"Nothing in biology makes sense except in the light of evolution." --Theodosius Dobzhansky

Introduction

@lukestokes has encouraged me to offer evidence supporting my claims about the biological origins of many of the observed differences between men and women, and to clarify where I fall on the nature versus nurture spectrum.  I hope to do that here.  

Let me begin by saying that my purpose in writing on this subject (both past and present) is definitely to persuade and not just to educate, so readers should consider my essays to be examples of "persuasive writing".  I've studied the biological differences between men and women extensively (as part of my research into the subject of free will), and I've developed some firm conclusions.  My conclusions are mainly consistent with mainstream "hard" science but they differ from some of the mainstream social sciences (which are influenced as much by politics as research).  

Folks who disagree with what I have to say are invited to voice their disagreements by commenting here or creating their own top level post, and to support their contrary conclusions however they best see fit.  Through the process of "hashing it out"--with me advocating one position and detractors another--readers will hopefully be exposed to both sides of the argument and can decide for themselves which one is better supported.  

Nature v. Nurture

The crux of the issue is the extent to which nature and nurture influence human outcomes in general and gender differentiated outcomes in particular.   There are essentially three possible positions to consider on this subject:  1) outcomes are only a result of nature, 2) outcomes are only a result of nurture, or 3) outcomes are a combination of the two.  With regard to the latter, nature could account for as little as 1 percent of the outcome or as much as 99 percent with nurture making up the difference.  

My position, and the one that I believe to be best supported by the science, is that nature plays a far more significant role than most laypersons imagine in determining the opportunity set of potential outcomes.  From there, nurture significantly influences which particular opportunity from that set is realized.   

An Easy Example

When it comes to physical attributes like height, nobody seriously doubts my contention that nature sets the bounds of possibilities  while nurture influences which possibility is realized.  Almost everyone recognizes the obvious genetic contribution to height:  Someone without a genetic predisposition to be taller than average cannot generally be made to grow to great heights via simple manipulation of their environment.  One's maximum theoretical height is largely a function of his genes, as is the "opportunity set" of potential heights based on varying environmental factors.   Furthermore, most everyone's genetic opportunity set is slightly, or in some cases markedly, different.  

(Is the height difference between Cindy and me a consequence of nature or nurture?)

For instance, consider the case of North Koreans who are several inches shorter on average than their South Korean kin despite sharing mostly the same gene pool.  Far fewer North Koreans than South Koreans approach their maximum genetic potential height due to environmental factors, namely malnutrition during their growth years, even though the genetic opportunity set for both populations is essentially identical.  So clearly environment influences height, but only to a point:  Not even well-fed South Koreans approach the heights of well-fed folks from the Netherlands (where men average six feet).  Why?  Genes.

 

It's my contention that what we know about height and many other physical attributes extends more or less to most other measurable attributes, even psychological ones--that is, genes almost exclusively determine the maximum theoretical potential outcome and the environment simply influences how much of the given potential is achieved in each instance.  

Bell Curves Everywhere

In fact, regardless of the human attribute we measure, the measured outcomes almost always fall on a normal bell curve distribution (given a sample set of sufficient size).  Professor Steven Pinker discusses this tendency (and many other things) here:

For instance, the mean height for males and females varies only by a few inches.  However, as with nearly other measurable human attribute, the variance among men is significantly greater than the variance among women. Take a sample of sufficient size and both tallest and shortest person measured is very likely to be male.  Also, take a sample of sufficient size and both the smartest and dumbest person measured (as determined by IQ) is likely to also be male.  Quite simply, regardless of the attribute measured, the male bell curve is almost always a little "flatter" and a little wider than the female one.  (Nobody is really sure why this is, though I have a theory that I'll save for another day.) 

Understanding Bell Curves is Critical 

Note that when comparing any two bell curves, a modest difference in the mean of each produces significant differences at the tails (the far right or left end of the curve), especially when one curve is flatter and wider than the other.  Pinker explains this quite well in the video above.  

