Sex differences in human psychology

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"Sex differences" are sexually dimorphic traits hypothesized to be evolved consequences of sexual selection.[1][2] Research spans several scientific disciplines investigating psychological and behavioral differences between men and women due to:

  • Genetics and epigenetics.[3]
  • Psychological traits such as emotion, motivation, cognition, and sexuality.[8][9][10][11][12]

Since all behaviors are phenotypes—complex interactions of nature and nurture—researchers especially investigate how biology and environment interact to produce sex differences.[1][15][16][17]

History[edit]

In his 1859 book On the Origin of Species Charles Darwin hypothesized that sexually dimporhic traits were an evolved product of the process he termed sexual selection. Darwin also proposed that, like physical traits, psychological traits evolve:

In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation.

—Charles Darwin, The Origin of Species, 1859, p. 449.

He explored the evolution of sexually dimorphic psychological traits in two of his later books The Descent of Man, and Selection in Relation to Sex in 1871 and The Expression of the Emotions in Man and Animals in 1872. The The Descent of Man and Selection in Relation to Sex includes 70 pages on sexual selection in human evolution, and 500 pages on sexual selection in other animals.[18]

Genetics and epigenetics[edit]

Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits. In mammals, the female is the homogametic sex, with two X chromosomes (XX), while the male is heterogametic, with one X and one Y chromosome (XY). Genes on the X or Y chromosome are called sex-linked.

It is important to distinguish between sex-linked characters, which are controlled by genes on sex chromosomes, and two other categories.[19] Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body.[20] Even in a homozygous dominant or recessive female the condition may not be expressed fully. Example: baldness in humans. Sex-limited traits are characteristics only expressed in one sex. They may be caused by genes on either autosomal or sex chromosomes.[20]

Sex-linked genes have been found to differentially affect the brains of males and females,[21] and epipgenetic effects have also been found to cause sex differences in the brain.[22]

Brain structure and function[edit]

Studies have found many similarities but also differences in brain structure, neurotransmitters, and function.[23] However, some argue that innate differences in the neurobiology of men and women have not been conclusively identified.[24][25] The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large.[24][26]

A 2004 review in Nature Reviews Neuroscience stated that the brain's sexual dimorphism is probably determined by genes on the sex chromosomes. They likely do so by genes in cells in the gonads causing the gonads to produce sex hormones that travel to the brain which affect brain cells and also by genes in brain cells directly affecting these brain cells.[27] In the human brain, a difference between sexes has been observed in regarding the PCDH11X/Y gene pair which is unique to Homo sapiens.[28]

In adults, men's brains are an average of 11–12% heavier than women's brains.[29] However, men's bodies are also larger and heavier than women's bodies. In the United States, for example, adult men are an average of 18% heavier than adult women.[30] Some researchers propose that the brain-to-body mass ratio does not differ between the sexes.[31][32] However, some argue that the brain-to-body mass ratio tends to decrease as body size increases, and a sex difference in brain size still exists between men and women of the same size.[citation needed] A 1992 study of 6,325 Army personnel found that men's brains had an average volume of 1442 cm3, while the women averaged 1332 cm3. These differences were shown to be smaller but to persist even when adjusted for body size measured as body height or body surface, such that women averaged 100g less brain mass than men of equal size.[33]

Though statistically there are sex differences in white matter and gray matter percentage, this ratio is directly related to brain size, and some argue these sex differences in gray and white matter percentage are caused by the average size difference between men and women.[34][35][36][37] Others argue that these differences remain after controlling for brain volume.[23] A 2013 study used diffusion MRI to map functional (rather than structural) differences in male and female brains.[38] In the cerebral cortex it was observed that, in comparison with the opposite sex, there was greater intra-lobe neural communication in male brains and greater inter-lobe (between the left and right hemispheres of the cerebral cortex) neural communication in female brains.[39][40]

Hormones[edit]

Sex hormones, such as testosterone, estrogen, progesterone, oxytocin, and vasopressin have differential psychological effects on men and women. For example, they may affect sexual motivation.

Testosterone appears to be a major contributing factor to sexual motivation in male primates, including humans. The elimination of testosterone in adulthood has been shown to reduce sexual motivation in both male humans and male primates.[41] Male humans who had their testicular function suppressed with a GnRH anatagonist displayed decreases in sexual desire and masturbation two weeks following the procedure.[42] It is also suggested that levels of testosterone in men are related to the type of relationship in which they are involved. Men involved in polyamorous relationships display higher levels of testosterone than men involved in either a single partner relationship or single men.[43]

Females at different stages of their menstrual cycle have been shown to display differences in sexual attraction. Non-pill using heterosexual females who are ovulating (high levels of estrogens) have a preference for the scent of males with low levels of fluctuating asymmetry.[44] Ovulating heterosexual females also display preferences toward masculine faces and report greater sexual attraction to males other than their current partner.[45] From an evolutionary perspective, increases in estrogens during fertile periods in females may direct sexual motivation toward males with preferential genes (the good genes hypothesis).

