CHAPTER 7 The Biology of Sex and Gender The Sex and the Stimulus Gender-Related Behavioral and Cognitive Differences
Choosing mate by scent? • Each animal has unique genetically determined odor • Mothers/babies identify one another from birth • Families can identify eachother’s smell • Important in sexual arousal • Pheromones • airborne chemicals released by an animal • 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.
Pheramones: An important Stimulus factor • Evidence that odor influences mate choice • May be to avoid genetic inbreeding • Women rated men’s smells as more pleasant when men had greater difference in major histocompatability complex (MHC) than the women raters • MHC contributes to autoimmune function • Many animals “scent mark” • Your cat has glands just above eyes • When animal rubs/pees, etc. is scent marking • Marking territory, leaving scent around • Fire hydrants = doggie facebook!
Olfactory system • Olfaction = chemical sense • responds to chemical stimulation • Airborne odorous materials enter nasal cavity • Dissolve in mucous layer overlapping receptor cells • Odorous particles then stimulate receptor cells when contact with receptor site on cell dendrites • Axons from olfactory receptors pass through opening in base of skull to enter olfactory bulb • Olfactory bulb overlies nasal cavity
Olfactory Pathway • Neurons follow olfactory nerve to olfactory cortex • Olfactory cortex in inner folds of temporal lobe • In other animals: is separate cortex area • Humans can distinguish approximately 10,0000 odors • Between 500-750 different receptors • Only ¼ to ¾ are functional • Brain identifies odor by combination of stimulation
The Biological Determination of Sex • Sex: term for the biological characteristics that divide humans and other animals into the categories of male and female. • Gender: behavioral characteristics associated with being male or female. • Gender role: set of behaviors society considers appropriate for people of a given biological sex. • Gender identity: aperson’s subjective feeling of being male or female.
Chromosones and hormones • We typically start out as XX or XY • XX = female • XY = male • Are exceptions • Develp gonads: undifferentiated • For the first month, XX and XY fetuses are identical. • Later, primitive gonads develop into appropriate primary reproductive organ: • XY = male = testes • XX = female = ovaries
Becoming a female • The default sex: what happens if no testostrone burst • At about 6 weeks gestation: Müllerian ducts develop into • Uterus • fallopian tubes • inner vagina. • Wolffian ducts that would become the male organs wither and are absorbed. • Approximately 3 weeks later: the undifferentiated external genitals become • clitoris, • outer segment of the vagina, • the labia, which partially enclose the entrance to the vagina.
Becoming male • Receive Y chromosome from the father • SRY gene on that chromosome causes the primitive gonads to develop into testes • 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 must be turned ON • develop into the seminal vesicles: stores semen, • Also the vas deferens: carries semen from the testes to the penis. • A derivative of testosterone, dihydrotestosterone, masculinizes the external genitals.
Sexual development timeline Fetal Crown AGE rump (weeks) length (mm) Sex differentiating events 0 blastocyst Inactivation of one X chromosome 4 2-3 Development of wolffian ducts 5 7 Migration of primordial germ cells in the undifferentiated gonad 6 10-15 Development of müllerian ducts 7 13-20 Differentiation of seminiferous tubules 8 30 Regression of müllerian ducts in male fetus 8 32-35 Appearance of Leydig cells. First synthesis of testosterone 9 43 Total regression of müllerian ducts; . Loss of sensitivity of müllerian duct in female 9 43 First meiotic prophase in oogonia 10 43-45 Beginning of masculinization of external genitalia 10 50 Beginning of regression of wolffian ducts in the female fetus 12 70 Fetal testis is in the internal inguinal ring 12-14 70-90 Male penile urethra is completed 14 90 Appearance of first spermatogonia 16 100 Appearance of first ovarian follicles 17 120 Numerous Leydig cells. Peak of testosterone secretion 20 150 Regression of Leydig cells. Diminished testosterone secretion 24 200 First multilayered ovarian follicles. Canalisation of the vagina 28 230 Cessation of oogonia multiplication 28 230 Descent of testis
Brain and hormones • Sexual development at puberty • Average age of menarche for girls: 10.5-12 years • Average age for boys: 12.5-14 • What triggers puberty? • Still unknown • Family history • Environmental hormones? • Hypothalamus plays critical role
Brain and hormones • Preoptic area of Hypothalamus: POA • At puberty, begins to secrete gonadotrophin=releasing factor: GRF • Carried by bloodstream to anterior pituitary • At anterior pituitary: encourages release of LH and FSH (follicle stimulating hormone) • Effect is: • Flood of release of estrogen from ovaries • Flood of release of testosterone from testes • Result = maturation of primary sexual organs and development of secondary sex characteristics
Brain and hormones • The presence of testosterone masculinizes certain brain structures. • Actually, is estradiol plus testosterone • a derivative of estrogen • 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. • During critical period for brain masculinization:aromataseincreases in the areas that are to be masculinized.
Organizing and activating effects • 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 • may come and go with hormonal fluctuations • or be long lasting, • are reversible. • Helps explain some gender differences
Hormones responsible for Cognitive Differences? • Masculinization of prenatal brain: Birds and Birdsong • Bird songs generally fall into 2 classes • Male calls designed to attract females • Territorial calls designed to warn away competitor males • Most females have limited song range • In birds: specialized circuitry for song • Provides nice opportunity for studying sexual development • Can study Sexual maturation • Nature/nurture effects
Hormones responsible for Cognitive Differences? • Finches: • Male learns song by listening to father • Amount of singing dependent on testosterone levels • More testosterone = more singing • Castrated birds do not sing • Can make female sing if give high doses of testosterone
Hormones responsible for Cognitive Differences? • Activational effect • Need sexual circuitry • Need exposure to correct cues and experience • Must develop circuits at birth • Must turn on circuits at puberty • What does this mean? Need nature AND nurture to take on adult male role.
Gender-Related Behavioral and Cognitive Differences • Eleanor Maccoby and Carol Jacklin, 1974 • 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: • Documented gender differences: • Girls have overall greater verbal ability than boys. • Boys have overall greater visual-spatial ability. • Boys excel in mathematical ability. • Boys are more aggressive than girls. • Why? The great unknown • Nature? • Nurture?
Gender-Related Behavioral and Cognitive Differences • More recent data show reducing gender gap • Cognitive differences = rather limited • Females higher in verbal fluency and writing • No difference in in reading comprehension or vocabulary. • Male scores higher than females’ on most on tasks requiring • mental rotation of a three-dimensional object and • But not on other spatial tasks. • Females are better at computation • Males do better on standardized tests of broad mathematical ability (e.g., ACT, SAT) • More women go to college, graduate from college • Is this nature or nurture?