Module 7: Gender Through a Physiological Psychology Lens

3rd edition as of August 2023

Module Overview

We often hear of books such as “Men are from Mars and Women are from Venus” and “Men Are Like Waffles- Women Are Like Spaghetti: Understanding and Delighting in Your Difference” which suggest men and women are opposites. While there is some disagreement as to how similar and different men and women truly are, the purpose of this module is to examine the biological differences in observed cognition, behavior, and gender roles with regards to one’s genes, hormones, and structure/function of the brain. We know that genetic make-up, hormones, and brain anatomy are some of the factors which impact behavior. This module explores the differences and similarities between men and women through the physiological lens.

Module Outline

Module Learning Outcomes

Describe the relationship between DNA, genes, and chromosomes.
List and describe the most common chromosomal abnormalities.
Explain how the endocrine system functions and clarify how the production of (or lack thereof) hormones impact ones social, cognitive, and behavioral development.
Clarify gender differences in brain function and how this might impact differences in behavior.

7.1. Basic Building Blocks

Section Learning Objectives

Explain how genetic information is transferred from generation to generation.
List and describe both sex and non-sex-linked chromosomal abnormalities.

7.1.1. DNA

DNA, or deoxyribonucleic acid, is the most basic hereditary material in most organisms. Nearly every cell in your body contains some DNA which is comprised of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Each DNA base attaches to another- A with T and C with G; these attached bases form base pairs. Each strand of DNA contains a structural component comprised of alternating sugar and phosphate molecules. This structural component is often referred to as the “backbone” of DNA. The combination of a base pair and this structural backbone is called a nucleotide. The nucleotides form two long strands that twist in a ladder-like structure forming the shape of a double helix (National Institute of Health, 2019).

7.1.2. Genes

While DNA neatly packages the hereditary material, genes are the basic physical and functional unit of heredity (National Institute of Health, 2019). DNA within each human gene varies from a few hundred DNA bases to more than 2 million. Of all the genes contained humans, only about 1% differ between individuals, making us much more physiologically alike than different (National Institute of Health, 2019). “Mistakes” and changes sometimes occur within the large number of DNA bases in a gene. While some of these changes do not affect the individual, some can have notable consequences. We will discuss this in more detail in section 7.1.3.1.

7.1.3 Chromosomes

We’ve already discussed that base pairs and a sugar/phosphate backbone create nucleotides. Several nucleotides form together to create strands of DNA. Thousands to millions of DNA sequences create a gene. Hundreds to thousands of genes are packaged into chromosomes which are thread-like structures located inside the nucleus of a cell (National Institute of Health, 2019).

In humans, each cell contains 23 pairs of chromosomes for a total of 46. Twenty-two of these pairs are autosomes, and one pair is an allosome. The 22 pairs of autosomes are the same in males and females, however, the allosome chromosome, also known as the sex-chromosome, differs between males and females. This chromosome determines whether a fetus will become genetically male or female.

How does this happen? In humans, males have one X chromosome and one Y chromosome, whereas females have two X chromosomes. Some cells are produced through mitosis, but gametes, or sex cells, are produced through meiosis in which cells divide. This meiotic division results in gametes which contain only half the number of chromosomes as the parent (National Institute of Health, 2019). Each parent, therefore, contributes one gamete to their offspring, either an X or a Y. Since females only have two X chromosomes, they will always contribute an X to the offspring. Males, however, contain both an X and a Y, and will therefore determine the sex of the offspring by contributing either one or the other.

7.1.3.1. Chromosomal abnormalities. Chromosomal abnormalities occur when there is an anomaly, aberration, or mutation of chromosomal DNA (Genetic Alliance, 2009). This can occur during either egg/sperm development or during the development of fetus. These abnormalities can occur either numerically or structurally. A numerical abnormality occurs when a whole chromosome is either missing or an extra chromosome is attached to the pair. A structural abnormality occurs when part of an individual chromosome is missing, extra, or switched to another chromosome (Genetic Alliance, 2009). The range of effects of chromosomal abnormalities vary depending on the specific abnormality, ranging from minimal developmental delays to the inability to survive.

We will now briefly discuss a few of the most common chromosomal abnormalities. While the first two chromosomal abnormalities are not sex-linked abnormalities, they are the most commonly observed, and therefore, worth mentioning. The final two chromosomal abnormalities are sex-linked and therefore, occur within specific genders.