Psychological traits[edit]

Development of gender identity[edit]

An ongoing debate in psychology is the extent to which gender identity and gender-specific behavior is due to socialization as opposed to genetic factors. The mainstream view is that both factors play a role, but the relative importance of each is contentious.[citation needed]

Because of the wide acceptance of gender roles, it is difficult to execute a study which controls for the influence of such socialization. Individuals who are sex reassigned at birth offer an opportunity to see what happens when a child who is genetically one sex is raised as the other. The largest study of such individuals was conducted by Reiner & Gearhart on 14 children born with cloacal exstrophy and reassigned female at birth. Upon follow-up between the ages of 5 to 12, 8 of them identified as boys, and all of the subjects had at least moderately male-typical attitudes and interests, however these tests were not double blind as the parents (and often the subjects) knew the biological sex of children they were raising.[46] The procedure of sex reassignment and vaginoplasty on intersex children assigned to be females also changes their experiences in ways not analogous to typically developed females, including vaginal dilatation by parents on toddlers (routine widening of the vagina through insertion of a device), and testing of clitoral scar tissue for sensation by doctors.[47]

Girls with congenital adrenal hyperplasia and thus exposed to high androgen level during pregnancy play more with boy toys and less with girl toys.[48][49]

One study showed that at birth girls gaze longer at a face, whereas suspended mechanical mobiles, rather than a face, keep boys' attention for longer, though this study has been criticized as having methodological flaws.[24]

Sexual behavior[edit]

The Sexual Strategies Theory by David Buss and David P. Schmitt is a comprehensive evolutionary psychology theory regarding female and male short-term and long-term mating strategies which are argued to be dependent on several different goals and vary depending on the environment. Men and women are predicted to have both similar and different strategies depending on the circumstances. The theory included many predictions that could be empirically tested. The theory is argued to have received extensive empirical support in subsequent research. It has also been developed further.[50] Terri D. Conley et al. has argued that other empirical evidence support smaller or non-existing gender differences and social theories such as stigma, socialization, and double standards.[51]

Intelligence[edit]

Historical perspectives[edit]

Thomas Gisborne argued (1801) that women were naturally suited to domestic work and not spheres suited to men such as politics, science, or business. He argued that this was because women did not possess the same level of rational thinking that men did and had naturally superior abilities in skills related to family support.[52]

In 1875, Herbert Spencer argued that women were incapable of abstract thought and could not understand issues of justice, and only had the ability to understand issues of care.[53] In 1925, Sigmund Freud also concluded that women were less morally developed in the concept of justice, and, unlike men, were more influenced by feeling than rational thought.[53] Early brain studies comparing mass and volumes between the sexes concluded that women were intellectually inferior because they had smaller and lighter brains.[24] Later studies with better equipment have confirmed this brain size difference.[54] Many believed that the size difference caused women to be excitable, emotional, sensitive, and therefore not suited for political participation.[55] Today, others argue that brain size correlates with intelligence and/or personality.[24] The correlation is around 0.44 for brain size and IQ. This together with the brain size difference between sexes have caused some scholars to propose a sex difference in IQ/intelligence.[54]

In the nineteenth century, whether men and women had equal intelligence was seen by many as a prerequisite for the granting of suffrage.[55] Leta Hollingworth argues that women were not permitted to realize their full potential, as they were confined to the roles of child-rearing and housekeeping. From the late twentieth century onwards, researchers have investigated the possibility of environmental factors in perceived sex differences.[53]

During the early twentieth century, the scientific consensus held that gender plays no role in intelligence.[56] In his research, psychologist Lewis Terman found "rather marked" differences on a minority of tests. For example, he found boys were "decidedly better" in arithmetical reasoning, while girls were "superior" at answering comprehension questions, though he concluded that sex plays no role in general intelligence. He also proposed that discrimination, denied opportunities, women's responsibilities in motherhood, or emotional factors may have accounted for the fact that few women had careers in intellectual fields.[57]

Current research on general intelligence[edit]

According to the 1994 report "Intelligence: Knowns and Unknowns" by the American Psychological Association, "Most standard tests of intelligence have been constructed so that there are no overall score differences between females and males." Differences have been found, however, in specific areas such as mathematics and verbal measures.[48]

When standardized IQ tests were first developed in the early 20th century, girls typically scored higher than boys until age 14, at which time the curve for girls dropped below that for boys.[57][58] As testing methodology was revised, efforts were made to equalize gender performance.[58][59][60]

The mean IQ scores between men and women vary little.[48][61][62][63][64] The variability of male scores is greater than that of females, however, resulting in substantially more males than females in the top and bottom of the IQ distribution.[65]

Several meta-studies by Richard Lynn between 1994 and 2005 found mean IQ of men exceeding that of women by a range of 3–5 points.[66][67][68][69] Lynn's findings were debated in a series of articles for Nature.[70][71] Jackson and Rushton found males aged 17–18 years had average of 3.63 IQ points in excess of their female equivalents.[72] A 2005 study by Helmuth Nyborg found an average advantage for males of 3.8 IQ points.[73] One study concluded that after controlling for sociodemographic and health variables, "gender differences tended to disappear on tests for which there was a male advantage and to magnify on tests for which there was a female advantage."[74] A study from 2007 found a 2-4 IQ point advantage for females in later life.[75] One study investigated the differences in IQ between the sexes in relation to age, finding that girls do better at younger ages but that their performance declines relative to boys with age.[76] Colom et al. (2002) found 3.16 higher IQ points for males but no difference on the general intelligence factor (g) and therefore explained the differences as due to non-g factors such as specific group factors and test specificity.[64] A study conducted by Jim Flynn and Lilia Rossi-Case (2011) found that men and women achieved roughly equal IQ scores on Raven's Progressive Matrices after reviewing recent standardization samples in five modernized nations.[77] Irwing (2012) found a 3 point IQ advantage for males in g from subjects aged 16–89 in the United States.[78]

Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier et al. found in a 2004 study that: "Men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance.[79] For men, the gray matter volume in the frontal and parietal lobes correlates with IQ; for women, the gray matter volume in the frontal lobe and Broca's area (which is used in language processing) correlates with IQ.[23]

Some studies have identified the degree of IQ variance as a difference between males and females. Males tend to show greater variability on many traits including tests of cognitive abilities,[80][81] though this may differ between countries.[82][83][84][85] A 2005 study by Ian Deary, Paul Irwing, Geoff Der, and Timothy Bates, focusing on the ASVAB showed a significantly higher variance in male scores, resulting in more than twice as many men as women scoring in the top 2%. The study also found a very small (d' ≈ 0.07, less than 7%, of a standard deviation) average male advantage in g.[65] A 2006 study by Rosalind Arden and Robert Plomin focused on children aged 2, 3, 4, 7, 9 and 10 and stated that there was greater variance "among boys at every age except age two despite the girls’ mean advantage from ages two to seven. Girls are significantly over-represented, as measured by chi-square tests, at the high tail and boys at the low tail at ages 2, 3 and 4. By age 10 the boys have a higher mean, greater variance and are over-represented in the high tail."[86]

The variability hypothesis still evokes controversy, but recent data and analyses may bring some closure to the debate […] Data from a number of representative mental test surveys, involving samples drawn from the national population, have become available in the past twenty years in the USA. These have finally provided consistent results. Both Feingold (1992b) and Hedges and Nowell (1995) have reported that, despite average sex differences being small and relatively stable over time, test score variances of males were generally larger than those of females. Feingold found that males were more variable than females on tests of quantitative reasoning, spatial visualisation, spelling, and general knowledge. […] Hedges and Nowell go one step further and demonstrate that, with the exception of performance on tests of reading comprehension, perceptual speed, and associative memory, more males than females were observed among high-scoring individuals.

John Archer and Barbara Lloyd[87]

A psychological study was conducted where about 1200 high school graduates were recruited to take tests looking at each of their verbal, reasoning, spatial abilities, and general scholastic knowledge.[88] Male and female performances were compared through these tests. As a result of this testing, it was discovered that males had a higher mean score on all four tests than the mean score of the females who participated in the study.[88] In 1995, it was suggested by Charles Lewis and Warren W. Willingham that patterns of gender differences on IQ scores can change because of the selectivity of the sample itself. They argued two factors played in giving the males an advantage: the greater male variability and the sampling of a greater proportion of women.[89]

Another study on intelligence came up with similar findings. Young adolescents were asked to volunteer in this study and completed various assessments including ones looking at language, math, and sciences skills as well as the Toulous-Pieron test of attention and the Dominoes test.[90] While one sample of children completed these assessments, another sample completed these plus another handful. The test results from both samples show a null sex difference in general intelligence in young adolescents. Researchers concluded that since g does not differ through academic and cognitive abilities in young adolescents, male or female, and that some other factor must be responsible for the variance between the sexes.[90]

It was believed at one point that Gf, or fluid intelligence, can be used to systematically detect sex differences in general intelligence if there are any.[91] The PMA Inductive Reasoning Test, Cattell’s Culture-Fair Intelligence Test, and the Advanced Progressive Matrices were used to test a group of about 4000 high school graduates. Through the results of these tests, researchers discovered that females perform better in the PMA Inductive Reasoning Test and males perform better in the Advanced Progressive Matrices assessment.[91] There was no sex difference noted from the results of the Culture-Fair Test. Sex difference in fluid intelligence was proven to be non-existent in this study.[91]

While research has shown that males and females do indeed each excel in different abilities, math and science might be an exception to this.[92]

While some think that IQ scores are the best way to reach conclusions about cognitive sex differences, this theory is not used consistently to measure intelligence. These tests have been complied over the years so there is no sex difference in average IQ. This was done in order to keep one sex from gaining an unfair advantage over the other in performance. Although this would imply that males and females have about the same IQ scores on average and most researchers maintain this view, some researchers have concluded that men have slightly higher IQ scores than women.[93]

Mathematics performance[edit]

Girl scouts compete in the USS California Science Experience at Naval Surface Warfare. In 2008, the National Science Foundation reported that, on average, girls perform as well as boys on standardized math tests, while boys are overrepresented on both ends of the spectrum.

