Below are some facts concerning some gender differences between male and female humans and/or between males and females of related species.  It is important to keep at least the following caveats and cautions in mind when considering them (see also the final section):

(1) It is not possible to infer that because some species related to humans has a trait, that we will also have the trait. Ultimately, it needs to be shown directly that humans have the trait.  Humans are obviously different from lower species, most notably in our linguistic, cognitve, emotional and cultural abilities, all of which are reflected in our brains and minds.

Despite this restriction, evidence that a related species has a trait does provide us with some reason to believe that that trait, or an analogous trait, will be found in humans.  This provides the basis for much scientific and medical research.

(2) The fact that we have a trait does not mean that that trait was selected for, i.e., that we possess it becuase it was biologically adaptive.  Some traits are side effects of other traits, which were selected for.  it is easy to tell a story about how some perceived trait was adaptive to our ancestors.  These stories often reflect more about preconceptions than about the facts.

(3) Many (if not most) of the psychological and physiological gender differences discussed below are relatively small, as compared to the gender differences in other species, for example.  Furthermore, intragroup differences are greater than intergroup differences; e.g., the variation among men is greater than the variation between men and women. (Both of these facts rule out any justification of gender-based exclusionary policies on the basis of these differences; e.g., preventing women from being soldiers or engineers, or men from being nurses.) Nevertheless, the differences are reliable, and in many (if not most) cases show up across cultures, ages, and other variables.

(4) The gender differences emerge against a background of massive similarity in form and function between the genders; i.e., men and women seem to be basically similar overall.

(5) Experience and culture undoubtedly shape the way in which biological factors manifest themselves, and can lead directly to changes in brain structure or function, so a discovery that some feature of the brain is correlated with a behavior is not by itself an argument that that the feature is “innate” or “biologically determined” independently of experience.

(6) We have just begun to understand the brain and gender differences therein.  Hence, these “facts” are open to future revision, reinterpretation, or criticism on the basis of present or future evidence.  They do not represent an established “consensus” about the origins, nature, or significance of gender differences, although the experimental facts are widely confirmed.  Indeed, some claim that there are no differences between the genders that are not socially constructed.
 

Gender Differences

1. Cognition

A. Spatial abilties

M--mental rotation; navigation using geometrical coordinates; perceptual disembedding (recognizing a perceptual object “embedded” in a visual background)

F--memory for object location; navigation using landmarks
 

B. Motor skills

M--accuracy in aiming projectiles (from 3-5yrs. onward)

F--fine motor skills; small amplitude coordination; speech rate
 

C. Math Skills

M--problem solving (including word problems) involving abstract reasoning; aptitude tests

F--computational/calculational problem solving
 

D. Verbal Abilities

F--earlier maturation of nearly every dimension of verbal ability; verbal memory; fluency; spelling; grammar
 
 

2. Emotion

Male and female sexuality are controlled by different areas of the brain and different, but overlapping chemical systems, and much of our social bonding is connected (evolutionarily and neurally) to sexual processes
 

M--Angio-vasopressin (AVP) (bonding, intermale aggression, mating persistence, territoriality, jealousy)

F--Oxytocin (sexual arousal/receptivity and satiety, bonding, nurturant behaviors, social memories)
 (Oxytocin cells and their receptors have some remarkable dynamic properties in the female brain: e.g., hormonal changes prior to sex or nursing result in changes in the number, range, and form of contact  between oxytocin cells (a shift from chemical to electrical synapses))

males and females release oxytocin and opioids during sexual acitivty which reduces aggression and facilitates social bonding

There is no society in which men are the primary caregivers
 
 
 

Males are generally more aggressive in most mammals (a notable exception: the female hyena, which also has high T-levels and M-type external genitalia: see below)

In most mammals, male sexuality requires assertion and/or aggression at some point in the mating process

Play behavior correlates with hormonal levels:
male: rough-and-tumble
female: infant-carrying behavior
 

Different forms of aggression (fear-induced, maternal, irritable, sex-related, terretorial, intermale, predatory)

At least some forms of aggression appear to be controlled by different neural structures or pathways

BNST mediates “affective attack” (testosterone sensitizes, and estrogen desensitizes this region)

Neural circuits for M-type sexual behavior overlap those for affective aggression and have been shown to be coactivated in some species

AVP stimulation increases aggressive and persistence behaviors, while drugs that block AVP diminish it

Males have more AVP circuits than females, and they are more closely tied to the regions that govern male sexuality than those that govern female sexuality

Estrogen, progesterone, and oxytocin reduce aggressiveness
 

3. Perception

Females are generally more sensitive in all sensory modalities except for some aspects of vision (i.e., they can detect lower levels of stimulation)
 
 

Developmental Mechanisms of Gender Differences

Y chromosome determines the formation of the testes and their activity determines male development

It is widely thought that the “default” course of development for any fetus (XX or XY) is female, and that it is only the action (or lack thereof) of the testes which determines sex.  There is, however, some evidence that the ovaries make a positive contribution to female development, though less seems to be known about this.