Down syndrome (Trisomy 21) occurs when there is an extra chromosome on chromosome pair 21, hence the term trisomy 21. Trisomy 21 is the most common chromosomal condition in the United States and occurs in roughly 1 out of every 700 babies, affecting males and females equally (Parker et al., 2010). Individuals with Down syndrome have distinct physical characteristics that include: flattened face, small head, short neck, protruding tongue, upward slanting eyelids, poor muscle tone, excessive flexibility, and shortened stature (National Library of Medicine, 2019). Additionally, individuals with Down syndrome are more susceptible to congenital heart defects, gastrointestinal defects, sleep apnea, and dementia, with symptoms appearing around age 50. The lifespan of individuals with Down syndrome has increased significantly, to 60 years (National Library of Medicine, 2019).

The effects of an extra chromosome range from moderate to severe. Intellectual and developmental difficulties range from mild to severe, however, research routinely supports the effectiveness of early intervention programs for reducing developmental issues. Similarly, delayed developmental milestones related to low muscle tone are common. Early interventions with occupational, physical, and speech therapists have been shown to reduce delay in both physical and speech development (National Library of Medicine, 2019).

While researchers are still unclear about the cause of this chromosomal abnormality, advanced maternal age of over 35 has been identified as a risk factor for having a child with Down syndrome (National Library of Medicine, 2019). A screening test can inform parents of their risk of having a child with Down syndrome, and there is no risk of miscarriage from the screening. Prenatal genetic diagnosis can alert parents to the existence of Down syndrome in a developing fetus and is 99% accurate, with only a 1% risk of miscarriage (UCSF Health, 2023).

Trisomy 18 (Edwards syndrome) occurs when there is a third chromosome on chromosome 18. A Trisomy 18 error occurs in about 1 out of every 2500 pregnancies in the US and 1 in 6000 live births (National Library of Medicine, 2019). The number of total births is higher because it includes a significant number of stillbirths that occur in the 2nd and 3rd trimesters of pregnancy.

Individuals with Trisomy 18 have significant medical complications that are potentially life-threatening, which is why this chromosomal abnormality is associated with a high mortality rate. In fact, only 50% of babies with Trisomy 18 that are carried to term will be born alive, with girls surviving more often than boys (National Library of Medicine, 2019). Girls with Trisomy 18 also out-perform baby boys in the neonatal intensive care unit (NICU). Those born alive have a low birth rate due to slowed intrauterine growth. Physical abnormalities such as a small, abnormally shaped head, small jaw and mouth, clenched fists with overlapping fingers, as well as many other organ abnormalities are common in individuals with Trisomy 18 (Trisomy18 Foundation, 2019). Due to the severity of these abnormalities, only 5-10% of the surviving infants live to their first birthday. There have been rare cases of individuals with Trisomy 18 living into their twenties, however, they are unable to live independently without full time caregiving due to their significant developmental delays (Trisomy18 Foundation, 2019).

Klinefelter syndrome is a rare sex chromosome disorder in males that occurs in the presence of an extra X chromosome. Individuals with Klinefelter syndrome have the normal XY chromosomes, plus an extra X chromosome for a total of 47 Chromosomes (XXY; National Library of Medicine, 2019). It is believed that the activity from the extra copy of multiple genes on the X chromosome disrupts many aspects of development, from sexual development to physical and intellectual development.

Occurring in about 1 in 650 newborn boys, Klinefelter syndrome is among the most common sex chromosome disorder. Symptoms can be so mild that the condition is not diagnosed until puberty or adulthood. In fact, researchers believe that up to 75% of affected individuals are never diagnosed (National Library of Medicine, 2019).

Individuals with Klinefelter syndrome typically have small testes that produce a reduced amount of testosterone. Because of the reduced hormone production, individuals with Klinefelter syndrome may have delayed or incomplete puberty, causing infertility. Unless treated with hormone replacement, the lack of testosterone can lead to breast enlargement, decreased muscle mass, decreased bone density, and a reduced amount of facial and body hair (National Library of Medicine, 2019).

Developmentally, individuals with Klinefelter syndrome often have learning disabilities, particularly with speech and language development. Receptive language skills appear to supersede expressive language skills, so individuals with Klinefelter syndrome are likely to understand speech but have difficulty communicating and expressing themselves (National Library of Medicine, 2019). Due to this language disruption, individuals with Klinefelter syndrome also often have difficulty learning to read.

While there are additional physical characteristics associated with Klinefelter syndrome, they are subtle. As adolescents and adults, these individuals may be taller than their peers. Children may have low muscle tone and problems with motor development such as sitting, standing, walking (National Library of Medicine, 2019). Similar to individuals with Down syndrome, early intervention programs are helpful in reducing the delay of motor development.