Large, representative studies of US students show that no sex differences in mathematics performance exist before secondary school. During and after secondary school, historic sex differences in mathematics enrollment account for nearly all of the sex differences in mathematics performance. However, a performance difference in mathematics on the SAT exists favoring males, though differences in mathematics course performance measures favor females.[94] In 1983, Benbow concluded that the study showed a large sex difference by age 13 and that it was especially pronounced at the high end of the distribution.[95] However, Gallagher and Kaufman criticized Benbow's and other reports finding males overrepresented in the highest percentages as not ensuring representative sampling.[94]

In a 2008 study paid for by the National Science Foundation in the United States, researchers found that "girls perform as well as boys on standardized math tests. Although 20 years ago, high school boys performed better than girls in math, the researchers found that is no longer the case. The reason, they said, is simple: Girls used to take fewer advanced math courses than boys, but now they are taking just as many."[96][97] However, the study indicated that, while on average boys and girls performed similarly, boys were overrepresented among the very best performers as well as among the very worst.[98][99]

Kiefer and Sekaquaptewa proposed that a source of some women's underperformance and lowered perseverance in mathematical fields is these women's underlying "implicit" sex-based stereotypes regarding mathematical ability and association, as well as their identification with their gender.[100] Some psychologists believe that many historical and current sex differences in mathematics performance may be related to boy's higher likelihood of receiving math encouragement than girls. Parents were, and sometimes still are, more likely to consider a son's mathematical achievement as being a natural skill while a daughter's mathematical achievement is more likely to be seen as something she studied hard for. This difference in attitude may contribute to girls and women being discouraged from further involvement in mathematics-related subjects and careers.[101] Stereotype threat has been shown to affect performance and confidence in mathematics of both males and females.[24][94] However, a review of stereotype threat literature found most studies couldn't be replicated or suffered methodological problems and concluded "that although stereotype threat may affect some women, the existing state of knowledge does not support the current level of enthusiasm for this as a mechanism underlying the gender gap in mathematics."[102]

Two cross-country comparisons have found great variation in the gender differences regarding the degree of variance in mathematical ability. In most nations males have greater variance. In a few females have greater variance. Hyde and Mertz argue that boys and girls differ in the variance of their ability due to sociocultural factors.[82][103][104]

Spatial abilities[edit]

Examples of figures from mental rotation tests.
A man playing a video game at the Japan Media Arts Festival. Spatial abilities can be affected by experiences such as playing video games, complicating research on sex differences in spatial abilities.

Some studies investigating the spatial abilities of men and women have found no significant differences,[105][106] though metastudies show a male advantage in mental rotation and assessing horizontality and verticality,[48][107] and a female advantage in spatial memory.[108][109]

A proposed hypothesis is that men and women evolved different mental abilities to adapt to their different roles in society.[110][111] This explanation suggests that men may have evolved greater spatial abilities as a result of certain behaviors, such as navigating during a hunt.[112] Similarly, this hypothesis suggests that women may have evolved to devote more mental resources to remembering locations of food sources in relation to objects and other features in order to gather food.[113]

A number of studies have shown that women tend to rely more on visual information than men in a number of spatial tasks related to perceived orientation.[114][115] However, 'visual dependence' has been found to be task specific and not a general characteristic of spatial processing that differs between the sexes. Here an alternative hypothesis suggests that heightened visual dependence in females does not generalize to all aspects of spatial processing but is probably attributable to task-specific differences in how male and females brains process multisensory spatial information.[116]

Results from studies conducted in the physical environment are not conclusive about sex differences, with various studies on the same task showing no differences. For example, there are studies that show no difference in 'wayfinding'.[117] One study found men more likely to report having a good sense of direction and are more confident about finding their way in a new environment, but evidence does not support men having better map reading skills.[118] Women have been found to use landmarks more often when giving directions and when describing routes.[119] Additionally, a study concludes that women are better at recalling where objects are located in a physical environment.[118] Women show greater proficiency and reliance on distinctive landmarks for navigation while males rely on an overall mental map.[120][121]

Performance in mental rotation and similar spatial tasks is affected by gender expectations.[24][122] For example, studies show that being told before the test that men typically perform better, or that the task is linked with jobs like aviation engineering typically associated with men versus jobs like fashion design typically associated with women, will negatively affect female performance on spatial rotation and positively influence it when subjects are told the opposite.[123][124][125][126] Experiences such as playing video games also increase a person's mental rotation ability.[117][127] A study from the University of Toronto showed that differences in ability get reduced after playing video games requiring complex mental rotation. The experiment showed that playing such games creates larger gains in spatial cognition in females than males.[128]

The possibility of testosterone and other androgens as a cause of sex differences in psychology has been a subject of study. Adult women who were exposed to unusually high levels of androgens in the womb due to congenital adrenal hyperplasia score significantly higher on tests of spatial ability.[129] Many studies find positive correlations between testosterone levels in normal males and measures of spatial ability.[130] However, the relationship is complex.[131][132]

A study was done to compare the relationship between mental rotation ability and gender difference specifically with the SAT-Math. Cognitive gender differences are apparent and findings of a male advantage in certain mathematical domains have been demonstrated cross-culturally. These gender differences found are largely in geometry and word problems and tend to be in countries with the highest achieving students and with the largest gender gap in experience.[133] Smaller differences were noted in countries with lower achieving students in mathematics which includes the United States. Moore and Smith state that within the United States, poorly educated female students outperform their male peers, but as the level of education increases, the male advantage in mathematics emerges.[133]

Spatial ability may be responsible in part for facilitating gender differences in math aptitude. Casey et al. (1995) looked at the relationship of mental rotation ability and the SAT-M among four samples. The four samples were: (1) undergraduates at two liberal arts colleges in the Northeast that were tested on their mental rotation ability in groups of 10-20, (2) a group of mathematically talented preadolescents participating in a summer math and science training in the Midwest which included seventh to ninth graders who were either recruited from a national talent search program or statewide teacher selection program, (3) a high ability group of college bound students who were enrolled in a middle-income suburban high school in the Northeast and elected to take the SAT, and (4) a low ability group of college bound students who were enrolled in a middle-income suburban high school in the Northeast and elected to take the SAT. The data used were SAT math and verbal scores and mental rotation scores. Mental rotation was assessed using the Vandenberg Test of Mental Rotation.[133] Students were asked to match two out of four choices to a standard figure.

