Study Notes on Chapter 10: Sexual Behavior

Behavioral Neuroscience

Chapter 10: Sexual Behavior

10.1 Sexual Development

  • Sex and Gender

    • Sex refers to biological sex.

    • Gender refers to social, learned, and personal aspects of sex.

  • Gender Identity

    • Defined as the sense of being male or female, which is independent of genetic sex or physical appearance.

Sexual Development

  • Match of Gender Identity and Appearance

    • Most individuals have a gender identity that aligns with their physical appearance.

    • Some individuals may experience dissatisfaction with their assigned gender.

The Genetics of Sex

  • Inheritance of Sex Chromosomes

    • Sex is determined by sex chromosomes inherited from parents.

    • Mothers always contribute an X chromosome.

    • Fathers contribute either an X or a Y chromosome.

    • Genetic combinations:

    • XX combination results in a female.

    • XY combination results in a male.

  • Prevalence of Chromosomal Abnormalities

    • Occur in about 1 in every 426 children.

    • Variations may lead to mild to severe differences in development.

    • A longitudinal study involved more than 13,000 newborns with three sex chromosomes.

Turner Syndrome

  • Condition

    • Occurs when an individual receives only a single X chromosome.

  • Characteristics

    • Normal female external genitalia but abnormal ovarian development.

    • Associated with short stature and webbed neck deformity.

    • Cognitive abilities typically within normal limits but may have deficits in spatial relationships and memory.

Klinefelter Syndrome

  • Condition

    • Characterized by a phenotypically male with XXY genotype.

  • Characteristics

    • Experience reduced fertility and often require hormone treatment at puberty.

    • Normal intelligence but may demonstrate social awkwardness and delayed verbal skills.

    • Higher prevalence of left-handedness.

XYY Syndrome

  • Also known as Jacob Syndrome

    • Affects males with XYY genotype.

  • Characteristics

    • Tend to be tall, lean, and may have more acne.

    • Exhibit mild physical abnormalities of the eye, elbow, and chest.

    • Slightly lower IQ could be noted.

    • Language difficulties and tendencies towards autism spectrum disorder are common.

    • Generally fertile, but a higher proportion of sperm may have sex chromosome abnormalities.

    • A controversial link exists between this syndrome and antisocial behavior.

Issues of Gender Assignment and Rearing

  • Current Guidelines

    • Honesty with individuals and families about gender assignment.

    • Gender identification should base on predominant external genital appearance.

    • Aim to rear the child under a consistent gender identity while remaining open to potential deviations later in life.

    • Avoid surgical alterations to ambiguous external genitalia, as this can severely impair erotic sensation.

Three Stages of Prenatal Development

  • Processes Involved

    • The development of male and female reproductive structures consists of three distinct processes:

    1. Development of gonads.

    2. Formation of internal organs.

    3. Development of external genitalia.

    • In rare cases (intersex conditions), elements of both male and female developmental paths may occur within the same fetus.

The Development of the Gonads

  • Primordial Gonads

    • Until the sixth gestational week, male and female embryos possess identical primordial gonads that can develop into ovaries or testes.

  • Role of the SRY Gene

    • Around 6 weeks of gestation, a gene on the Y chromosome, known as the Sex-Determining Region of Y (SRY), is activated in male embryos.

    • The Testis-Determining Factor (protein encoded by SRY) initiates the transformation of the primordial gonads into testes.

    • For female embryos, other genes direct the primordial gonad to develop into ovaries.

    • Experimental evidence: Chromosomally male mice that lack the SRY gene develop ovaries; conversely, female mice with the SRY gene develop testes.

Differentiation of the Internal Organs

  • Embryonic Development

    • Up until the third gestational month, both male and female embryos have the Wolffian (male) and Müllerian (female) systems.

  • Male Development

    • In males:

    • The Wolffian system develops into seminal vesicles, the vas deferens, and the prostate.

  • Female Development

    • In females:

    • The Müllerian system forms the uterus, upper part of the vagina, and fallopian tubes.

Differentiation of Internal Organs

  • Hormonal Influence

    • During the third month, the testes begin to secrete testosterone and anti-Müllerian hormone.

    • Testosterone: Promotes the development of the Wolffian system.

    • Anti-Müllerian Hormone: initiates degeneration of the Müllerian system.

  • Female Development

    • In absence of androgens, the Müllerian system will naturally develop while the Wolffian system regresses, with remnants that may persist in adult females.

  • Androgen Insensitivity Syndrome (AIS)

    • Caused by a defective gene that produces dysfunctional androgen receptors in males.

    • Despite normal secretion of androgens and anti-Müllerian hormone by testes, lack of functional androgen receptors prevents development of the Wolffian system, leading to the typical female external appearance.

Development of the External Genitalia

  • Comparison of Genitalia

    • Male external genitalia: penis and scrotum.

    • Female external genitalia: labia, clitoris, and outer part of the vagina.

  • Prader Scale

    • A tool used by health care providers for evaluating the extent of masculinization of external genitalia.

  • Hormonal Stimulation

    • Development of male external genitalia relies on recognition of the androgen 5-alpha-dihydrotestosterone by receptor sites.