Psychiatrically, individuals with Klinefelter syndrome often experience anxiety, depression, and impaired social skills. Those with Klinefelter syndrome have a higher rate of ADHD and Autism Spectrum Disorder than that of the general public. Medically, they also experience complications related to metabolic issues (National Library of Medicine, 2019). Half of men with Klinefelter syndrome develop conditions such as type 2 diabetes, hypertension (high blood pressure), and high cholesterol. They are also at an increased risk for developing osteoporosis, breast cancer, and autoimmune disorders compared to unaffected men (National Library of Medicine, 2019).

Unlike Klinefelter syndrome where there is an additional X chromosome, Turner syndrome occurs when there is one normal X chromosome and the other sex chromosome is missing or altered. Due to the altered X chromosome and lack of Y chromosome, individuals with Turner syndrome are genetically female. Turner syndrome is equally as rare as Klinefelter syndrome and occurs in about 1 in 2,500 newborn girls (National Library of Medicine, 2019).

Due to the altered or absence of the 2nd X chromosome, girls with Turner syndrome have a short stature which becomes apparent in early elementary years. Additional physical characteristics include low hairline at back of the neck, swelling of hands and feet, skeletal abnormalities, and kidney problems. Additionally, one third to half of girls born with Turner syndrome are born with a heart defect (National Library of Medicine, 2019).

Early developmental problems in girls with Turner syndrome vary significantly, with some experiencing developmental delays, nonverbal learning disabilities, and behavioral problems and others not requiring any early intervention. Despite these early developmental issues, girls and women with Turner syndrome typically have normal intelligence (National Library of Medicine, 2019).

Due to the altered sex chromosomes, women with Turner syndrome often experience early loss of ovarian function. While early prenatal development of ovaries is normal, egg cells die prematurely and the majority of ovarian tissue degenerates before birth (National Library of Medicine, 2019). Due to ovarian loss, many affected girls do not undergo puberty unless they undergo hormone replacement therapy. Even with the hormone treatment, most women with Turner syndrome are unable to conceive children.

7.2. Endocrine System

Section Learning Objectives

Identify key organs involved in the endocrine system.
Describe the function of the endocrine system.
Clarify why the endocrine system is important in behavior.

7.2.1. Anatomy and Function

The endocrine system is made up of a network of glands that secrete hormones into the circulatory system that are then carried to specific organs (Tortora & Derrickson, 2012). While there are many glands that make up the endocrine system, it is often helpful to organize them by location. The hypothalamus, pituitary gland, and the pineal gland are in the brain; the thyroid and parathyroid glands are in the neck; the thymus is between the lungs; the adrenal glands are on top of the kidneys; and the pancreas is behind the stomach. Finally, the ovaries (for women) or testes (for men) are located in the pelvic region.

While all the organs are important, two main organs are responsible for the execution of the entire system: the hypothalamus and the pituitary gland. The hypothalamus is important because it connects the endocrine system to the nervous system. Its main job is to keep the body in homeostasis, or a balanced state (Johnstone et al., 2014). When the body is out of balance, it is the job of the hypothalamus to identify the need (i.e., food to increase energy, water to increase hydration, etc.), and through the pituitary gland, identify the way to achieve balance once again.

The pituitary gland is the endocrine system’s master gland. Through the help of the hypothalamus and the brain, the pituitary gland secretes hormones into the blood stream which “transmits information” to distant cells, regulating their activity (Johnstone et al., 2014). Non- sex-related hormones that are released from the pituitary gland include: Growth hormone (GH), the hormone that stimulates growth in childhood and impacts healthy muscles and bones; Adrenocorticotropin (ACTH), the hormone responsible for production of cortisol which is activated in a stress response, and Thyroid-Stimulating hormone (TSH), the hormone responsible for regulating the body’s metabolism, energy balance, and growth. The pituitary gland also produces sex-related hormones that are involved in reproduction. For example, the pituitary gland is responsible for producing prolactin and oxytocin, which are both implicated in milk production for new mothers. Oxytocin may also play an important role in bonding between mother and child. Additional hormones including Luteinizing hormone (LH), which stimulates testosterone production in men and ovulation in women and Follicle-Stimulating hormone (FSH), which promotes sperm production in men and develops eggs in women, are also maintained by the pituitary gland. LH and FSH work together to produce normal function of ovaries and testes. Deficits in any of these hormones may impact reproductive ability.

