on hypothalamus
Alpha-fetoprotein protects females from seeing these changes in their hypothalamus due to exposure to their own estrogen
Testosterone also encourages development of muscles around the penis
Video

Sex Differences in Nonreproductive Characteristics
Aggressiveness in males
Dependent on prenatal testosterone
Infant Care
Estrogen-dependent
Longevity
Estrogen-dependent
Preference for sex-specific toys
Females exposed to high levels of testosterone show great preference for male-typical toys and activities

Activating Effects of Sex Hormones
Activate Sexual Behavior
Testosterone results in male sexual behavior
Estrogen followed by progesterone results in female sexual behavior
Male-typical sex behavior
Erection and orgasm dependent on dopamine
Female-typical sex behavior
Lordosis and orgasm dependent on dopamine

Sexual Behavior in Humans-MEN
Sexual excitement is highest when testosterone is highest
Cyproterone blocks binding of testosterone and can be used to decrease male sexual behavior
Triptorelin-is a long-lasting testosterone blocking drug that has been beneficial in reducing deviant sexual fantasies and abnormal sexual behavior

Sexual Behavior in Humans-WOMEN
High estrogen levels increase sexual behavior
May determine what women find attractive in men (women pick a more feminized face when approaching menstruation)

Figure 11.3 Blood levels of four hormones during the human menstrual cycle
Note that estrogen and progesterone are both at high levels during the midluteal phase, but drop sharply at menstruation, a short time later.

Figure 11.4 Interactions between the pituitary and the ovary
FSH from the pituitary stimulates the ovary to release and develop a follicle, which produces estradiol, triggering a release of a burst of FSH and LH from the pituitary.Those hormones cause the follicle to release its ovum and become a corpus luteum. The corpus luteum releases progesterone while the ovary releases estradiol.

Nonsexual Behaviors
Estrogen
Stimulates growth of dendritic spines in hippocampus
Increase DA2 receptors and 5HT2A receptors-may increase mood
Improves memory and fine motor skills
Decrease spatial performance
Testosterone
Improve spatial tasks

Puberty
Defined-onset of sexual maturity
Onset occurs when the hypothalamus begins to release luteinizing hormone (LH) releasing hormone
LH and FSH stimulate the release of gender-specific hormones from the gonads
Sex Hormones stimulate growth and secondary sex characteristics

Parental Behavior
Maternal Behavior-stimulated by prolactin and oxytocin (these hormones act on the medial preoptic area)
Vomeronasal organ and response to pheromones from infants
(maintains connection between mother and their young)
Paternal behavior-regulated by testosterone and prolactin

Variations in Sexual Development and Orientation
Definitions of Gender Identity
Gender Identity-how we identify sexually and what we call ourselves
Gender role-activities and dispositions that a particular society encourages for one sex or the other

Variations in Sexual Development
Determinants of Gender Identity
Physical Development
Testicular Feminization/Androgen Insensitivity-a genetically male fetus develops as female
Hermaphrodites-genitals do not match the genetic sex pseudohermaphrodites/intersexes-sexual development is ambiguous
Prenatal Hormones-Ex: male children in the Dominican
Republic raised as females
Social Influences-Stereotypes

Possible Biological Bases of Sexual Orientation
Hormones
Stress during last week of pregnancy results in endorphins having an antitestosterone effect on the fetus (rats)
Genetics
Evidence for contribution but there also appear to be other factors involved
Possibly a gene carried on the X chromosome

Figure 11.11 Sexual orientations in adult relatives of a homosexual man or woman
Note that the probability of a homosexual orientation is highest among monozygotic twins of a homosexual individual, lower among dizygotic twins, and still lower among adopted brothers or sisters. These data suggest a genetic contribution toward the development of sexual orientation.

Possible Biological Bases of Sexual Orientation
Brain Structure
Interstitial Nucleus connects to medial preoptic area and it is larger in men than women also smaller in homosexual men
Suprachiasmatic Nucleus-unclear role anterior commissure-unclear role
The data indicating a role for the interstitial nucleus has been questioned. Future studies will need to be done to confirm or deny these results.