For instance, while the difference in average height between men and women isn't all that great (meaning there are nearly as many females as males at the mean male height, and vice versa), this doesn't hold true as we move away from the average toward the tails of the curve.  The slight "right shift" in average male height, combined with a greater variance (that is, a flatter an longer curve), means that we observe only one female at 5'10" for every thirty-five males.  At 6'0" there is only one female for every 2000 males!  The further we go out on the tail the more disparate the outcomes between men and women.  

We observe this same phenomenon if we plot mental attributes like IQ scores (though perhaps to a lesser degree)--men significantly overrepresented among both the most intelligent and the least intelligent--or most any other attribute. 

In short, the measured differences in the means and variances between men and women in the physical domain, such as height and even brain size, suggests strongly that, on average, the maximum (and often the minimum) genetic potential for men and women vary measurably.  Again, there is virtually no debate on this subject when it comes to physical qualities like height, speed, strength, dexterity, etc.  We are perfectly willing to accept that men are, on average, genetically taller, stronger and faster than women while women are, on average, genetically more dexterous (to name just one).   

The Significance of the Bell Curve and the Burden of Proof

Importantly, we observe very similar differences in bell curve distributions between men and women, and similar variances in outcomes, when we move from plotting physical attributes to plotting some mental or psychological ones.   As noted by Steven Pinker in the above video, there are at least six mental/psychological areas where the measured means and variances between men and women diverge appreciably.  

Despite the fact that the shapes of these psychological/mental bell curves are in many cases essentially indistinguishable from those observed when measuring physical qualities, humans feel compelled to rationalize away the possibility, indeed the likelihood, that differences in maximum genetic potential between males and females explain psychological/mental observations equally as well as physical ones.  Instead, there's a tendency to insist that the environment plays a larger role in the psychological domain than the physical, despite much evidence to the contrary.   The reasons for this are more political and emotional than logical or scientific and are to some degree anchored in our "persistent illusion" of free will.   

Before I move on to examine the specific overwhelming evidence suggesting that differences in maximum genetic potential explain psychological differences between men and women every bit as well as physical ones, let me first say this:  Given how unequivocal the evidence in the physical realm is, given how similar the bell curves in the physical and psychological realms are, and given the unescapable "logic" of bell curve distributions, the burden of proof should really be on those who contend that psychological observations require different explanations than physical ones--that is, on those who insist that genetic influence over outcomes mysteriously "stops at the neck".   

Those who insist that evolution stops at the neck offer no testable explanation for how or why this is so.  They remind me of Intelligent Design (ID) advocates who insist that "microevolution" is possible but "macroevolution" (or speciation) is not.  These ID advocates hypothesize some mysterious unobserved stop on evolution's power that prevents it from from going "too far", from creating new species, despite the fact that no testable explanations for how such a stop might work, or why it might exist, are offered. 

Truth is, nothing in biology makes sense except in the light of evolution, and nothing in psychology does either. The idea that psychology is rooted in anything other than biology has no scientific support.   Virtually nobody in the "hard sciences" believes it, and increasingly fewer in the "social sciences" do either.  

By contrast, evolutionary psychology offers simple and elegant answers to questions that have plagued social scientists for decades, and it makes testable predictions that validate those answers.   Here's an example of just one testable, and tested, prediction from evolutionary psychology (note that the words "fitness variation" and "fitness ceiling" below refer to the theoretical maximum potential number of offspring of each gender):

One of the strongest pieces of evidence that anisogamy, the internal gestation, and the consequent greater fitness variance among men than among women lead to the sex differences in behavior comes from the proverbial “exception that proves the rule.” While males have a greater fitness ceiling than females in most species, there are a few exceptional species for which this is not true. Among some fish, frog, and bird species, the males carry the fertilized eggs during gestation, and as a result, the females have a higher fitness ceiling than males do. Females of these species can continue reproducing while the males are “pregnant” with the young. As predicted by evolutionary biology, among these species, females are more aggressive, competitive, and violent than males. Among these species, the females compete fiercely with each other for sexual access to the coy males. These exceptions therefore prove the rule that it is the fitness variance that determines which sex is more competitive and aggressive. (Why Beautiful People Have More Daughters)

For a sampling of the many hundreds of other successful predictions made by evolutionary psychology, please read Why Beautiful People Have More Daughters.  

