Mamawawa

Hormones and Sex What’s Wrong with the Mamawawa?

13.1 Neuroendocrine System

13.2 Hormones and Sexual Development of the Body

13.3 Hormones and Sexual Development of Brain and Behavior

13.4 Three Cases of Exceptional Human Sexual Development

13.5 Effects of Gonadal Hormones on Adults

13.6 Neural Mechanisms of Sexual Behavior

13.7 Sexual Orientation and Sexual Identity

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the fact that this approach to hormones and sex is inconsis- tent with the evidence, its simplicity, symmetry, and com- fortable social implications draw us to it. That’s why this chapter grapples with it throughout. In so doing, this chapter encourages you to think about hormones and sex in new ways that are more consistent with the evidence.

Developmental and Activational Effects of Sex Hormones Before we begin discussing hormones and sex, you need to know that hormones influence sex in two fundamentally different ways (see Phoenix, 2008): (1) by influencing the development from conception to sexual maturity of the anatomical, physiological, and behavioral characteristics that distinguish one as female or male; and (2) by activat- ing the reproduction-related behavior of sexually mature adults. Both the developmental (also called organizational) and activational effects of sex hormones are discussed in different sections of this chapter. Although the distinction between the developmental and activational effects of sex hormones is not always as clear as it was once assumed to be—for example, because the brain continues to develop into the late teens, adolescent hormone surges can have both effects—the distinction is still useful (Cohen-Bendahan, van de Beek, & Berenbaum, 2005).

13.1 Neuroendocrine System

This section introduces the general principles of neuroen- docrine function. It intro- duces these principles by focusing on the glands and hormones that are directly in- volved in sexual development and behavior.

The endocrine glands are il- lustrated in Figure 13.1. By con- vention, only the organs whose primary function appears to be the release of hormones are re- ferred to as endocrine glands. However, other organs (e.g., the

This chapter is about hormones and sex, a topic thatsome regard as unfit for conversation but that fasci-nates many others. Perhaps the topic of hormones and sex is so fascinating because we are intrigued by the fact that our sex is so greatly influenced by the secretions of a small pair of glands. Because we each think of our gender as fundamental and immutable, it is a bit disturb- ing to think that it could be altered with a few surgical snips and some hormone injections. And there is some- thing intriguing about the idea that our sex lives might be enhanced by the application of a few hormones. For whatever reason, the topic of hormones and sex is always a hit with my students. Some remarkable things await you in this chapter; let’s go directly to them.

Men-Are-Men-and-Women-Are-Women Assumption Many students bring a piece of excess baggage to the topic of hormones and sex: the men-are-men-and-women-are- women assumption—or “mamawawa.” This assumption is seductive; it seems so right that we are continually drawn to it without considering alternative views. Unfor- tunately, it is fundamentally flawed.

The men-are-men-and-women-are-women assumption is the tendency to think about femaleness and maleness as discrete, mutually exclusive, opposite categories. In thinking about hormones and sex, this general attitude leads one to assume that females have female sex hormones that give them female bodies and make them do “female” things, and that males have male sex hormones that give them male bodies and make them do opposite “male” things. Despite

328 Chapter 13 ■ Hormones and Sex

Thinking CreativelyThinking Creatively

Pineal

Hypothalamus

Pituitary

Thyroid

Parathyroid

Thymus

Adrenal

Pancreas

Ovary

Testis FIGURE 13.1 The endocrine glands.

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Biopsychology, Eighth Edition, by John P.J. Pinel. Published by Allyn & Bacon. Copyright © 2011 by Pearson Education, Inc.

stomach, liver, and intestine) and body fat also release hor- mones into general circulation (see Chapter 12), and they are thus, strictly speaking, also part of the endocrine system.

Glands There are two types of glands: exocrine glands and en- docrine glands. Exocrine glands (e.g., sweat glands) re- lease their chemicals into ducts, which carry them to their targets, mostly on the surface of the body. Endocrine glands (ductless glands) release their chemicals, which are called hormones, directly into the circulatory system. Once released by an endocrine gland, a hormone travels via the circulatory system until it reaches the targets on which it normally exerts its effect (e.g., other endocrine glands or sites in the nervous system).

Gonads Central to any discussion of hormones and sex are the gonads—the male testes (pronounced TEST-eez) and the female ovaries (see Figure 13.1). As you learned in Chapter 2, the primary function of the testes and ovaries is the pro- duction of sperm cells and ova, respectively. After copulation (sexual intercourse), a single sperm cell may fertilize an ovum to form one cell called a zygote, which contains all of the information necessary for the normal growth of a complete adult organism in its natural envi- ronment (see Primakoff & Myles, 2002). With the excep- tion of ova and sperm cells, each cell of the human body has 23 pairs of chromosomes. In contrast, the ova and sperm cells contain only half that number, one member of each of the 23 pairs. Thus, when a sperm cell fertilizes an ovum, the resulting zygote ends up with the full com- plement of 23 pairs of chromosomes, one of each pair from the father and one of each pair from the mother.

