Gender Differences in Cognitive Functioning

by

Heidi Weiman

 

 

Introduction

Over the past decade, the differences between males and females have been highlighted in the media, based on such popular literature as, "Men Are from Mars, Women Are from Venus" (Gray, 1992), the television show of the same name, as well as the one-man play, by Rob Becker, "Defending the Caveman". Much of the information about the sexes that has been disseminated in pop culture appears to be based on sex role stereotyping, gender bias, or myth. The scope of this paper is to examine the scientific research on the differences between the sexes, particularly in regard to the brain and cognitive functioning in young people.

 

Controversies

Differences or Deficits: Interpretations and Policy Implications

 

The underscoring of human differences, rather than similarities, is a delicate matter. Historically, many societies have been inclined to define differences as deficits. All too often, this kind of interpretation of variance has led to the demarcation of the norm as "superior" and to the labeling of differing groups as "inferior". In the first half of the 20th century, the Eugenics Movement and the Holocaust were examples of how the devaluing of human differences can be taken to horrific extremes, resulting in government policies that condoned the sterilization, persecution, and death of millions of innocent people. Unfortunately, today, the world is not free of discrimination or of the oppression of differing groups, as exemplified by the "ethnic cleansing" of Muslims in the Balkans, as well as the severe restrictions placed on women in countries across the globe, such as Afghanistan, where women were stripped of most of their liberties, including their right to an education.

In the United States as well, there has been a long history of prejudice against those who differ from the white, male, European, middle-income, Protestant norm. Though constitutionally guaranteed, America's foundation in equality and justice is considered by some to be rather tenuous, since restrictive practices that severely limited opportunities for differing groups, including women and African-Americans, were outlawed and abandoned only within the latter half of the twentieth century. The Equal Rights Amendment, first introduced in 1923 and supported by the National Organization for Women (NOW) since 1967, was proposed in order to constitutionally assure the equal treatment of females in America, however, it has yet to gain popular support across the nation. In the 1990s, the implications of the social policy recommendations of American researchers who emphasized between group differences, in performance on intelligence tests, effectively identified historically marginalized groups as unworthy of equitable treatment, essentially due to perceived biological inferiority (Hernstein & Murray, 1992). Numerous hate groups, including self-proclaimed racists and misogynists, have already established links on their web pages to research that identifies differences in the cognitive functioning between the races and the sexes, as if to justify their biases and intolerance. Today, with statewide movements organized to repeal affirmative action, the maintenance of a "level playing field" ensured by nondiscriminatory legislation is questionable, at best, particularly when considering the persistence of hate crimes, the "glass ceiling", and other hurdles that remain for women and people of color.

The literature on sex differences is replete with warnings about the difficulties of teasing out "nature" from "nurture". Research underscores how little is certain about biological gender differences and their possible ramifications. Thus, caution is strongly urged in the interpretation of research findings on such sensitive and potentially volatile matters.

 

Sex vs. Gender: Clarification of Terms

            In the literature, the terms "sex" and "gender" often refer to distinct and separate constructs. Traditionally, many researchers have used the term "sex" in regard to physiological traits, while "gender" has been commonly used in relation to socially and psychologically mediated states. However, this is not the case in all scientific literature, therefore, the terms will be used interchangeably, here, rather than implying two distinct meanings.

 

 

Similarities & Differences

There is much overlap in the cognitive abilities of males and females. Today, measures of intelligence, such as the Wechsler tests, have been constructed to avoid gender bias. Research indicates that, on general measures of intelligence, the sexes perform about equally the same. Many researchers emphasize that there are more similarities than differences between the cognitive abilities of males and females, as effect sizes that do demonstrate differences, though statistically significant, tend to be rather small.

There is also much variability within gender groups, and this is particularly true for males (Halpern, 2000). Boys are over-represented at both tails of the distribution, with higher percentages identified as having disabilities, including dyslexia, dysfluency, delayed speech, mental retardation, and attention deficit/hyperactivity disorder, as well as giftedness --especially in math and science. However, there is also evidence that referral bias plays a role in undercounting girls with dyslexia (Pennington, 1991), as well as girls who are gifted.

Over the past few decades, much attention has been paid to righting the historical inequities and lack of opportunities available to females. Today, increasing evidence indicates that girls out-perform boys at all levels of schooling. Schools must now address the fact that boys are at higher risk for school failure.