Under the control of the hypothalamus and the pituitary gland, the remaining glands are responsible for manufacturing specific hormones that are carried throughout the body to carry out specific functions. While it is beyond the scope of this course to identify all of the hormones and functions of the endocrine system, it is important to identify the five main functions of the endocrine system (Johnstone et al., 2014):

Maintain homeostasis through the regulation of nutrient metabolism, water, and electrolyte balance.
Regulate growth and production of cells.
Control the responses of the body to external stimuli, especially stress.
Control reproduction.
Control and integrate circulatory and digestive activities with the autonomic nervous system.

7.2.2 Hypothalamus-pituitary-adrenal (HPA) Axis

As mentioned above, the endocrine system is involved in a lot of different body functions, however, one of the most important aspects of the endocrine system related to psychology is the HPA Axis. The HPA axis connects the central nervous system (brain and spinal cord) to the hormonal system. While there are many functions of this system, the one we will focus on is the stress-response system.

When in stress, the hypothalamus releases corticotropin-releasing hormone (CRH). CRH then activates the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH travels down to the adrenal gland on top of the kidneys which initiates the secretion of glucocorticoids from the adrenal cortex. The most common type of glucocorticoid in humans is cortisol, which plays a critical role in providing energy when presented with stressful or threatening situations (Kudielka & Kirschbaum, 2005). Elevated levels of cortisol produce a negative feedback loop, signaling brain functions to shut off the stress response system. A good video showing this response system can be found here:

https://www.neuroscientificallychallenged.com/glossary/hpa-axis.

As you will see more in other modules, particularly Module 9 when discussing clinical disorders, the HPA axis is responsible for keeping the body at homeostasis during stressful situations. A dysfunctional HPA axis has been associated with psychosomatic and mental health disorders. More specifically, HPA hyperactivity (too much activity) has been linked to major depression, whereas hypoactivity (too little activity) is associated with a host of autoimmune disorders, as well as fibromyalgia and chronic fatigue syndrome (Kudielka & Kirschbaum, 2005). Chronic HPA axis dysregulation has also been associated with the development of mood and anxiety disorders that will be discussed in more detail in Module 9.

7.2.2.1. Gender Differences in HPA Axis. Studies exploring the HPA axis hormonal response to stress among men and women have yielded conflicting findings. Kirschbaum and colleagues (1995a, b) identified higher cortisol and ACTH responses in men than women. Additional studies also found that men yielded greater cortisol and ACTH response to a psychological challenge (i.e. public speaking) than women. With that said, other studies have reported no significant gender differences in response to stress.

Some studies have indicated that a woman’s menstrual cycle may be implicated in gender differences among activation of the HPA axis. For example, Kirschbaum and colleagues (1999) showed that free cortisol responses were similar between men and women in the luteal phase of their menstrual cycle, however, women in the follicular phase or those taking oral contraceptives showed less free cortisol compared to males. While there appears to be some biological differences in men and women’s activation of the HPA axis, one cannot rule out other factors such as cognitive appraisals that may also implicate individual differences in the stress response. These additional factors will be discussed in more detail in Module 9.

7.3. Hormones

Section Learning Objectives

Describe hormones and their role in the body.
Differentiate estrogen and androgens.
Define and describe intersex conditions.
List and describe the hormone disorders.
Clarify the effect hormones have on behavior.
Clarify the effect hormones have on cognition.

The word hormone is derived from the Greek word meaning “arouse to activity.” Hormones are the body’s “chemical messengers.” Produced via the endocrine system, hormones travel throughout one’s bloodstream helping tissues and organs carry out their respected functions. Because there are so many types of hormones, they are often categorized by their function such as: reproduction/sexual differentiation; development and growth, maintenance of the internal environment, and regulation of metabolism/nutrient supply (Nussey & Whitehead, 2001). It should be noted that although hormones are categorized into these main groups, there are many hormones that affect more than one of these functions and serve multiple purposes.

7.3.1. Estrogens vs. Androgens

There are two classes of sex-related hormones: estrogens and androgens. Estrogens are hormones associated with female reproduction, whereas androgens (i.e. testosterone) are associated with male reproduction. Males and females have both estrogen and androgen in their bloodstream—the difference is the amount of each hormone in each gender. For example, females have higher amounts of estrogen and lower amounts of androgens; males have higher amounts of androgens and lower amounts of estrogen.

