Male & Female Biological Differences
Compiled and Adapted by Steven Fearing
Evolutionary psychologists argue that males and females faced different pressures in primeval environments and that adaptation during human evolution produced differences between males and females.
The following information is based on a meta-analysis of research from developmental neuroscience, medical genetics, evolutionary biology, and cross-cultural psychology. It was reported in “The Truth About Sex Differences”, Psychology Today (November/December 2017) by David Schmitt, personality and evolutionary psychologist, and Director of the Centre for Culture and Evolution at London’s Brunel University. (Note: differences in brain structure and hormones, especially testosterone, will be explained in a separate document and blog post.)
Overview of Physical and Behavioral Differences between Men and Women
Behavioral and physical differences between men and women are found in the following domains:
- strength and size
- pubertal timing
- hunting vs. gathering
- childrearing
- risk-taking
- mortality
- reproductive requirements
Degree of Difference Matters
Research almost always approaches sex differences as group averages. No matter how big or small a sex difference, there is almost always a significant overlap across distributions of men and women. It is the size of the differences that makes the differences worth talking about.
Psychologists use a “d” statistic to express the size of sex difference; “d” numbers drive the categories below of “small,” “medium,” “medium/large” and “large” differences.
Women’s Differences from Men – aggregate data/meta-analysis:
Large Differences (occurs much more in women than in men)
- amount of body fat
- cooking among foragers
- early onset of puberty
- empathy
- interest in people over things
- preference for female-typical toys
- preference for a taller mate
- the primary caretaker of children among foragers
- sexual disgust
- vulnerability to depression
- lack of interest in casual sex
Medium/Large Differences
- preference for status in a mate
- tender-mindedness
Medium Difference
- tendency to smile
Small Differences
- conformity
- general verbal ability
- indirect aggression (gossip)
- interpersonal trust
- sensitivity to negative emotions
- spatial location ability (locate an object in the visual field)
Men’s Differences from Women – aggregate data/meta-analysis:
Large Differences (occurs much more in men than in women)
- deep pitch voice
- early mortality
- grip strength
- height (in U.S., the average adult woman is as tall as average 14-yr. old boy)
- likelihood of homicide
- preference for rough and tumble play
- throwing ability
- upper-body strength
- vulnerability to psychopathy
- interest in casual sex
- interest in engineering as an occupation
- absence of sexual disgust
Medium/Large Differences
- mental rotation ability
- physical aggression
- preference for a physically attractive mate
Medium Differences
- 3-D geometry ability
- high blood pressure
- risk-taking
- sex drive
- task-orienting leadership (vs. maintaining harmony)
Small/Medium Differences
- impulsivity
- sexual jealousy
Biological Sex, Gender, and Sexual Identity
It is most correct to call the differences between men and women as “sex differences,” not “gender differences.” Biological sex differences include:
- gamete size
- genital morphology
- presence or absence of Y chromosome
- sex hormone levels
Sexual identity refers to whether you identify as a man or a woman. Differences in how self-identified men or women think, feel, and behave are psychological sex differences.
According to the American Psychological Association, “gender” refers to the attitudes, feelings, and behaviors that a given culture associates with biological sex. “Gender identity” is whether a person is typically masculine and/or feminine as defined by their local culture.
Differences in Emotions, Behavior, and Cognition
The meta-analysis of a large number of studies have found several differences in the realm of emotions: Women are somewhat more empathic. Men tend to more powerfully experience emotional jealousy.
In the cognitive realm, men are better able to rotate a dimensional object, whereas women excel at locating an object in a visual field.
Behaviorally, men are more physically aggressive and homicidal with same-sex others, and women tend to choose older and wealthier partners for marriage.
Biological Beginnings
All humans start out as female. The Y chromosome of human males begins to masculinize the body during the first two months and the brain during the first trimester. Male brains in the second trimester are usually altered by exposure to androgens that influence psychological sex differences, predicting the degree and kind of postnatal play preferences (rough and tumble), personality traits (thrill-seeking, and aggression), and cognitive abilities (mental rotation). Psychological sex differences emerge before extensive gender socialization has taken place.
Development and Environment Matters
Men and women’s psychologies are not binary – there is an overlap in the distribution of traits. The existence of biological predispositions does not mean sex differences are fixed and unchangeable after birth; most biological mechanisms that have emerged over time have been designed to be responsive to key elements in the environment.
Humans are developmentally sensitive to features of the family, social structures, and local ecologies. The input from the environment almost always affects the degree of sexual differentiation.