Despite that the burden of proof should logically rely on those who, irrationally and without evidence, insist that evolution stopped at the neck, I will now proceed with offering much evidence supporting my contentions to the contrary.  

The Role of Genes in Our Psyches

Before I can convince you that the observed differences in male and female psyches is largely a function of differences in genetic maximum potential rather than culture or socialization, I must first convince you that psychological attributes are themselves significantly genetically influenced (regardless of gender)--that is, that evolution most definitely does not "stop at the neck".  Thankfully, evidence on this point is abundant.   Some of it is summarized in the video link above to Steven Pinker's talk.   

For now let's focus on a single well-documented mental attribute:  Intelligence.   This excellent talk, also by Steven Pinker, summarizes the compelling evidence that genes govern each individual's maximum theoretical potential intelligence: 

The evidence offered in the video supports the contention that environmental factors simply influence how much of one's intelligence potential is realized but that the bounds of that potential vary among individuals as consequence of genes.

It's Beyond Our Control

So far we've established that any individual's maximum ability to express a given attribute, whether physical or mental/psychological, is regulated in large part by genes.  Before I can convince you that the differences in outcomes observed between groups of men and groups of women is strongly influenced by genes, I must first establish that our personal preferences in general, which translate into ultimately into societal preferences or "standards", are largely beyond our control (regardless of gender).  Let's do that by considering something near and dear to all of us--sexual arousal and disgust. 

Scientists have shown conclusively that what arouses us most, or disgusts us most, is not a conscious decision and cannot be controlled by willpower. Common human paraphilias (odd sexual fixations) include feet, the stumps from amputated limbs, horses, corpses, children, "little people", teeth, shoes...etc.  The list is literally endless.

(Anyone have a paraphilia for...feet?)

(Or maybe a paraphilia for ladies' shoes?)

The list of potential paraphilias is so long, in fact, that nearly everyone, or at least every male, has some sort of unusual "kink".  And yet, when interviewed, kinksters are often unable to explain these sexual preoccupations in any rational way.  Most indicate that they've had them as long as they can remember.  Others indicate that it simply came to them out of the blue and can't be banished even with great effort.  In any event, none remember ever being taught or socialized to be pedophiles (to pick an extreme example).  

This inability to banish our paraphilias is important evidence that they are an emergent property of genes rather than socialization.   In other words, if these sexual fixations can't be socialized out of us, then they almost certainly were never socialized into us.

And it's not just our physical arousal patterns that are beyond our control, our mental reactions to them are also.  For instance, when aroused, our figurative "gag reflex" lowers uncontrollably, making things we previously perceived to be "disgusting" suddenly quite appealing:

Indeed, more recent scholars have found that our willingness (and sometimes our eagerness) to let others’ body products make contact with our lips and tongues or even to slip down our throats entirely is a product of our fluctuating arousal levels. When we’re horny, we’re happy to dip into someone else’s organic buffet. Really. There are even data on it.  (Perv:  The Sexual Deviant in All of Us)

(Saliva is erotic or disgusting depending upon one's state of arousal)

This near total inability to control either our experiences of arousal or disgust suggests strongly that decisions made under the influence of these urges are similarly not a matter of free will:  

Lust is a sort of mental fog in which men temporarily lose, or at least lower, their standards. And by “standards,” I mean in the sense of whom (or what) a highly aroused [person] is willing to have sex with as well as the moral values that he’ll place into temporary quarantine just to satisfy his pressing urges. (Perv:  The Sexual Deviant in All of Us)

In short, we have little to no control over what arouses us.  And in an aroused state, things that usually disgust us actually entice us, causing us to act in ways we otherwise would not.   Not only is our potential for arousal or disgust hardwired rather than conditioned--for instance, men don't "learn" to get an erection when viewing an attractive naked woman, they just do--but our personal urges and "put offs", and the decisions we make as a result of them, are in large part dictated by these uncontrolled states of arousal or disgust.   

The Genetic Origins of Arousal

Most all scientists today recognize that sexuality is innate.  Not only can't it be socialized out of us, most consider it both immoral and unethical to even try.  

If sexuality can't be socialized out of us, then what evidence do we have that it was every socialized into us?  Not much.  The social sciences struggle to explain the societal origins of sexual preferences (most having actually conceded them to be innate), of aversions, of personal preferences, etc.  