Occasionally, there can be a disruption in hormone production causing an excess or reduction of hormone level. This disruption can lead to changes in the brain as well as physical changes. This can be especially problematic in sex-linked hormones as it can alter the production of both primary and secondary sexual characteristics depending on when the hormone imbalance occurs.

Primary sexual characteristics are those an individual is born with, including sex organs needed for sexual reproduction. Secondary sexual characteristics are features which develop during puberty and imply sexual maturation. Physical characteristics such as developing breasts, increased pubic hair, facial hair, widening of hips (women) and deepening of voice (males) are among of the most common secondary sexual characteristics. Due to hormonal disruption, occasionally there are situations where there is a discrepancy between chromosomal sex and phenotypical sex (or external genitals). Known as intersex conditions, these situations have allowed researchers to study the effects of hormones on various behaviors.

7.3.2. Hormone Disorders

As previously discussed, occasionally there is a disruption in hormone production. This is seen in many medical disorders such as hyperthyroidism, dwarfism, and Cushing’s syndrome to name a few; however, sometimes there is a disruption in sex-related hormones. Below we will discuss the two most common types of conditions where sex-related hormones are affected and assess the implications of these disorders.

7.3.2.1. Congenital adrenal hyperplasia. One of the most common types of intersex conditions is Congenital Adrenal Hyperplasia (CAH). CAH is a genetic disorder that affects the adrenal glands and the production of hormones. More specifically, individuals with CAH have a significant enzyme deficiency resulting in impaired synthesis of cortisol and aldosterone. The consistently low levels of cortisol leads to an increase of ACTH by the pituitary gland, which in response, causes an increase in synthesis of steroid precursors, resulting in high androgen levels. While the hormonal effects of CAH can be of varying degrees, the most common, also known as Classic CAH, results in a complete lack of cortisol and an overproduction of androgens.

An individual with classic CAH will experience symptoms related to too little sodium in the body such as dehydration, poor feeding, low blood pressure, heart rate problems, and low blood sugar at birth (Mayoclinic, 2019). Due to the extensive nature of these symptoms, they are generally detected days or weeks from birth. In addition to the low cortisol related symptoms, individuals also experience effects related to high levels of androgens. Newborn females may present with ambiguous external genitalia despite having normal internal reproductive organs, whereas newborn males often have enlarged genitalia (Mayoclinic, 2019). Individuals with classic CAH will also experience significantly early onset of puberty—females may fail to menstruate or have irregular menstrual periods. Infertility in both males and females is also common.

Congenital Adrenal Hyperplasia has allowed researchers to study the effects of excess sex hormones on an individual’s behavior. While studied more extensively in females due to the fact that women do not usually develop high levels of androgens, findings suggest that prenatal exposure to excess androgen may influence the development of regions in the brain responsible for sex difference behaviors (Dittman et al., 1990). For example, some studies have found higher levels of energy and aggressive behaviors, increased participation in sports, and increased interest in traditionally masculine games and behaviors in girls with CAH (Berenbaum & Hines, 1992; Berenbaum & Snyder, 1995; Berenbaum, 1999).

These findings have been replicated over the years with CAH females routinely displaying more male-typical play behaviors in childhood. Assessment of CAH females as they age into adulthood also suggest differences in sexual identity. More specifically, CAH females report less satisfaction with their female sex assignment as well as less heterosexual interest than unaffected women. When assessing the relationship between childhood play and adult sexual preference in CAH females, a significant relationship was observed between increased male-typical play in childhood and decreased satisfaction with the female gender. These findings were also found between increased male-typical play in childhood and reduced heterosexual interest in adulthood (Hines, Brook, & Conway, 2004). Studies assessing behavior and sexual orientation in males with CAH have failed to identify any significant differences between males with CAH and unaffected males. These results are not surprising given the fact that unaffected males have higher levels of androgens than unaffected females.

7.3.2.2. Complete androgen insensitivity syndrome. Unlike CAH where there is an overproduction of androgens, Complete Androgen Insensitivity Syndrome (CAIS) is a rare condition that inhibits boys from responding to androgens. Occurring in approximately 2-5 per 100,000 births, individuals with CAIS are genetically male (XY), however, due to the body’s inability to respond to androgens, they display mostly female external sex characteristics. Despite the external female sex characteristics, these individuals are still genetically male and therefore, lack a uterus but do have undescended testes. While genetic testing in fetuses has expanded over the years, many individuals with CAIS are not diagnosed until menses fail to develop at puberty. While gender identity issues are likely, individuals with this syndrome are often raised female due to the external sexual characteristics at birth.