 

Behavior

Before the advent of advanced brain imaging technologies, a large body of research accumulated on some of the behavioral characteristics indicative of the sex-related cognitive abilities of males and females. Behavioral studies, along with brain imaging research, autopsies, and animal research have begun to provide converging lines of evidence for some biological differences in the cognitive functioning of the sexes.

The differences between the intellectual capacities of the sexes appear to be in patterns of ability, rather than in overall intellectual functioning (Kimura, 1992). Attention and perception, which occur at the earliest stages of information processing, appear to differ between the sexes and may ultimately provide some clues in regard to differences that occur later on in cognitive processing.  Infant girls have been found to gaze longer at visual stimuli than boys, and males are much more likely to be diagnosed with attention related problems. Baker's review of sex-related perceptual differences (as cited in Halpern, 2000) suggests that there are variations in all of the sensory systems. Males tend to be more adept at dynamic visual acuity, which involves the ability to detect slight movements in the field of vision. Males are also more adept than females in temporal cognition, the ability to recognize the passage of time. Females tend to be more sensitive to touch, odors, taste, and sounds --much of which is detectable shortly after birth.

Males have consistently shown an advantage in visual-spatial abilities, such as aiming at stationary or moving targets, as well as throwing and intercepting projectiles (Kimura, 1992). Males also perform better, and differently, than females in navigation. Whereas females are inclined to use landmarks as guides, males tend to rely on direction, distance, and geometric shapes for navigating their way through a route. Males also excel at quantitative problem solving, and mental rotation, or tasks involving the underlying cognitive processes of maintaining and manipulating a visual image in working memory (Halpern, 2000). It has been theorized that, evolutionarily, many of these abilities would have been important for survival when humans lived in hunter-gatherer societies, where males navigated unfamiliar terrain while hunting, and females foraged more nearby areas gathering food.  An evolutionary theory regarding ADHD has been proposed as well. According to this theory, the ability to vigilantly scan the horizon, on alert to novel stimuli, such as stampeding buffalo, would have served the prehistoric hunter well (Hartmann, 2001). Recent genetic research suggests that there is scientific evidence to support this theory (Seay, 2002). It is conceivable that some cultures would value and reinforce different kinds of skills and behaviors, including perseverance and novelty-seeking, especially when advantageous to survival. Additionally, evidence on the evolution of the cerebral cortex suggests brain-behavior relationships, particularly in regard to the development of the prefrontal lobes, seat of the Executive Functions, including planning and organization, maintenance and flexibility of mental set, and self-regulation, such as delayed gratification and the inhibition of impulses --deficits often associated with ADHD.

In 1995, Shaywitz et al. identified evidence for gender differences in the functional organization of the brain for language, in a functional magnetic resonance imaging (fMRI) study. Behaviorally, females have consistently shown an advantage for verbal abilities, including earlier language acquisition and longer attention spans than males for conversation (as cited in Kruger, 2001). Females also tend to excel at memory tasks, including associational fluency, which includes generating synonyms, as well as color naming, or listing items beginning with a designated letter (Halpern, 2000; Kimura, 1992). The underlying cognitive process appears to involve rapid retrieval of information from memory. Females also tend to excel at tasks involving manual dexterity and perceptual speed, such as visually identifying matching items.

 

Physiology

There is increasing evidence for neuroanatomic differences contributing to the cognitive functioning of males and females, although the literature is by no means conclusive, as there are numerous questions remaining and vast amounts of information still to be learned about the human brain and sex-related abilities. It has been well established that genes play a key role in sexual differentiation. Research suggests that the genetic blueprint for sex is female (XX), because it is the absence of the Y chromosome triggering the release of male hormones, or androgens, which causes a female to develop.  Researchers have identified critical periods when the release of sex hormones impacts the brain. Hormone levels are high during prenatal development, beginning around the 6th or 7th week of gestation, and continuing through the perinatal period. It is believed that the release of hormones during these periods contributes to both sex differentiation and brain differentiation. Hormone levels rise again during puberty, and there are fluctuations in hormonal levels across the life span, including diurnally, seasonally, during the menstrual cycle, and after menopause. Researchers have begun to examine the contribution of hormones to behavior and learning. Girls who were known to have been exposed to high levels of testosterone in utero, due to congenital adrenal hyperplasia (CAH), appear to demonstrate  better spatial skills than other girls and they are more likely to behave aggressively --similar to boys (Collaer & Hines, 1995). Hormone Replacement Therapy has been implicated in improved attention and reading for post-menopausal women (Shaywitz, et al., 1999). It has been speculated that naturally occurring increases of estrogen during puberty may account for documented improvements in the reading skills of some adolescent girls with dyslexia (Institute of Medicine, 2001). 

