April 9, 2021
PSY 340 Brain and Behavior
Class 23: Sex and Hormones [Outline]
March 17, 2005: The human X chromosome has been fully decoded and the results published in the journal, Nature. The "X" chromosome is comprised of
ca. 1.5 million base-pairsca. 154 million base-pairs coding for ca. 800 proteins in every cell in the human body (vs. ca. 59 million base-pairs in the "Y" chromosome coding for ca. 70 proteins).
- two X chromosomes in females and an X and Y chromosome in males.
A. Becoming male or female (that
is, sexual differentiation)
Development in males
- Gonads: the SYR (Sex Region Y) --> testes (sperm-producing organs).
- Testes --> testosterone & Müllerian inhibiting hormone (MIH) which cases the Müllerian ducts to disappear.
Development in females
- Wolffian ducts
- Ovaries (egg-producing organs)
- Müllerian ducts --> uterus & upper vagina
Hormones: Androgens & Estrogens
- Hormones = chains of amino acids that are secreted in the blood stream and tend to work (see diagram below) by way of
- Direct effects on the nucleus
- "Second messenger" effects
- Steroid hormones (= molecules built of four carbon rings & derived from cholesterol
- Androgens = testosterone (the best known androgen) & other "male" hormones.
- Estrogens = estradiol (the best known estrogen) & other "female" hormones.
- Cortisol --> adrenal gland cortex (not the sex glands or gonads,
Organizing Effects of Sex Hormones
1. Sex Differences in the Hypothalamus
Organizing Effects Causes permanent structural differences to develop gradually in the body.
Activating Effects Causes temporary behavior to occur over seconds, minutes, hours, or days following exposure to hormone(s).
- Testosterone --> dihydrotestosterone (DHT)
- Absence of Androgens in Males
- Sexually dimorphic nucleus (in medial preoptic area (POA) of the anterior hypothalamus)
- M>F for both volume and number of cells
- Testosterone --> converts to estradiol inside neurons which then "masculinizes" the hypothalamus
- Sexually dimorphic nucleus (in medial preoptic hypothalamus) is smaller in females than males.
- Female hypothalamus --> monthly cycle of hormone release
- Alpha-fetoprotein: protects against own estradiol masculinizing brain
2. Sex Differences in Childhood Behavior
- Boy vs. Girl toy and activity choices & tendencies (by age 3)
- Very strong socialization pressures are certainly there. But...
- Young infants (3-8 months old) show some sexual differences
- Chemicals in mother's blood during pregnancy may affect fetal brain and subsequent behavior
- Girls whose mothers had higher levels of testosterone in pregnancy showed slightly elevated interest in boys toys at 3.5 years.
- Phthalates --> sons who were less interested in playing with boys' toys at ages 3 to 6 years.
- These types of chemicals are found in many plastic products and in many cosmetic products like perfume, etc.
- Toxicology & Endocrine Disrupting Chemicals (EDCs)/Hormone Disruptors
- Work of epidemiologist, Shana Swan, includes showing significant changes in human sexual and reproductive capacities over the past 50 years
- Male sperm concentration and count decreased <50% in West
- Genital malformation of newborn males related to EDC exposure
- Genital size in males lower and adult male testicular volume decreased in exposure to phthalates
- Phthalates linked to various reproductive problems in women
- Maternal EDC exposure related to increased anxiety and shyness in male children and impulsivity in female children
- Phthalate exposure is associated with lowered levels of psychomotor development in infants and young children.
1. Sexual Behavior in Humans
- Activating effects of hormones tend to influence sexual behavior in short time periods (e.g., 15 minutes)
- Oxytocin, a hormone secreted by the pituitary gland, has multiple effects in terms of sexuality and reproduction: (1) stimulating contractions during delivery; (2) stimulating mammary gland to produce milk; and (3) the pleasure and relaxation associated with sexual orgasm. HOWEVER, NOTE: OXYTOCIN ≠ A "CUDDLE" OR "LOVE" HORMONE. Multiple effects both positive and negative in social interactions.
- Testosterone --> dopamine in medial preoptic area. Sexual responsiveness. Serotonin can inhibit dopamine release.
- SSRI antidepressant medications (but not other kinds) associated with mild to severe sexual dysfunction.
- Sexual excitement is highest when testosterone is highest (ages 15-25 years)
- Decreased testosterone generally decreases sexual activity
- Treatment of sex offenders with medications (e.g., Depo-Provera [progestin or medroxyprogesterone acetate (MPA)])
In females: the menstrual cycle
Interactive effects of hypothalamus and pituitary gland
- Days 1-5: Menstruation ("menstrual period")
- Days 5-15: Follicular Phase
- follicle-stimulating hormone (FSH)
- estradiol: endometrium thickens
- luteinizing hormone (LH)
- Days 16-28: Luteal Phase: The FSH & LH causes the follicle to release an ovum (egg), i.e. ovulation. The remnant of the follicle (corpus luteum) releases progesterone which prepares endometrium for implantation of fertilized ovum, prevents development of new follicle, & inhibits uterine contractions.
- Birth Control Pills
- Periovulatory period
- Premenstrual Syndrome (PMS & "Premenstrual Dysphoric Syndrome") [Not in book]
Sex Hormones & Nonsexual Characteristics
- Beware of arguments that the brains of men and women are extremely different. They are not.
- Specific brain differences include
- "Males [have] higher cortical and sub-cortical volumes, cortical surface areas, and white matter diffusion directionality; females [have] thicker cortices and higher white matter tract complexity." (Ritchie et al, 2017, Abstract)
- Connectivity between different regions of brain [in resting state fMRI] (see above)
- M>F between sensorimotor, visual, and rostral lateral prefrontal areas
- F>M Default Mode Network
- But, human brains cannot be categorized into two distinct classes: male brain/female brain
- Dr. Janet Shibley Hyde's Gender Similarity Hypothesis = women & men are very similar on most (though not all) psychological variables.
- Recognition of Facial Expression of Emotion. Females > males
1. Behavior in Mammals
- Among many mammals, parental behavior appears to depend both upon hormones and upon other factors.
- oxytocin and prolactin = onset of maternal behavior in female rats.
- Later stages of maternal behavior are not dependent on hormones
- In male rats, testosterone levels drops after the delivery of their pups
2. Human Parental Behavior (this is not in text but required)
- No obvious or dependable parallels between parenting in other mammals and in humans
- Humans give birth to semiprecocial offspring (mostly dependent on parent but still capable of some expressive behaviors, e.g., crying, eyes open). Many animals (e.g., Canada geese, horses, mountain goats) give birth to precocial offspring who cope quickly with environment after birth. Other animals at birth are even more helpless than human babies and these are called altricial offspring.
- Fathers are more affectionate & attentive to infants if they have lowered testosterone levels.
- Human Parent-Infant Care-giving
- Attachment as basis of social organization: parent-infant bond forms family
- Rapid formation of attachment between mother & child under influence of oxytocin
- Behavioral synchrony of mother and infant
- Human culture shapes goals of parents & interaction patterns
- Central role of maternal sensitivity and responsiveness to infant involving multiple brain systems
- emotional regulation
- parental empathy
- executive control
- Deficient sensitivity/responsiveness in mother may reflect (1) mother's own experience of maternal warmth, (2) pre-birth affective disorders, or (3) postpartum maternal psychopathology
This page was first posted March 20, 2005.