Physically, individuals with CAIS are generally taller than women without the disorder, but shorter than males. It is believed that part of this increased height is due to the effect of the growth controlling region on the long arm of the Y chromosome. There is little research on the psychological gender development of individuals with CAIS, however, the limited information available suggests that individuals with CAIS usually assume a gender identity and sexual orientation in line with their female sex rearing (Wisniewski et al., 2000). Individuals with CAIS report maternal interests and report high femininity from childhood to adulthood on global rating scales (Wisniewski et al., 2000). Psychologically, individuals with CAIS report similar levels of psychological well-being and overall quality of life as unaffected women. Similarly, there were no differences in psychological and behavioral domains suggesting CAIS women and unaffected women experience similar levels of psychological and behavioral symptoms (Hines, Ahmed, & Hughes, 2003).

7.3.3. Effects of Hormones on Behavior

We just briefly discussed how atypical hormone levels via hormone disorders can have an effect on behavior, but what about the effect of typically producing hormones behaviors? Let’s take a look at how estrogen and testosterone can impact the way we behave!

7.3.3.1 Estrogen. Though researchers historically neglected the roles of primary female hormones, modern research focuses on estrogens as a crucial component of the sexual desire of women (Cappelletti & Wallen, 2016). Changing levels of estrogen across the reproductive lifespan have been associated with changes in incidence of anxiety in females. More specifically, women are more at risk for developing an anxiety disorder during onset of puberty, which is also associated with an increase of circulating estradiol from prepubertal to adult levels (Ojeda & Bilger, 2000). However, estrogen has been shown to be an emotionally protective factor, reducing fear responses. When estrogen is low, as it is during certain phases of the menstrual cycle, women are at higher risk for the development of PTSD. Men may have a lower risk for the development of PTSD, because in male brains, testosterone is converted into estrogen and remains stable rather than fluctuating monthly (Beck, 2019). Therefore, anxiety may be exacerbated by fluctuations in estrogen, rather than the presence of it. Increases in anxiety symptoms are also observed when estrogen levels drop post-menopause (Sahingoz, Ugus, & Gezginc, 2011). In women with anxiety disorders, there is an increase in anxiety symptoms during the luteal phase of the menstrual cycle, which is characterized by a dramatic decline in circulating estrogen levels (Cameron, Kuttesch, McPhee, & Curtis, 1988). Therefore, there appears to be a strong link between anxiety related behaviors and estrogen levels in women. In rats, estrogen decreased anxiety and increased exploratory behavior, learning, and memory (Khaleghi et al., 2021). This has important implications for estrogen treatments for post-menopausal women experiencing decreased estrogen production (Khaleghi et al., 2021).

7.3.3.2. Testosterone. Misunderstandings about testosterone abound. One stereotype is that testosterone is related to sexual activity, but this has been shown to be false, with the exception of elderly men. As long as the testosterone levels of a male falls within a normal range, it is unrelated to sexual frequency (Gray et al., 2005). Testosterone has also been commonly associated with aggression, but just as in the case of estrogen and anxiety, care should be taken when assigning causation. Even though there is a statistical relationship between aggression and testosterone, this could mean that either testosterone increases aggression, that aggression increases testosterone, or that another factor increases them both. Popular opinion suggests that testosterone causes aggressive, violent, and other machismo behaviors, however, there is little empirical support for these assumptions (Booth, et al., 2006). In fact, the relationship between aggression and testosterone is bi-directional and depends on several individual factors, as well as the environment (Sapolsky, 1997). Testosterone does not cause aggression, but it can enhance it if it’s already there (Sapolsky, 1997). In fact, it has been demonstrated that the presence of testosterone is not even necessary for aggression to occur (Mims, 2007).

Other factors related to testosterone include competition. In examining aggressive behaviors during competitive video gaming, researchers found men made higher unprovoked attacks during the game than women. Furthermore, individuals with higher levels of testosterone also completed higher unprovoked attacks than those with lower levels of testosterone. Furthermore, men in securely-partnered relationships, or those who are not competing, have lower testosterone levels than those who are still competing for mates. Consistently, decreased testosterone production follows reproductive success, which is not surprising, considering high levels of testosterone are detrimental for paternal care and pair-bonding (Anders et al., 2007; Puts et al., 2015). Researchers propose that situational factors, such as a threat to status or competition, interact with hormones to produce aggressive behaviors (McAndrew, 2009).