The study found that that when mental rotation is used as a predictor of Math aptitude for female students, the correlations between mental rotation and SAT-Math scores ranged from 0.35 to 0.38 whereas males showed no consistent pattern. Male correlations ranged from -0.03 to 0.54.[133] However, an interesting finding was that in the three high ability samples, there was a significant gender difference in SAT-Math scores alone. This difference favored males. In the three high ability samples, males scored higher than females in mental rotation ability. Interesting enough, for the verbal aptitude test on SAT, there was a significant difference in verbal ability for the low ability college bound sample favoring girls.[133]

Dyslexia[edit]

Dyslexia is a learning disability that impairs a person’s fluency or comprehension accuracy in being able to read.[134] The cause of this disability is associated with abnormal brain anatomy and function. Gray matter deficits have been demonstrated in dyslexics using structural magnetic resonance imaging.[135] This deficit has been found in specific regions within the left hemisphere involved in language.

There is higher prevalence of dyslexia in males than in females.[136] However, different abnormalities are found in female brains as opposed to male brains. In a study that examined gray matter volume in dyslexic females, it was found that there was less gray matter volume in the right precuneus and paracentral lobule/medial frontal gyrus.[136] In males, there was less gray matter volume in the left inferior parietal cortex.[136] This study shows that dyslexia in females does not involve the left hemisphere regions involved in language as it does in males. Instead, it affects the sensory and motor cortices such as the motor and premotor cortex and primary visual cortex.[136]

Memory[edit]

The results from research on sex differences in memory are mixed and inconsistent, with some studies showing no difference, and others showing a female or male advantage.[109] Most studies have found no sex differences in short term memory, the rate of memory decline due to aging, or memory of visual stimuli.[109] Females have been found to have an advantage in recalling auditory and olfactory stimuli, experiences, faces, names, and the location of objects in space.[108][109] However, males show an advantage in recalling "masculine" events.[109] A study examining sex differences in performance on the California Verbal Learning Test found that males performed better on Digit Span Backwards and on reaction time, while females were better on short-term memory recall and Symbol-Digit Modalities Test.[74]

A study was conducted to explore regions within the brain that are activated during working memory tasks in males versus females. Four different tasks of increasing difficulty were given to 9 males and 8 females. Functional magnetic resonance imaging was used to measure brain activity. The lateral prefrontal cortices, the parietal cortices and caudates were activated in both genders.[137] With more difficult tasks, more brain tissue was activated. The left hemisphere was predominantly activated in females’ brains, whereas there was bilateral activation in males’ brains.[137] This suggests some sort of gender difference in the brain organization involved in working memory.

Aggression[edit]

Although research on sex differences in aggression show that males are generally more likely to display aggression than females, how much of this is due to social factors and gender expectations is unclear. Aggression is closely linked with cultural definitions of "masculine" and "feminine." In some situations women show equal or more aggression than men; for example, women are more likely to use direct aggression in private, where other people cannot see them, and are more likely to use indirect aggression in public.[107] Eagly and Steffen suggested in their meta-analysis of data on sex and aggression that beliefs about the negative consequences of violating gender expectations affect how both genders behave regarding aggression.[138] Men are more likely to be the targets of displays of aggression and provocation than females. Studies by Bettencourt and Miller show that when provocation is controlled for, sex differences in aggression are greatly reduced. They argue that this shows that gender-role norms play a large part in the differences in aggressive behavior between men and women.[139] Psychologist Anne Campbell argues that females are more likely to use indirect aggression, and that "cultural interpretations have 'enhanced' evolutionarily based sex differences by a process of imposition which stigmatises the expression of aggression by females and causes women to offer exculpatory (rather than justificatory) accounts of their own aggression."[140]

The relationship between testosterone and aggression is unclear, and a causal link has not been conclusively shown.[141][142][143][144] Some studies indicate that testosterone levels may be affected by environmental and social influences.[145] The relationship is difficult to study since the only reliable measure of brain testosterone is from a lumbar puncture which is not done for research purposes and many studies have instead used less reliable measures such as blood testosterone. In humans, males engage in crime and especially violent crime more than females. The involvement in crime usually rises in the early teens to mid teens which happen at the same time as testosterone levels rise. Most studies support a link between adult criminality and testosterone although the relationship is modest if examined separately for each sex. However, nearly all studies of juvenile delinquency and testosterone are not significant. Most studies have also found testosterone to be associated with behaviors or personality traits linked with criminality such as antisocial behavior and alcoholism.[146]

In species that have high levels of male physical competition and aggression over females, males tend to be larger and stronger than females. Humans have modest general body sexual dimorphism on characteristics such as height and body mass. However, this may understate the sexual dimorphism regarding characteristics related to aggression since females have large fat stores. The sex differences are greater for muscle mass and especially for upper body muscle mass. Men's skeleton, especially in the vulnerable face, is more robust. Another possible explanation, instead of intra-species aggression, for this sexual dimorphism may be that it is an adaption for a sexual division of labor with males doing the hunting. However, the hunting theory may have difficulty explaining differences regarding features such as stronger protective skeleton, beards (not helpful in hunting but they increase the perceived size of the jaws and perceived dominance which may helpful in intra-species male competition), and greater male ability at interception (greater targeting ability can be explained by hunting).[147]

There are evolutionary theories regarding male aggression in specific areas such as sociobiological theories of rape and theories regarding the high degree of abuse against stepchildren (the Cinderella effect).