Of particular interest in the context of this chapter is the pair of chromosomes called the sex chromosomes, so named because they contain the genetic programs that di- rect sexual development. The cells of females have two large sex chromosomes, called X chromosomes. In males, one sex chromosome is an X chromosome, and the other is called a Y chromosome. Consequently, the sex chromo- some of every ovum is an X chromosome, whereas half the sperm cells have X chromosomes and half have Y chromo- somes. Your sex with all its social, economic, and personal ramifications was determined by which of your father’s sperm cells won the dash to your mother’s ovum. If a sperm cell with an X sex chromosome won, you are a fe- male; if one with a Y sex chromosome won, you are a male.

You might reasonably assume that X chromosomes are X-shaped and Y chromosomes are Y-shaped, but this is incorrect. Once a chromosome has duplicated, the two products remain joined at one point, producing an X shape. This is true of all chromosomes, including Y chromo- somes. Because the Y chromosome is much smaller than

the X chromosome, early investigators failed to discern one small arm and thus saw a Y. In humans, Y-chromo- some genes encode only 27 proteins; in comparison, about 1,500 proteins are encoded by X-chromosome genes (see Arnold, 2004).

Writing this section reminded me of my seventh-grade basketball team, the “Nads.” The name puzzled our teacher because it was not at all like the names usually fa- vored by pubescent boys—names such as the “Avengers,” the “Marauders,” and the “Vikings.” Her puzzlement ended abruptly at our first game as our fans began to chant their support. You guessed it: “Go Nads, Go! Go Nads, Go!” My 14-year-old spotted-faced teammates and I considered this to be humor of the most mature and so- phisticated sort. The teacher didn’t.

Classes of Hormones Vertebrate hormones fall into one of three classes: (1) amino acid derivatives, (2) peptides and proteins, and (3) steroids. Amino acid derivative hormones are hormones that are synthesized in a few simple steps from an amino acid molecule; an example is epinephrine, which is released from the adrenal medulla and synthesized from tyrosine. Peptide hormones and protein hormones are chains of amino acids—peptide hormones are short chains, and protein hormones are long chains. Steroid hormones are hormones that are synthesized from cholesterol, a type of fat molecule.

The hormones that influence sexual development and the activation of adult sexual behavior (i.e., the sex hor- mones) are all steroid hormones. Most other hormones produce their effects by binding to receptors in cell mem- branes. Steroid hormones can influence cells in this fash- ion; however, because they are small and fat-soluble, they can readily penetrate cell membranes and often affect cells in a second way. Once inside a cell, the steroid mole- cules can bind to receptors in the cytoplasm or nucleus and, by so doing, directly influence gene expression (amino acid derivative hormones and peptide hormones affect gene expression less commonly and by less direct mechanisms). Consequently, of all the hormones, steroid hormones tend to have the most diverse and long-lasting effects on cellular function (Brown, 1994).

Sex Steroids The gonads do more than create sperm and egg cells; they also produce and release steroid hormones. Most people are surprised to learn that the testes and ovaries release the very same hormones. The two main classes of gonadal hormones are androgens and estrogens; testosterone is the most common androgen, and estradiol is the most common estrogen. The fact that adult ovaries tend to re- lease more estrogens than they do androgens and that adult testes release more androgens than they do estrogens

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has led to the common, but misleading, practice of refer- ring to androgens as “the male sex hormones” and to es- trogens as “the female sex hormones.” This practice should be avoided because of its men-are-men-and- women-are-women implication that androgens produce maleness and estrogens produce femaleness. They don’t.

The ovaries and testes also release a third class of steroid hormones called progestins. The most common progestin is progesterone, which in women prepares the uterus and the breasts for pregnancy. Its function in men is unclear.

Because the primary function of the adrenal cortex— the outer layer of the adrenal glands (see Figure 13.1)—is the regulation of glucose and salt levels in the blood, it is not generally thought of as a sex gland. However, in addi- tion to its principal steroid hormones, it does release small amounts of all of the sex steroids that are released by the gonads.

Hormones of the Pituitary The pituitary gland is frequently referred to as the master gland because most of its hormones are tropic hormones. Tropic hormones are hormones whose primary function is to influence the release of hormones from other glands (tropic means “able to stimulate or change something”). For example, gonadotropin is a pituitary tropic hormone that travels through the circulatory system to the gonads, where it stimulates the release of gonadal hormones.

The pituitary gland is really two glands, the posterior pituitary and the anterior pituitary, which fuse during the

course of embryological development. The posterior pituitary develops from a small outgrowth of hypothala- mic tissue that eventually comes to dangle from the hypothalamus on the end of the pituitary stalk (see Figure 13.2). In contrast, the anterior pituitary begins as part of the same embryonic tissue that eventually devel- ops into the roof of the mouth; during the course of de- velopment, it pinches off and migrates upward to assume its position next to the posterior pituitary. It is the ante- rior pituitary that releases tropic hormones; thus, it is the anterior pituitary in particular, rather than the pituitary in general, that qualifies as the master gland.

Female Gonadal Hormone Levels Are Cyclic; Male Gonadal Hormone Levels Are Steady Although men and women possess the same hormones, these hormones are not present at the same levels, and they do not necessarily perform the same functions. The major difference between the endocrine function of women and men is that in women the levels of gonadal and go- nadotropic hormones go through a cycle that repeats itself every 28 days or so. It is these more-or-less regular hor- mone fluctuations that control the female menstrual cycle. In contrast, human males are, from a neuroendocrine per- spective, rather dull creatures; males’ levels of gonadal and gonadotropic hormones change little from day to day.