7.3.4. Effects of Hormones on Cognition

Sex hormones influence cognition at many stages of life, however, the focus of most of the research is the relationship between estrogen and testosterone and the decline of cognition in older age. General findings suggest that estrogen may serve as a protective factor in cognitive decline in elderly women, whereas lack of testosterone in men may be linked to a general decline in cognition. In this section we will discuss the implications of hormones on men and women’s cognitive functioning throughout the lifespan.

Studies on women have identified a relationship between specific brain regions and estrogen. More specifically, the prefrontal cortex and the hippocampus have been identified as areas that improve in function due to increased estrogen (Hara, Waters, McEwen & Morrison, 2015). The hippocampus, the brain region responsible for memory and learning, appears to be affected by stress differently in men and women. Researchers found that women have a heightened sensitivity to stress within the hippocampus region. For example, ten days of a significant stressor in men causes the opioid system within the hippocampus to “shut down,” whereas in women, the system is “primed.” This priming encourages excitement and learning when the individual is exposed to activation of the opioid system again (Marrocco & McEwen, 2016). As you will see in Module 9, this may have implications for the development of psychological symptoms after stressful situations (i.e. depression, anxiety, PTSD) as heightened sensitivity of systems to specific situations in women may be due to the “primed” opioid system.

Endocrine changes appear to be largely responsible for age-related cognitive decline in both men and women (Henderson, 2008). In women, the most significant change in hormones occurs during menopause. While menopause can occur naturally, it can also be medically induced via the removal of the ovaries and uterus due to a variety of reasons, such as cancer or pregnancy complications. Research examining cognitive effects in women experiencing either natural or medically induced menopause indicates that regardless of the menopause method, women are at an increased risk for cognitive decline once menopause is “complete.” Interestingly, cognitive decline in women who undergo menopause due to medical necessity respond to estrogen replacement, whereas those who undergo menopause naturally do not respond as favorably to estrogen support (Phillips & Sherwin, 1992). It should be noted that although cognitive declines due to reduced estrogen are observed, they are often mild and are generally observed as deficits in concentration and processing speed (Kok et al., 2006).

When examining the relationship between men and cognitive decline, testosterone has been identified as a variable that may significantly impact performance on a variety of cognitive tasks. For example, men with low levels of testosterone have been shown to perform lower on cognitive tasks such as memory (Barrett-Connor et al., 1999), executive functioning (Muller et al., 2005), and attention (Cappa et al., 1998). The effects of testosterone on these cognitive tasks appear to have a greater effect when assessed in elderly men; results on the effects of testosterone and cognition do not appear to impact young men (Yaffe et al., 2002; Barret-Connor et al., 1999).

Similarly in women, researchers have examined outcomes in performance with supplementation of testosterone in older men experiencing low levels of testosterone. Findings indicate that supplementation of testosterone is an effective method to improve working memory and other cognitive functioning in older men (Janowsky, Chavez, & Orwoll, 2000; Cherrier et al., 2001). It should be noted, however, that despite the support for increased testosterone and cognitive function, researchers are still unsure of how much testosterone is needed for “optimal” cognitive performance (Barrett-Connor et al., 1999).

7.4. The Brain

Section Learning Objectives

Explain what sexual dimorphism is and its importance.
Describe gender differences in the lateralization of the brain.
Describe gender differences in cortical thickness of the brain.
Describe gender differences in myelination of the brain.

Another attempt to explain sex differences is through the anatomy and function of the brain. Sexual dimorphism refers to structural differences between the sexes. In addition to dimorphic sexual organs, different sexes have sexual dimorphism in the brain. For instance, two of the interstitial nuclei of the anterior hypothalamus are about twice as large in the brains of males than females, an area associated with sexual behavior (Allen et al., 1989). The purpose of this section is to explore these brain differences and determine how they may impact behavior.

7.4.1 Lateralization

The brain is divided into two hemispheres and connected via the corpus collosum. The right hemisphere is thought to be dominant in spatial abilities whereas the left hemisphere is dominant in verbal tasks. While early researchers proposed that women were more “right-brained” and men were more “left-brained,” this hypothesis has been unsupported, with research showing both genders utilize the two hemispheres equally (Bishop & Wahlsten, 1997). However, as we will explain, some processes are more lateralized to particular areas of the brain than others, and they are lateralized differently in different brains.