Sex differences emerge long after prenatal experiences – appearing during puberty or other critical periods when genes become sensitive to activation by maturational events, such as sexual debut, parenting, and menopause.
Sex differences in the personality trait of neuroticism, (e.g. sensitivity to emotions and vulnerability to stress) do not reach full adult form until around age fourteen; indicating that puberty affects their development.
So, sex differences are specifically designed by evolution to arise developmentally, and only after particular milestones.
Direct Effects of Genes
Some psychological sex differences result from the direct effect of genes, apart from the sex chromosomes and their related hormones, and function differently in men’s and women’s brains. Serotonin transporter gene, 5-HTTLPR, exists in short and long versions; the short version is associated with higher negative emotionality and is more closely linked to neuroticism-related traits in women than in men. The involvement of genes found on the X chromosome (a mechanism called X-linkage), or involvement of estrogen-induced gene expression, may play a role in the higher prevalence of depression among women.
Psychological sex differences (even if originating from direct genetic effects) do not take precisely the same form, or manifest to the same degree, across all cultures. Human psychology is highly sensitive to developmental and socioecological contexts.
Culture and Mate Selection
Such environmental factors as malaria, infections, number of men to women in the local population, and the degree to which men and women must compete in order to find a mate and reproduce, can magnify or minimize sex differences. But across all cultures, men more than women tend to favor physical attractiveness in potential marriage partners, especially cues to youth and fertility, such as small waist and curvy hips. However, where pathogens are present and disease is a constant danger (e.g. India, Brazil) both men and women tend to emphasize physical attractiveness in a mate, because it is a reliable indicator of good health. In these environments, a woman’s preference for physical attractiveness is amplified, leading to a smaller sex difference in mate preference for attractiveness in societies with a lager pathogen burden.
Egalitarian Cultures and the Gender Equity Paradox
It is expected that sex differences will be smaller in cultures with higher levels of gender-related egalitarianism, as in Scandinavia where socialization and roles are more balanced between men and women and sociopolitical gender equity prevails. But this is not the case!
When scientists measure the big five personality traits (extroversion, agreeableness, consciousness, neuroticism, and openness to experience), dark triad traits (narcissism, Machiavellianism, and psychopathy), self-esteem, subjective well-being, and depression – sex differences are larger in cultures with more egalitarian gender roles. The same is true for cognitive traits. Scandinavian men and women, for instance, show a large psychological variance for preferring physically attractive mates – this is called the gender equity paradox.
“When you treat everyone the same, as in Nordic countries,” says David Schmitt, “it’s only genetic predispositions that produce the most observable individual differences. Extremes of sexual freedom beget larger psychological sex differences.” Israeli psychologists Shalom Schwartz and Tammy Rubel-Lifshitz explain that fewer gendered restrictions in a culture may allow “both sexes to pursue more freely the values they inherently care about.”
Sex and Gender Dials
Think of sex differences for a myriad of traits as dozens of interconnected sex/gender dials with endpoints ranging from extreme male-typicality/masculine to extreme female-typicality/feminine. If you are a man, it is likely you have a deeper voice and a stronger sex drive than most women. But not all women. There is a lot of evolutionary variation of sex and gender dimensions among sexually reproducing species.
In viewing sex differences as dimensional sex/gender dials (not switches), it is clear, says Schmitt, “that there is not one simple sex adaptation that gives rise to male and female psychologies. Rather, there are likely dozens, if not hundreds, of evolved functional mechanisms generating physical sex differences in height, strength, voice, and hirsuteness, and psychological differences in personality, play preferences, mates selection, erotic desires, personal values, and cognitive abilities. Each adaptation turns the respective sex/gender dials of men and women in oblique, context-sensitive ways; each contributes a small part in generating the panoply of the sex differences seen in our species all around the world.”
As science advances the fundamental understanding of sex differences and documents subtle ways biology and culture interact, this progress may come under assault. Consider the uproar over the Google memo and the censorship of Cassie Jay’s film on men’s rights. Cordelia Fine recently repudiated psychological sex difference altogether in her book, Testosterone Rex: Unmasking the Myths of our Gendered Minds. Fine has since been rebuked and criticized in several scientific journals.
Concluding Thoughts
Substantial evidence attests to the existence of many psychological sex differences. But even differences clearly stemming from prenatal hormone exposure (e.g. preference for rough and tumble play) – do not imply genetic determinism; these differences could still be modified by future developmental experiences. Now, and in the future, we may want to modify sex differences, such as autism in males and depression in women. In order to do this, we must first recognize and understand their biological origination
Related video resource: “50 real differences between men and women,” with Julia Tourianski