Personality traits[edit]

Cross-cultural research has shown gender differences on the domains and facets of the Big Five personality traits. For example, women consistently report higher Neuroticism, Agreeableness, warmth (an extraversion facet) and openness to feelings, and men often report higher assertiveness (a facet of extraversion) and openness to ideas as assessed by the NEO-PI-R.[148] Gender differences in personality traits are largest in prosperous, healthy, and egalitarian cultures in which women have more opportunities that are equal to those of men. Differences in the magnitude of sex differences between more or less developed world regions were due to differences between men not women in these respective regions. That is, men in highly developed world regions were less neurotic, extraverted, conscientious and agreeable compared to men in less developed world regions. Women, on the other hand tended not to differ in personality traits across regions. Researchers have speculated that resource poor environments (that is, countries with low levels of development) may inhibit the development of gender differences, whereas resource rich environments facilitate them. This may be because males require more resources than females in order to reach their full developmental potential.[149] The authors argued that due to different evolutionary pressures, men may have evolved to be more risk taking and socially dominant, whereas women evolved to be more cautious and nurturant. Hunter-gatherer societies in which humans originally evolved may have been more egalitarian than later agriculturally oriented societies. Hence, the development of gender inequalities may have acted to constrain the development of gender differences in personality that originally evolved in hunter-gatherer societies. As modern societies have become more egalitarian again it may be that innate sex differences are no longer constrained and hence manifest more fully than in less developed cultures. Currently, this hypothesis remains untested, as gender differences in modern societies have not been compared with those in hunter-gatherer societies.[149]

Demographics of the Myers-Briggs Type Indicator indicate that in the United States 65-76% of women prefer "feeling" and 55-67% of men prefer "thinking".[150]

A personality trait directly linked to emotion and empathy where gender differences exist (see below) is Machiavellianism. Individuals who score high on this dimension are emotionally cool; this allows them to detach from others as well as values, and act strategically rather than driven by affect, empathy or morality. In large samples of US college students males are on average more Machiavellian than females; in particular, males are over-represented among very high Machiavellians, while females are overrepresented among low Machiavellians.[151][152]

Empathy[edit]

Women perform better than men in tests involving emotional interpretation, such as understanding facial expressions, and empathy.[153][154][155][156]

Some studies argue that this is related to the subject's perceived gender identity and gender expectations.[24] Additionally, culture impacts gender differences in the expression of emotions. This may be explained by the different social roles men and women have in different cultures, and by the status and power men and women hold in different societies, as well as the different cultural values various societies hold.[157] Some studies have found no differences in empathy between men and women, and suggest that perceived gender differences are the result of motivational differences.[158][159] Some researchers argue that because differences in empathy disappear on tests where it is not clear that empathy is being studied, men and women do not differ in ability, but instead in how empathetic they would like to appear to themselves and others.[24][160]

An evolutionary explanation for the difference is that understanding and tracking relationships and reading others' emotional states was particularly important for women in prehistoric societies for tasks such as caring for children and social networking.[112]

Emotion[edit]

When measured with an affect intensity measure, women reported greater intensity of both positive and negative affect than men. Women also reported a more intense and more frequent experience of affect, joy, and love but also experienced more embarrassment, guilt, shame, sadness, anger, fear, and distress. Experiencing pride was more frequent and intense for men than for women.[157] In imagined frightening situations, such as being home alone and witnessing a stranger walking towards your house, women reported greater fear. Women also reported more fear in situations that involved "a male's hostile and aggressive behavior" (281)[157] In anger-eliciting situations, women communicated more intense feelings of anger than men. Women also reported more intense feelings of anger in relation to terrifying situations, especially situations involving a male protagonist.[161] Emotional contagion refers to the phenomenon of a person’s emotions becoming similar to those of surrounding people. Women have been reported to be more responsive to this.[162]

Women are stereotypically more emotional and men are stereotypically angrier.[157][163] When lacking substantial emotion information they can base judgments on, people tend to rely more on gender stereotypes. Results from a study conducted by Robinson and colleagues implied that gender stereotypes are more influential when judging others' emotions in a hypothetical situation.[164]

There are documented differences in socialization that could contribute to sex differences in emotion and to differences in patterns of brain activity. An American Psychological Association article states that, "boys are generally expected to suppress emotions and to express anger through violence, rather than constructively". A child development researcher at Harvard University argues that boys are taught to shut down their feelings, such as empathy, sympathy and other key components of what is deemed to be pro-social behavior. According to this view, differences in emotionality between the sexes are theoretically only socially-constructed, rather than biological.[165]

Context also determines a man or woman's emotional behavior. Context-based emotion norms, such as feeling rules or display rules, "prescribe emotional experience and expressions in specific situations like a wedding or a funeral," independent of the person's gender. In situations like a wedding or a funeral, the activated emotion norms apply to and constrain every person in the situation. Gender differences are more pronounced when situational demands are very small or non-existent as well as in ambiguous situations. During these situations, gender norms "are the default option that prescribes emotional behavior." (290-1)[157]