Testosterone is produced by the testes and converted to DHT by means of 5-alpha-reductase (5-a-r), or to estrogen by means of aromatase.
 

Two processes in male development:

(1) masculinization: the organization of M-typical traits and behaviors; relies on the conversion of testosterone to DHT and DHT action on androgen receptors

(2) defeminization: suppression of “default” F-typical traits and behaviors; relies on the aromatization of testosterone into estrogen and estrogen action on its receptors
Note: male development depends on estrogen

(females possess alpha-feroprotein in their bloodstream, a substance which absorbs estrogen during development, preventing its entry into the brain and its masculinizing effects there)

Masculinizing effects of sex hormones appear to operate on all behaviors and traits that are sexually dimporphic (differ between the sexes), although different regions of the body and brain mediate different capabilities and have different sensitive or “crucial” periods for development

The hormones that trigger the masculinization of the male brain (testosterone aromatized to estrogen) are distinct from those that trigger the organization of the male body (testosterone converted to DHT by 5-a-r)

I.e., the formation of the external genitals depends on dihydrotestosterone (DHT), while masculinization of the brain depends on estrogen.

higher T-levels in males: 8-24 wks.; birth -5 mo.; puberty
 
 

Maternal stress can desynchronize or prevent masculinization:

Stress causes the prenatal rush of testosterone to occur too early

Testosterone received too early (before the critical period) does not have its normal effect on the body

Stress impairs aromatase activity which prevents the conversion of testosterone to estrogen and the subsequent effects of testosterone on the brain

Effect seems to be due to an excess of opioids (brain opiates like beta-endorphin), which are released during stress, since the effect can be also obtained by external administration of opiates to pregnant rats

Mother rats who have been stressed have a high number of homosexual offspring

Homo-bisexual male rats exhibit more maternal behavior

Both male and female virgin rats can be induced to perform nurturant behavior by exposure to rat pups and/or hormonal priming. Mature virgin females adopt the behavior more quickly than juvenille females, and juvenille males exhibit the behavior more readily than mature males.  An opposite pattern is seen in pups of stressed mothers
 
 

Androgen insensitivity syndrome: XY individuals who lack androgen sensitivity

appear female
testes present in abdominal cavity
F-type psychological profile
 

Guevodoces: XY individuals with a genetic deficit in 5-a-r (predominantly in the Dominican Republic)

appear female during childhood
testes present in abdomen
testosterone produced in childhood, but converted to estrogen
raised as girls
testes descend at puberty
undergo a psychological and social shift toward an M-type pattern
normal, well adapted, most are married (though infertile)
 

Absence of fetal estrogen with sufficient DHT leads to F-type brain in M-type body (converse of guevodoces)
 

Congenital adrenal hyperplasia (CAH): individuals (male or female) exposed to an excess of androgens from their adrenal glands, due to a deficiency in an enzyme that converts converts adrenal secretions to cortisol, which increases androgen hormone production.

female: appear normal and are raised as girls, but exhibit certain M-type characteristics
higher rates of homosexuality
tomboyish
engage in more rough-and-tumble play
more likely to play contact sports
M-typical toy preferences
enhanced spatial abilities

Male:
diminished spatial abilities (androgen levels too high)
 
 

Female infants of mothers given injections of diethylstilbestrol (DES) (an estrogenic hormone which prevents miscarriages) show some M-type traits and behaviors (too much estrogen masculinizes the female brain)

Estrogen injection in female rats produces a masculinizing effect and leads to M-type navigational pattern

In general, females with high T-levels have better spatial abilities
 
 

Kleinfelter’s syndrome: XXY males who lack some masculine traits and possess some feminine traits

XYY males are reported to be more aggressive than normal males
 
 

Aphasia: loss of speaking ability

Female speech ability appears to be (a) more bilaterally organized, and to be (b) organized differently within each hempisphere:

(a) aphasia is less likely to happen in women than in men after damage to the left hemisphere of the brain (where lanaguage abilties are “usually” found)

(b) aphasia in women following left-hemisphere damage usually results from damage to more forward (anterior) areas, whereas in men it follows from damage to regions toward the rear (posterior)

(Some spatial and motor abilities may also show differential intra-hemisperic organization as above)
 

