Human Mating Systems, Sexual Differentiation, and Evolutionary Psychology

A comprehensive vocabulary review of human sexual selection, anatomical and physiological dimorphism, endocrine mechanisms of sexual development, gender identity, sexual orientation, and evolutionary mate choice theories based on the lecture notes.

Physiological Investment

Biological parental contribution to offspring that only females can provide, specifically through gestation and lactation.

Economic Investment

The type of parental investment contributed by males toward mates and offspring, consisting of resources, socioeconomic status, and parental care rather than physiological gestation or lactation.

Human Sexual Dimorphism in Size and Strength

Morphological differences between human sexes showing that males are $15\text{--}20\%$ heavier, $8\%$ taller, possess $61\%$ more muscle mass, and have significantly greater hand strength than females.

Neotenous Features

Juvenile physical traits that human females retain even after puberty, such as rounder faces, larger eyes, and small noses.

Cohen's d

A standardized metric used to quantify the difference between two means, guided by conventional benchmarks where $d = 0.2$ is small, $d = 0.5$ is medium, and $d = 0.8$ is large.

Sociosexuality

An individual's willingness to engage in uncommitted, casual sexual relationships.

SRY Gene

The gene located on the Y chromosome that encodes testis determining factor (TDF), triggering fetal gonads to develop into testes.

Testis Determining Factor (TDF)

The substance encoded by the SRY gene on the Y chromosome that causes fetal gonads to differentiate into testes.

Organizational Effects

Relatively permanent developmental and structural effects exerted by hormones on the body and brain, such as those occurring during puberty.

Activational Effects

Temporary physiological and behavioral effects that disappear once the hormone leaves the body, such as menstruation and ovulation.

Müllerian Ducts

Embryonic precursors to the female reproductive tract present in both sexes prior to differentiation that develop into the uterus, cervix, and fallopian tubes, while falling away in males.

Wolffian Ducts

Embryonic precursors to the male reproductive tract that develop into the vas deferens and seminal vesicles.

Lordosis

A female-typical sexual posture in rats displayed when male rats are castrated, reflecting female-like gene expression.

Mounting

A male-typical copulatory behavior in rats that is restored in castrated males or induced in females following testosterone administration.

Complete Androgen Insensitivity Syndrome (CAIS)

A condition in XY individuals with non-functional androgen receptors whose testes develop and secrete androgens, but the target cells cannot receive them, preventing typical masculinization of the body and brain.

Congenital Adrenal Hyperplasia (CAH)

A condition in which the enzyme required to produce cortisol is absent, causing cortisol precursors to convert into androgens and resulting in elevated androgen levels that masculinize toy preferences, play patterns, and sexual orientation in females.

Idiopathic Hypogonadotropic Hypogonadism (IHH)

A disorder characterized by the absence of GnRH-releasing cells in the hypothalamus, resulting in low gonadal hormone production in utero and absence until hormone replacement therapy is initiated around puberty.

Gender

How an individual identifies socially based on masculine and feminine characteristics (such as man, woman, or nonbinary), distinct from biological sex determined by gonads, genetics, and physiology.

Childhood Gender Nonconformity (CGN)

A developmental pattern in which a child's toy choices, play styles, athletic activities, role models, and career ambitions align more closely with the opposite gender.

Alyha

Biological males within the Mohave (North American Indian) culture who deviate from male-typical behavior and social gender roles.

Travestis

Biological males in Brazilian society who adopt feminine identities and deviate from male-typical behavior and presentation.

Kinsey Scale

An instrument measuring sexual orientation across four dimensions (identification, attraction, fantasy, behavior) on a scale ranging from exclusively heterosexual ($0$) to exclusively homosexual ($6$).

Gynephilia

The sexual attraction toward female genitals.

Androphilia

The sexual attraction toward male genitals.

INAH-3

A sexually dimorphic brain nucleus that is larger in males than in females, larger in heterosexual men than in homosexual men, and whose adult volume is potentially influenced by prenatal androgen exposure.

Kin Selection (Helper in the Nest)

An evolutionary explanation proposing that genes contributing to homosexuality persist because homosexual individuals compensate for lower personal reproduction by altruistically investing time, energy, and resources into genetic relatives.

Antagonistic Pleiotropy

An evolutionary mechanism where a gene yields competing fitness effects, reducing reproductive success in one context or sex (e.g., homosexual individuals) while increasing reproductive success in another (e.g., heterosexual relatives or the opposite sex).

Intergenomic Sexual Conflict

An evolutionary situation with an unresolved optimum for a shared trait between sexes, such that a gene or allele is beneficial to one sex but causes suboptimal fitness in the other.