Though both male and female brains show lateralization of function, research shows that male brains are generally more lateralized than those of females. This means that male brains are more functionally and structurally asymmetrical (Shaywatz et al., 1995). Different degrees and locations of functional asymmetry in male and female brains could account for gender advantages in language and visuospatial skills. Males slightly outperform females in visuospatial tasks, during which, there is lateralization in the right hemisphere, whereas females show activity in both hemispheres during the task. Whereas, females slightly outperform males in language skills tasks, during which, more lateralization is seen in the female left hemisphere than in males (Tomasi & Volkow, 2012).

7.4.2 Cortical Thickness

Cortical thickness, or the tissue volume and tissue composition of the cerebrum, has long been explored as a possible explanation for behavioral differences in men and women. Magnetic Resonance Imaging (MRI) studies have shown that gray matter, white matter, and brain size are smaller in women than men, even after controlling for body size, though there is no relationship between brain volume and intelligence. When both gray and white matter normalize, adult men have a greater proportion of white matter, whereas women demonstrate a greater proportion of gray matter (Allen et al., 2003; Gur et al., 1999). Women also demonstrate significantly greater global and regional cortical thickness, while no significant thickening is observed in men. This significant cortical thickening in women is localized in anatomical regions consistent with studies that support sexual dimorphism (Kiho et al., 2006).

During childhood and adolescence, white matter volume increases faster in boys than in girls. When examining specific brain regions, greater diffusivity was found in the corticospinal tract and the frontal white matter in the right hemisphere for boys, whereas greater diffusivity was found in the occipital-parietal regions and the most superior aspect of the corticospinal tracts in the right hemisphere in girls (Rabinowicz, Dean, Petetot, & de Courten-Myers, 1999). Coincidently, girls show a greater organization in the right hemisphere compared to the left hemisphere for boys. These differences in brain matter and diffusivity may indicate differing developmental trajectories for both boys and girls, as well as possibly explain gender-specific abilities and/or behavioral differences between sexes.

7.4.3 Myelination

Myelination, or the development of an insulating myelin sheath around nerves so they can transmit information quicker, develops earlier in boys than girls. More specifically, by the age of two, myelination of long fiber tracks in the brain is more developed in males than females, thus allowing information to transmit faster in males.

One study examined brain density changes in girls and boys through childhood and adolescents. The findings from the study indicated that boys showed significantly greater loss of grey matter volume and an increase in both white matter and corpus collosum area compared with girls over a similar age range. Girls did show significant developmental changes with age, but at a slower rate than boys (DeBellis et al., 2001). The researchers argue that grey matter decreases are likely to reflect dendritic pruning which typically occurs during puberty. Dendritic pruning essentially eliminates extra neurons and synaptic connections to increase the efficiency of neuronal transmissions. It is suspected that the white matter density increase is related to increased myelination and/or axonal size, which also helps improve the efficiency of neuronal transmission.

Another aspect of myelination that appears to be different in men and women is related to Multiple Sclerosis (MS). MS is a chronic inflammatory disease of the central nervous system that causes inflammation, demyelination and axonal damage, leading to a wide range of neurological symptoms. It is found to be more prevalent in women. In fact, women are two to three times more likely to be diagnosed with MS than men. While the ultimate cause of MS is damage to the myelin, nerve fibers, and neurons in the brain and spinal cord, the onset of this degeneration is unknown. There is reason to believe that it is a combination of both genetic and environmental factors, however, further research is needed on this disease (National Multiple Sclerosis Society, 2019).

Regardless of the anatomical differences between males and females, it is important to note that differences in brain structure do not necessarily translate into differential performance (DeVries & Sodersten, 2009). Men and women might use different strategies to complete the same task, activating different brain areas, with similar competence. Finally, it is important to remember that the brain is plastic, with not only brain activation influencing behavior, but behavior also influencing brain activation.

Module Recap

Module 7 explored the biological differences between males and females. We are all comprised of billions of cells that contain DNA, genes, and chromosomes. While most of our genetic make-up is the same, there are some small differences that lead to significant physical differences. We learned that occasionally, cell division can go awry, and chromosomal abnormalities can occur. We briefly discussed some of the most common sex and non-sex linked chromosomal abnormalities.

We discussed the importance of the endocrine system and how the HPA axis responds to stressful situations. We identified different anatomy that is involved in regulating hormones- both for sexual reproduction and basic bodily function (homeostasis). Hormones can have significant implications on behavior, and we discussed the literature on the relationship between sex hormones and men and women’s behaviors and cognition. Finally, we discussed differences in brain structure and function in men and women. Although sex differences in brain anatomy and function are not clear, there are some implications for differences in male and female brains that may account for behavior differences between genders.

3rd edition