Activating Effects: Hormonal effects on sexual development that can occur at any time in an individual’s life; their duration depends on the presence of the hormone.

Androgen Insensitivity Syndrome: A form of 46,XY intersex involving insensitivity to androgen because of a genetic absence of androgen receptors. The person has male sex chromosomes and internal sex organs, but external sex characteristics are feminized or ambiguous.

Androgens: A class of hormones responsible for a number of male characteristics and functions.

Castration: Removal of the gonads (testes or ovaries).

Central Subdivision of the Bed Nucleus of the Stria Terminalis (BSTC): A structure that is larger in males than in females and in MtF transsexual individuals; it is closely connected to the amygdala, hippocampus, and prefrontal cortex, and in rats it mediates pheromone signaling.

Congenital Adrenal Hyperplasia (CAH): A form of 46,XX intersex characterized by XX chromosomes, female internal sex organs, and ambiguous or masculinized external sex characteristics. It is caused by excess production of androgens during prenatal development.

Coolidge Effect: An increase in sexual activity when the variety of sexual partners increases; named after former president Calvin Coolidge.

Dihydrotestosterone: A derivative of testosterone that masculinizes the genitals of males.

Estrogens: A class of hormones responsible for a number of female characteristics and functions; produced by the ovaries in women and, to a lesser extent, by the adrenal glands in males and females.

Estrus: A period when a nonhuman female animal is ovulating and sex hormone levels are high.

46,Xx Intersex: Possession of ambiguous or male external genitalia in a person with two X chromosomes in each cell, due to excess androgens or hormone treatment during gestation.

46,Xy Intersex: Possession of ambiguous or female external genitalia in a person with an X and a Y chromosome in each cell, due to incomplete masculinization.

Gender: The behavioral characteristics associated with being male or female.

Gender Dysphoria: The distress that people may feel when their gender identity does not match their sex at birth.

Gender Identity: A person’s subjective feeling of being male, female, a combination, or genderless.

Gender Role: A set of behaviors society considers appropriate for people of a given biological sex.

Gonads: The primary reproductive organs: testes in the male or ovaries in the female. An individual can have only one of the two types of gonads.

Hypogonadism: A disorder in which the gonads (testes or ovaries) fail to make normal levels of sexual hormones.

Medial Amygdala: The part of the amygdala that apparently responds to sexually exciting stimuli. In both male and female rats, it is active during copulation, and it causes the release of dopamine in the MPOA.

Medial Preoptic Area (MPOA): A part of the preoptic area of the hypothalamus that appears to be important for sexual performance, but not sexual motivation, in male and female rats.

Müllerian Ducts: Early structures that in the female develop into the uterus, fallopian tubes, and inner vagina.

Müllerian Inhibiting Hormone: A hormone released in the male that causes the Müllerian ducts to degenerate.

Organizing Effects: Hormonal effects of sexual development that occur during the prenatal period and shortly after birth and are permanent.

Ovaries: The female gonads, where the ova develop.

Oxytocin: A peptide hormone and neuropeptide transmitter involved in lactation, sexual arousal, and orgasm, as well as social recognition and bonding; sometimes called the “sociability molecule.”

Pheromones: Airborne chemicals released by an animal that have physiological or behavioral effects on another animal of the same species.

Progesterone: A hormone produced in greater abundance in females, in whom it controls reproductive functions, including sexual receptivity and desire; in males, it is needed to produce testosterone.

Sex: The term for the biological characteristics that divide humans and other animals into the categories of male and female.

Sexually Dimorphic Nucleus (SDN): A part of the MPOA important to male sexual behavior. It is larger in male rats, and their level of sexual activity depends on SDN size.

Testes: The male gonads that produce sperm.

Testosterone: The major sex hormone in males; a member of the class of androgens.

Third Interstitial Nucleus of the Anterior Hypothalamus (INAH3): A nucleus found to be half as large in gay men and heterosexual women as in heterosexual men and similar in size in women and male-to-female transgender individuals.

Transgender: The term for an individual whose gender identity differs from the sex assigned at birth; the person may dress and live as the other sex and may undergo surgery for sex reassignment. Also termed transsexual.

Ventromedial Hypothalamus (VMH): An area in the hypothalamus important for female sexual receptivity and male mating vigor in rats. It is also involved in eating behavior, and destruction in rats produces extreme obesity.

Vomeronasal Organ (VNO): A cluster of receptors in the nasal cavity that detect pheromones.

Wolffian Ducts: The early structures that in the male develop into the seminal vesicles and the vas deferens.