Scientists in the field distinguish between emotionality and the expression of emotion: Associate Professor of Psychology Ann Kring said, "It is incorrect to make a blanket statement that women are more emotional than men, it is correct to say that women show their emotions more than men." In two studies by Kring, women were found to be more facially expressive than men when it came to both positive and negative emotions. These researchers concluded that men and women experience the same amount of emotion, but that women are more likely to express their emotions.[166]

Women are known to have anatomically differently shaped tear glands than men as well as having more of the hormone prolactin, which is present in tear glands, as adults. While girls and boys cry at roughly the same amount at age 12, by age 18, women generally cry four times more than men, which could be explained by higher levels of prolactin.[101][167]

In a study where researchers wanted to concentrate on nonverbal expressions by just looking at the eyebrows, lips, and the eyes, participants read certain cue cards that were either negative or positive and recorded the responses. In the results of this experiment it is shown that feminine emotions happen more frequently and have a higher intensity in women than men. In relation to the masculine emotions, such as anger, the results are flipped and the women’s frequency and intensity is lower than the men’s.[168] Studies that measure facial expression by the use of electromyography recordings show that women are more adequately able to manipulate their facial expressions than men. Men, however can inhibit their expressions better than females when cued to do so. In the observer ratings women’s facial expressions are easier to read as opposed to men’s except for the expression of anger.[157]

Larry Cahill argues that neurobiological differences between men and women exist in brain lateralization and emotional processing.[169][170][171] Fine criticizes his conclusions as failing to account for size differences and failing to consider the possibility of environmental influences on brain activity, and in some cases relying on research about rats instead of humans.[24]

Women show a significantly greater activity in the left amygdala when encoding and remembering emotionally arousing pictures (such as mutilated bodies.[172]) Men and women tend to use different neural pathways to encode stimuli into memory. While highly emotional pictures were remembered best by all participants in one study, as compared to emotionally neutral images, women remembered the pictures better than men. This study also found greater activation of the right amygdala in men and the left amygdala in women.[173] On average, women use more of the left cerebral hemisphere when shown emotionally arousing images, while men use more of their right hemisphere. Women also show more consistency between individuals for the areas of the brain activated by emotionally disturbing images.[172] One study of 12 men and 12 women found that more areas in the brains of women were highly activated by emotional imagery, though the differences may have been due to the upbringing of the test participants.[174] When women are asked to think about past events that made them angry, they show activity in the septum in the limbic system; this activity is absent in males. In contrast, men's brains show more activity in the limbic system when asked to identify happy or sad male and female faces. Men and women also differ in their ability to recognize sad female faces: in one study, men recognized 70%, while women recognized 90%.[175] Responses to pain also reveal sex differences. In women, the limbic system, which is involved in the processing of emotions, shows greater activity in response to pain. In men, cognitive areas of the brain, which are involved in analytical processing, show higher activity in response to pain.[176] This indicates a connection between pain-responsive brain regions and emotional regions in women.

Avoidance or approach to moderately negative stimuli[edit]

Men and women differ on average how they respond to moderately negative stimuli which may have evolutionary causes as well as implications regarding (negative) news consumption and knowledge of public affairs.

Happiness[edit]

A 2003 worldwide survey by the Pew Research Center found that overall women stated that they were somewhat happier than men with their lives. Compared to the previous report five years earlier women more often reported progress with their lives while men were more optimistic about the future. Women were more concerned about home and family issues than men who were more concerned about issues outside the home. Men were happier than women regarding the family life and more optimistic regarding the children's future.[177]

Mental health[edit]

Childhood conduct disorder and adult antisocial personality disorder as well as substance use disorders are more common in men. Many mood disorders, anxiety disorders, and eating disorders are more common in women. One explanation is that men externalize stress while women internalize it. Gender differences vary to some degree for different cultures.[178] Women are more likely than men to show unipolar depression. One 1987 study found little empirical support for several proposed explanations, including biological ones, and argued that when depressed women tend to ruminate which may lower the mood further while men tend to distract themselves with activities. This may develop from men and women being raised differently.[179]

Men and women do not differ on their overall rates of psychopathology, however, certain disorders are more prevalent in women, and vice versa. Women have higher rates of anxiety and depression (internalizing disorders) and men have higher rates of substance abuse and antisocial disorders (externalizing disorders). It is believed that divisions of power and the responsibilities set upon each sex are critical to this predisposition. Namely, women earn less money than men do, they tend to have jobs with less power and autonomy, and women are more responsive to problems of people in their social networks. These three differences can contribute to women's predisposition to anxiety and depression. It is believed that socializing practices that encourage high self-regard and mastery would benefit the mental health of both men and women.[180]

One study interviewed 18,572 respondents, aged 18 and over, about 15 phobic symptoms. These symptoms would yield diagnoses based on criteria for agoraphobia, social phobia, and simple phobia. Women had significantly higher prevalence rates of agoraphobia and simple phobia, however there were no differences found between men and women in social phobia. The most common phobias for both men and women involved spiders, bugs, mice, snakes, and heights. The biggest differences between men and women in these disorders were found on the agoraphobic symptoms of “going out of the house alone” and “being alone”, and on two simple phobic symptoms, involving the fear of “any harmless or dangerous animal” and “storms,” with relatively more women having both phobias. There were no differences in the age of onset, reporting a fear on the phobic level, telling a doctor about symptoms, or the recall of past symptoms.[181]