  • Condition

    • Characterized by excessive androgen release from adrenal glands during the second trimester.

    • If female genetic backgrounds are exposed to excess androgens, external genitalia can become masculinized.

  • Behavioral Outcomes

    • Females affected by CAH often identify as tomboys, engage in male-typical play, and exhibit increased rates of bisexual and lesbian behavior.

Development at Mini-Puberty

  • Males experience a secondary surge of androgen exposure between ages one and three months, which decreases by six months.

Development at Puberty

  • Puberty is marked by additional hormonal changes leading to the maturation of genitals and secondary sex characteristics.

  • Secondary Sex Characteristics

    • Males: facial hair, deeper voice.

    • Females: wider hips, breast development.

Typical Age at Puberty

  • There has been a trend of declining average ages for puberty over the last 150 years.

    • Factors influencing this trend include increased weight and exposure to hormones or endocrine disruptors.

  • Environmental Influences

    • Compounds resembling female hormones found in meat, dairy products, shampoos, plastics, and insecticides may contribute to early puberty.

    • Estradiol—a type of estrogen—may be present in all edible animal tissues, suggesting historical exposure impacts sexual development.

Endocrine Disruptors and Puberty

  • Exposure to endocrine disruptors like phthalates, diethylstilbestrol (DES), and bisphenol A (BPA) might impact puberty timing.

    • Phthalates

    • Have been shown to expedite puberty onset in rats and alter male children's toy preferences based on maternal phthalate levels.

Interesting!!! Take Note!!!

  • Some estradiol-like compounds prevalent in plastic packaging expose nearly everyone to potential hormonal disruptions.

Hormone Changes at Puberty

  • Gonadotropin-Releasing Hormone (GnRH) released by the hypothalamus triggers the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland.

    • Both males and females produce these hormones, but they have different effects on sexual development.

Hormonal Effects on Males and Females

  • Males

    • In response to FSH and LH:

    • Testes produce additional testosterone.

    • Spermatogenesis is also regulated by testosterone levels.

  • Females

    • Ovaries produce estradiol in response to FSH and LH, leading to breast growth and maturation of external genitalia and uterus.

    • LH and FSH are also essential for regulating the menstrual cycle.

5-alpha-Reductase Deficiency

  • A rare condition affecting the maturation of male external genitalia due to a lack of the enzyme required to convert testosterone into the more potent 5-alpha-dihydrotestosterone.

    • Results in ambiguous genitalia at birth; individuals may be raised as females but will develop secondary male characteristics during puberty.

10.2 Sex Differences in Hormones, Brain Structure, and Behavior

  • The effects of genes on sex chromosomes initiate a complex interplay of continued expression involving brain structure, hormones, and epigenetics, directing individual development along diverse trajectories.

The Organizing Role of Sex Hormones

  • Both genders produce androgens and estrogens, but in varying amounts (e.g., adult females produce about 10% of the androgens produced by adult males).

Organization and Activation

  • Critical Periods

    • Hormonal influence during gestation and puberty are essential for sexual differentiation.

    • Organizational and activational effects from hormones dictate sexual and behavioral outcomes.

Brain Structure and Sex

  • Certain brain regions exhibit sexually dimorphic characteristics influenced by hormonal environments during prenatal development.

Contributions of Sex Chromosome Genes

  • Genes on the sex chromosomes contribute to neural structure and function even before the embryo starts producing sex hormones, influencing behavioral outcomes later in life.

Sex Differences in Behavior and Cognition

  • Social factors shape behavior, though biological contributions significantly interact with socialization processes, enriching our understanding of sex and gender.

Play Behavior

  • Notable sex differences have been documented in play behavior, with girls exposed to CAH demonstrating masculinity in toy preferences.

  • Infant Toy Preference Studies

    • Research has demonstrated that children begin to develop preferences for sex-typed toys between 12 to 18 months of age before recognizing societal associations.

  • Monkeys and Toy Choices

    • Male monkeys favor balls and toy cars, while females prefer dolls, showcasing inherent preferences not influenced by socialization.

Gender Identity

  • Defined as an individual's self-concept regarding being male or female, typically viewed as a product of extensive socialization.

  • In transsexuality, gender identity being inconsistent with biological sex may be resolved through sex reassignment surgery and hormone therapy.

Personality Traits

  • Research indicates females often display higher empathy levels, while males show increased physical aggression.

Cognition

  • Males have a slight advantage in visuospatial tasks, while females excel in verbal tasks and episodic memory.

    • Visuospatial skills correlate with current testosterone levels rather than prenatal exposure.

Psychological Disorders

  • Prevalence Rates

    • Distinct differences in incidence rates exist for various psychological disorders between genders:

    • PTSD, Panic Disorder, Anxiety Disorders, Major Depression, OCD: higher prevalence in women.

    • Schizophrenia, Parkinson's Disease, Drug Use Disorders: greater incidence in men.

Sexual Orientation

  • Defined as a stable pattern of attraction towards a particular sex.

  • Statistics indicate that approximately:

    • 1.8% of adult males identify as gay or lesbian.