Lek Paradox

The evolutionary problem questioning how genetic variation persists in a population when persistent female choice for males with the best genes should theoretically eliminate lower-fitness alleles.

Westermarck Effect

An evolved inbreeding avoidance mechanism characterized by a learned lack of sexual attraction between individuals who were reared together in early childhood.

Phenylketonuria (PKU)

A rare recessive genetic condition ($< 0.01$ carrier frequency) causing neurodevelopmental deficits, cognitive impairment, and seizures, which is $576\times$ more likely to occur in offspring of brother-sister matings.

Red Queen Hypothesis

The evolutionary model explaining host-parasite co-evolution as an ongoing arms race, where sexual reproduction enables hosts to generate constant genetic fluctuation to keep pace with evolving parasites.

Fluctuating Asymmetry (FA)

Deviations from bilateral symmetry in morphological traits; low levels reflect developmental stability, parasite resistance, and genetic quality, and predict higher facial attractiveness ratings.

Major Histocompatibility Complex (MHC)

A set of genes responsible for building the immune system, wherein mate preferences favor individuals with dissimilar genetic profiles to boost offspring disease resistance and avoid inbreeding.

What do relative parental investment and relative reproductive rates of males and females look like in humans?

Females: cost of reproduction is high, limiting the number of offspring ○ Higher investment includes gestation, lactation, and childcare

Males: lower level of of investment, reproductive rate is limited by access to fertile females

What mating system probably characterized ancestral humans? How is this reflected in marriage patterns across societies today?

Humans are somewhere between polygamy and monogamy, and tend to fall closer to monogamous overall ●

“Males maintain stable dominance hierarchies via agnostic encounters” and “High-frequency, high intensity male-male competition”

Are men or women more competitive for mates? Why?

Men– they invest less in offspring, reproduce quicker and reenter the mating pool faster than the slow sex, so they are forced to compete

Are men or women choosier about mates? Why?

Women— are the slower sex and invest more in offspring

Are men more or less choosy compared to most male mammals? Why?

More choosy– they invest in offspring more than other mammals, which slows their reproductive rates

2a. What do relative parental investment and relative reproductive rates of males and females

look like in humans?

● Females: cost of reproduction is high, limiting the number of offspring

○ Higher investment includes gestation, lactation, and childcare

● Males: lower level of of investment, reproductive rate is limited by access to fertile

females

○ Investment in offspring includes sperm contribution and parental care

○ Only females can provide physiological investment in offspring (via gestation and

lactation) while males can invest only economically in mates and offspring

2b. What mating system probably characterized ancestral humans? How is this reflected in

marriage patterns across societies today?

● Humans are somewhere between polygamy and monogamy, and tend to fall closer to

monogamous overall

● “Males maintain stable dominance hierarchies via agnostic encounters” and

“High-frequency, high intensity male-male competition”

2c. Using sexual selection theory, what predictions can we make about humans?

● Are men or women more competitive for mates? Why?

○ Men– they invest less in offspring, reproduce quicker and reenter the mating pool

faster than the slow sex, so they are forced to compete

● Are men or women choosier about mates? Why?

○ Women— are the slower sex and invest more in offspring

● Are men more or less choosy compared to most male mammals? Why?

○ More choosy– they invest in offspring more than other mammals, which slows

their reproductive rates

● Are females more or less competitive compared to most female mammals? Why?

○ More competitive– not all males are ideal, so those that are ideal are worth

fighting for. A male’s investment into a female makes his investment unavailable

to other females

2d. What lines of evidence support the idea that men are more competitive among themselves

for mates than are women?

● Size

○ Males are 15-20% heavier, 8% taller, and have 61% more muscle mass than

females

● Strength

○ Hand strength between the sexes is significant

● Aggression

○ Males commit nearly all the same sex homicides

● Maturity and life expectancy

○ Males mature later and die sooner

2e. What lines of evidence support the idea that women are more competitive mates compared

to other female mammals?

● Female body fat distribution

○ Other female mammals do not have the same body fat distribution as human

females

■ Extra fat stored in the hips and breasts

● Females tend to have neotenous features even after puberty

○ Rounder faces, larger eyes, small noses

2f. What are effect sized and Cohen’s d, and why are they relevant for quantifying sex

differences (no calculation needed for the exam)

● Used to measure the difference between two means

● It quantifies the difference so they can be compared, showing what differences are large

or small

● Conventional guidelines:

○ Large d = .8

○ Medium d = .5

○ Small d = .2

2g.

2h. What is sociosexuality? Explain the sex difference we see in sociosexuality.

● The willingness to have uncommitted sex

● Women are less likely to have uncommitted sex than men are because women are more