One study interviewed 2,181 people in Detroit, aged 18–45, seeking to explain gender differences in exposure to traumatic events and in the development or emergence of post traumatic stress disorder following this exposure. It was found that lifetime prevalence of traumatic events was a little higher in men than in women. However, following exposure to a traumatic event, the risk for PTSD was two times higher in women. It is believed this difference is due to the greater risk women have of developing PTSD after a traumatic event that involved assaultive violence. In fact, the probability of a woman developing PTSD following assaultive violence was 36% compared to 6% of men. The duration of PTSD is longer in women, as well.[182]

Men and women are both equally likely at developing symptoms of schizophrenia, but the onset occurs earlier for men. It has been suggested that sexually dimorphic brain anatomy, the differential effects of estrogens and androgens, and the heavy exposure of male adolescents to alcohol and other toxic substances can lead to this earlier onset in men. It is believed that estrogens have a protective effect against the symptoms of schizophrenia. Although, it has been shown that other factors can contribute to the delayed onset and symptoms in women, estrogens have a large effect, as can be seen during a pregnancy. In pregnancy, estrogen levels are rising in women, so women who have had recurrent acute episodes of schizophrenia did not usually break down. However, after pregnancy, when estrogen levels have dropped, women tend to suffer from postpartum psychoses. Also, psychotic symptoms are exacerbated when during the menstrual cycle, estrogen levels are at their lowest. In addition, estrogen treatment has yielded beneficial effects in patients with schizophrenia.[183]

Pathological gambling has been known to have a higher prevalence rate, 2:1, in men to women. One study chose to identify gender-related differences by examining male and female gamblers, who were using a gambling helpline. There was 562 calls placed, and of this amount, 62.1% were men, and 37.9% were women. Male gamblers were more likely to report problems with strategic forms of gambling (blackjack or poker), and female gamblers were more likely to report problems with nonstrategic forms, such as slots or bingo. Male gamblers were also more likely to report a longer duration of gambling than women. Female gamblers were more likely to report receiving mental health treatment that was not related to gambling. Male gamblers were more likely to report a drug problem or being arrested on account of gambling. There were high rates of debt and psychiatric symptoms related to gambling observed in both groups of men and women.[184]

There are also differences regarding gender and suicide. Males in Western societies are much more likely to die from suicide despite females having more suicide attempts.

The "extreme male brain theory" views autism as an extreme version of male-female differences regarding "systemizing" and empathizing abilities.[185] The "imprinted brain theory" argues that autism and psychosis are contrasting disorders on a number of different variables and that this is caused by an unbalanced genomic imprinting favoring paternal genes (autism) or maternal genes (psychosis).[186][187]

Differential Socialization[edit]

Division of labor between the sexes[edit]

One study argues that a division of labor between sexes developed relatively late, 45,000-10,000 years ago. It may have given humans an important advantage over Neanderthals who likely did not have a similar division of labor and who had similarly robust skeletons for both sexes. The paper also argues that gender roles varied across early human cultures and that the division of labor is not only due to innate differences between sexes but that much of it was learned.[188]

Controversies[edit]

In January 2005, Lawrence Summers, president of Harvard University, unintentionally provoked a public controversy when several attendees discussed with reporters some statements he made during his lunchtime presentation at an economics conference at the National Bureau of Economic Research.[189][190][191] In analyzing the disproportionate numbers of men over women in high-end science and engineering jobs he suggested that part of discrepancy may be due in part to the conflict between employers' demands for high time commitments and women's disproportionate role in the raising of children. He also suggested that well documented greater variability among men (in comparison to women) on tests of cognitive abilities [80][81][192] may be due to intrinsic factors,[189] adding that he "would like nothing better than to be proved wrong." The controversy generated a great deal of media attention; it contributed to the resignation of Summers the following year,[193] and led Harvard to commit $50 million to the recruitment and hiring of women faculty.[194] Stimulated by this controversy, in May 2005, Harvard University psychology professors Steven Pinker and Elizabeth Spelke debated "The Science of Gender and Science".[195]

In 2006, Danish psychologist and intelligence researcher Helmuth Nyborg was temporarily suspended from his position at Aarhus University, after being accused of scientific misconduct in relation to the documentation of a peer-reviewed paper appearing in the journal Personality and Individual Differences, in which he showed a 3.15-point IQ advantage of men over women.[73] This led to a review of his work by an investigative committee. Nyborg was defended — and the university criticized — by other researchers in the intelligence field.[101][196][197]

In July 2012, IQ researcher Jim Flynn was widely misquoted in the media as claiming that women had surpassed men on IQ tests for the first time in a century.[198] In a 2012 lecture, Flynn responded by denouncing the media reports as distortions, and made it clear that his data instead showed a rough parity between the sexes in a few countries on the Raven's Matrices for boys and girls between the ages of 14 and 18. Women, he argued, had previously scored lower than men on the Raven's tests, but reached equality with men in these nations as a result of exposure to modernity by entering the professions and being allowed greater educational access. Flynn stated that the minute variations that did appear were statistically negligible and were not attributable to differences in cognitive ability.[77][198]

See also[edit]

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