The male brain is approximately 10% larger than the brain of females, however, after adjustment for body size, the body-weight ratio for the female brain is actually greater (Drubach, 2000; Society for Neuroscience, 1998).  There has been much debate over the value of larger brains, since there have been many reports indicating a correlation between brain size and intelligence. Halpern (2000) argued rather persuasively that, if brain size was the most significant factor for determining intelligence, the earth would be governed by elephants or whales, not humans. Magnetic resonance imaging has revealed that the male brain contains more white matter and cerebrospinal fluid (CSF) than the female brain, and that the female brain contains a relatively greater proportion of gray matter (Gur et al., 1999). Gur speculated that increases in white matter may enable men to transfer information to further regions within the brain, thereby contributing to their spatial prowess, while the increased gray matter in women's brains may allow for more efficiency and a greater capacity for processing.

Animal research has revealed sexual dimorphisms in the brain, which are currently under investigation in humans, including the hippocampus, the hypothalamus, and the corpus callosum.  The right hemisphere of males tends to be larger and more dense. In utero, the right hemisphere develops more rapidly than the left, leaving the left hemisphere more vulnerable to adverse intrauterine conditions. It has been postulated that this might account for the high incidence of language-based learning problems in males, since language is lateralized in the left hemisphere in most people. It has been speculated that the female corpus callosum differs from that of males and that those differences may account for more efficient communication between the hemispheres of females.

 

School Performance

In school, males tend to excel at problem-solving, multiple choice tests, and they outperform females on the SATs --although there is much self-selection in taking the SATs and, as less able males are inclined to drop out of school, they are less likely to take the test (Halpern, 2000). Females excel in calculation, untimed and written tests, and they tend to have higher grades than males throughout their schooling. Until high school, females often exceed males on tests of math. It has been speculated that the reversal in girl's math performance may be due to both endogenous and exogenous factors, including a curriculum that includes more spatially oriented math at the high school level, such as geometry, as well as lower social expectations of females. Males tend to receive more teacher attention than girls, and teachers ask more higher order questions of them. Research also shows that students with learning difficulties often receive the least amount of teacher attention --which might contribute to high drop out rates for less successful males. Boys tend to participate more in whole group activities and they often dominate class discussions. Yet girls are advantaged by their verbal skills and increasing numbers have been enrolling in and graduating from college.

 

Environment

The increasing evidence for some biological differences between the sexes may seem to call into question the influence of environment on gender development. Some have argued that the infamous John/Joan sexual reassignment case might be a classic example of how powerful genetics is on gender development (PBS, 2001). In this case of a Canadian monozygotic twin infant boy, after a medical accident resulted in the loss of his penis, in 1967, the child’s parents were encouraged by psychologist John Money, from Johns Hopkins Medical Center, to have their son undergo sexual reassignment surgery. The zeitgeist of the time underscored the primacy of nurture over nature. Consistent with the prevailing notion, at the time, of gender neutrality at birth, it was believed that, with psychological guidance and hormonal supplements, gender could be socially mediated in this case. Though he underwent the surgery at less than two years of age, and, while growing up, the child was unaware of his birth as a male, “John/Joan” had great difficulty adapting to life as a female --and Money failed to report the challenges that the child was facing in the scientific literature. During puberty, the parents informed the adolescent of what had transpired during his early childhood. He chose to live his life as a male and underwent reconstructive surgery. He married, though his marriage didn't last. About a year after his twin died, he committed suicide.

 

Conclusion

Researchers have located an area of the brain that is distinctly different in males than in females, in a region of the hypothalamus that regulates hormones (PBS, 2001). Known as the sexually dimorphic nucleus, or SDN, it has been found to be larger in males than in females and transsexual males. In animal research, chromosomal females exposed to testosterone in utero behave similarly to males, and their SDNs are enlarged. Scientists do not yet know precisely what this brain difference means or how this might translate to human behavior. While the dominance of nurture over nature is no longer assumed, no one knows for certain the percentage of influence nature and nurture have on development and sexual identity. In the John/Joan case, researchers have asserted that undergoing sexual reassignment surgery while a toddler, as well as growing up with an identical male twin, might have confounded the situation for the child. Since sex-role stereotyping begins in the cradle, conceivably, lowered expectations, as well as decreased opportunities, could interact with genetics in determining gender identity and life outcomes.

 

 

 

References

 

 

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Created 08-4-01

Updated 5-10-04

 




 

 

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