“Unwanted” sex-change procedures: Edward Money’s patient (described by Diamond)

normal male infant
penis irreparably destroyed during circumcision (by a drunken doctor)
Money advised the parents to raise him as a girl and the parents did (including hormone therapy and reconstructive surgery)
showed M-type play and toy preferences
thought he was a boy
tried to urinate standing up
rebelled against estrogen therapy during puberty, after which his parents finally told him the truth
underwent reconstructive surgery to refashion a penis
now happily married, living as a man
 
 

Female-to-male transexuals: a gender-specific psychological shift toward an undergo M-type pattern with androgen therapy

Male-to-female transexuals: undergo a gender-specific psychological shift toward an F-type pattern with estrogen therapy
 
 

Body asymmetries

Some think that androgens enhance the development of the right hemisphere: there is some evidence that the gonads exert a direct neural influence on the hypothalamus on the same side of the brain (possibly via the vagus nerve); the hypothalamus appears to have control over autonomic functions on the same side of the brain and body, including the mammary glands.
 

Teste larger on the right in typical male
Left breast is larger in typical female
Hermaphrodites have a teste on the right and an ovary on the left

Fingerprint pattern: large genetic component; established by the fourth fetal month; males are R>, females are L>.

In general:

R> individuals (male or female) show M-type cognitive profile

L> individuals (male or female) show F-type cognitive profile
 
 

Cognitive abilities vary with (a) diurnal, (b) menstrual, and (c) seasonal changes in hormone levels

M--spatial ability is better later in the day and worse in the fall (when T-levels are high)

F--F-type abilities follow the estrogen cycle
 
 

Early castration of rats + administration of female hormones leads to F-type mating behavior and F-type navigational patterns
 

Conclusions and Open Issues

(1)  There appears to be a strong biological influence on the development of gender differences in the body and in the brain, which play a role in determining our cognitive and emotional profile, including our sexual orientation.  (Note!: as the case of maternal stress shows, not all biological factors are genetic, even for traits which are “innate” (literally, present at birth)).

(2) Many of the differences appear to be consonant with the conclusions about gender differences in moral reasoning and behavior offered by Gilligan.

(3)  The idea that there are only two genders appears to be oversimplistic. At the very least, we would seem to need to consider the gender of the body and brain as being to some degree separable, since we have seen that they follow partially independent paths of development.  This would yield a four-way division of individuals--male brain-female body, female brain-male body, and so on--which is recognized in many Native American tribes.  No doubt the categories of “body” and “brain” need to be further subdivided, which would give us a spectrum of genders.  Furthermore, gendered traits can in many cases be possessed as a matter of degree, which further spreads out the space of possible combinations found in any one individual.  Hence the classification of traits as “masculine” or “feminine” is based simply upon the  fact that they tend to correlate with statistically normal males or females.

(4) The claim that sexual orientation has a biological component has implications for our psychological and moral responses to gender preferences, but it does not by itself decide whether such behavior is desirable, moral, or even biologically adaptive.  For example, we could discover that some people have a biologically-based propensity to physical violence, but this would not change our opinion of physical violence.  It does, however, seem to undermine to some degree the claim that homosexual desires are themselves a voluntary moral failing.

(5) Similarly, the claim that men and women are biologically different in ways that are relevant to explaining our ways of thinking, feeling, and the social roles we have historically played, does not mean that those roles are desirable, moral, or adaptive under the present circumstances.  Indeed, many of these roles are based on false beliefs about the nature and scope of the biological differences between the genders; e.g., the view that women are not capable of leading because they are too emotional, or that men cannot be as nurturing as women.  In this sense, I believe, contrary to some, that the investigation and discovery of biological differences between the genders can be used to promote the flourishing and self-understanding of individuals of all genders, rather than “pigeon-holing”, stereotyping, and repressing them.
 

Some Readings

Sex and Cognition
Doreen Kimura

The Sexual Brain
Simon LeVay

Drive
Donald Pfaff

Affective Neuroscience
Jaak Panksepp

The Neurobiology of Affiliation
Sue Carter

Mother Nature: MAternal Instincts and How they Shape the Human Species
Susan Hrdy

“A Role for Ovarian Hormones in Sexual Differentiation of the Brain”
Behavioral and Brain Sciences (1998) 21, 311-352
Holly Fitch and Victor Denenberg
 

For an argument that there are no significant biologically-based psychological differences between the genders, see:

Myths of Gender: Biological Theories about Women and Men
Anne Fausto-Sterling
 

Gender in Native American society:

Changing Ones: Third and Fourth Genders in Native North America
Will Roscoe (ed.)