    • 1.5% of adult females identify as gay or lesbian.

    • 0.4% of men and 0.9% of women identify as bisexual.

Hormones and Sexual Orientation

  • Early exposure to sex hormones heavily influences adult sexual behaviors.

  • Birth Order

    • Males with older brothers have a higher likelihood of being homosexual, possibly due to maternal immune responses developing to Y chromosome proteins.

Brain Structure Differences by Sexual Orientation

  • Certain sexually dimorphic structures, particularly in the hypothalamus (INAH-3), differ according to sexual orientation in males.

    • Size Correlations

    • Smaller INAH-3 in women and homosexual men compared to heterosexual men.

    • Monkeys with lesions in INAH-3 showed impaired heterosexual behaviors while retaining overall sex drive.

Genetic Influences on Sexual Orientation

  • Genetics are believed to play a role in shaping sexual orientation, though specific mechanisms remain unclear.

    • Studies report 20-25% concordance among homosexual male twins and approximately 50% for monozygotic twins.

Evolutionary Perspectives on Sexual Orientation

  • An evolutionary explanation for the persistence of homosexual genes suggests kin selection, whereby homosexual relatives may assist in enhancing the survival of their relatives' offspring.

Sexual Orientation and Cognition

  • Research shows males tend to excel in visuospatial tasks, with heterosexual males outperforming homosexual males.

  • In contrast, homosexual and bisexual females show similar performance and outscore heterosexual females.

Sexual Orientation and Verbal Fluency

  • Results reveal that homosexual males outperform heterosexual females, while both groups surpass heterosexual males and homosexual females regarding verbal fluency tasks.

10.4 Biological Influences on Adult Sexual Behavior

  • The role of hormones significantly stimulates sexual behavior from puberty onward.

Regulation of Sex Hormones

  • The hypothalamus regulates sex hormone secretions via releasing Gonadotropin-Releasing Hormone (GnRH).

  • GnRH stimulates the pituitary gland to release LH and FSH, critical for reproductive functions.

Mood, Menstruation, and Childbirth

  • Around 5-8% of women report premenstrual syndrome (PMS); severe cases are categorized as premenstrual dysphoric disorder (PMDD).

  • Postpartum depression affects 10-15% of women within the year following childbirth, influenced by factors such as age, marital status, education, and socio-economic status.

Hormonal Influence on Sexual Interest

  • Human Females

    • Estrus and cycling affect sexual interest, with higher receptivity reported during ovulation.

  • Human Males

    • Variation in cultural influences can affect testosterone levels and sexual frequency, particularly noted in older men.

Managing Sexual Behavior

  • Testosterone management has been explored for sex offender control but comes with drawbacks such as potential side effects and issues with treatment adherence.

Female Initiated Activities

  • Variability in female-initiated sexual activities correlates with hormonal cycles, with a noticeable peak around ovulation.

Recognizing Emotional Expression

  • Women reportedly outshine men in recognizing emotional expressions based on facial cues.

  • Studies imply women's capabilities in identifying emotional expressions surpass expectations based on securities in socialization.

Male Sexual Behavior

  • Patterns of sexual interest often inversely correlate with testosterone levels, particularly post-childbirth when paternal roles take precedence.

Testosterone and Competition

  • Research indicates fluctuations in testosterone surrounding competitive scenarios where winning raises levels while losing decreases them.

  • Observation of competition can similarly influence hormonal levels.

Attraction and Romantic Love

  • Early identification of attraction appears innate, being activated by perceptions of beauty independent of cultural influences.

  • Variability in cultural definitions of attractiveness exists significantly across societies.

Evolutionary Explanations of Mating Behavior

  • According to Darwin's principles, success in survival leads to favorable gene propagation.

    • Sexual selection explains differing mating behaviors between genders.

Interest in Multiple Mates

  • Data suggests men are generally more inclined to seek multiple sexual partners, particularly short-term.

    • Evolutionary explanations highlight reproductive success through spreading genes.

  • Women's Perspectives

    • Evidence suggests women also gain advantages from multiple partners; however, genetic preference impacts mate selection predominantly.

Jealousy Metrics

  • Men typically exhibit stronger jealousy responses to female infidelity, attributed to reproductive uncertainty.

  • Cultural influences shape perceptions of infidelity with a universal trend disfavoring female extracurricular relationships.

Mate Preferences

  • In mate selection, common preferences include health, intelligence, honesty, and physical attractiveness.

    • Gender-specific Preferences

    • Women: Candidates as good providers.

    • Men: Prefer younger partners, while women may prioritize older men for financial stability.

Elements of Physical Attractiveness

  • Symmetry is a critical factor perceived as beautiful, with a tendency for males to favor youthful features in females.

    • Preferences also indicate variability for long-term versus short-term relationships, where different traits might be prioritized.

MHC Genes and Attraction

  • The Major Histocompatibility Complex (MHC) genes function in attraction, with individuals encouraged to select mates differing from their own profiles for optimal offspring immunity.

    • Couples in fertility treatments often share similar MHC profiles, raising concerns about potential conception difficulties in these scenarios.