By contrast we have considerable evidence that this lack of control over what arouses and disgusts us has genetic origins.  For instance males are nearly 100 times more likely to have paraphilias than females, with most of paraphilias beginning well before puberty, usually between the ages of five and nine.  Yep, for every female kinster, there are 100 male ones who developed their odd fixations in some cases long before they even knew what sex was!  

Evolutionary psychologists recognizes that this kinster differential is in large part a function of the fact that male sexual interests (in all cultures at all times, and even in many animals) is innately much more focused than female sexual interests.  This male/female disparity in the variety of their sexual interests is observed even across muiltiple species of animals:

Whether it involves livestock or people, this sexual-imprinting process (in which a highly circumscribed set of erotic targets is stamped early into the individual’s brain) appears to be a decidedly male characteristic. By contrast, the female sheep and goats that were able to “go both ways” after their intensive cross-rearing experiences, equally aroused by both their own biological kind and members of their adoptive species, were exhibiting “erotic plasticity” (in which one can be sexually excited by a wide range of stimuli). Interestingly enough, erotic plasticity is also strikingly more apparent in human females than it is in human males. Another way to say this is that a girl’s developing sexuality is more fluid or labile (and for once that’s not a pun) than a boy’s; it’s less prone to getting locked onto a specific category of erotic target during childhood.  (Perv:  The Sexual Deviant in All of Us)

It's this male-specific "imprinting" at an early age that contributes to paraphilias.  While the specific object that gets imprinted into the male sexual psyche is obviously a function of each individual's environment combined with some element of random chance, the tendancy of males to sexually imprint at rates 100 times higher than females has obvious genetic origins (there's no know social explanation for this phenomenon).  Males clearly have a higher maximum genetic potential for sexual imprinting.  In fact, all indications are that such imprinting is related to the male sex hormone, testosterone. 

Just as males are genetically predisposed to sexually imprint/fixate, females are genetically predisposed to to the opposite--sexual flexibility.  The superior sexual plasticity of the female (the ability to be aroused by a wider variety of things) has been studied and documented time and again:

Work by the psychologist Meredith Chivers also illustrates a greater female erotic plasticity. In several studies, Chivers has found that both straight women and lesbians exhibit vaginal vasocongestion (or increased blood flow to the genitals, a response specific to female sexual arousal) to a surprising assortment of sexual stimuli. For example, a woman’s genitals will respond this way not only to her preferred gender (which is to say, men for self-identified straight women and women for self-identified lesbians) but also to naked pictures of her nonpreferred gender. They’ll even become demonstrably aroused at this physiological level to video footage of other species having sex, notably graphic scenes of bonobo intercourse. That last finding has been replicated, so it wasn’t just some quirky, happenstance overrepresentation of female zoophiles in the study. Chivers clarifies that women aren’t always consciously aware of their arousal to such stimuli. At least they report not feeling as turned on as the objective state of their genitalia would otherwise suggest. (Perv:  The Sexual Deviant in All of Us)

By contrast, in similar tests, men who identify as heterosexual generally do not become physically sexually aroused (as measured by blood flow to their penis) when viewing naked men or scenes of homosexual male intercourse.   Nor do generally they become aroused from watching monkeys or horses screw.  Are we to believe that women were conditioned by culture or society to become sexually aroused by watching monkeys screw but men weren't?  This "nurture" explanation seems highly unlikely, and the social sciences have no testable explanation for it.  

On the other hand, evolution offers a simple and compelling one:

The evolutionary interpretation of these sex differences in arousal—and I’m aware of no other explanation that has been proposed—is that female genital hyper-responsiveness was biologically adaptive in the ancestral past. Back on the savannah tens of thousands of years ago, even the most unappealing sex cues would have often been followed by actual intercourse, so whether the woman wanted intercourse (it was consensual) or not (basically, rape), the capacity to become so easily physiologically aroused offered a sort of insurance policy against possible damage. Specifically, Chivers’s “preparation hypothesis” posits that a woman’s ready-for-anything genital arousal reduced physical injury to her reproductive organs by vaginal lubrication. “The costs of non-responding [genitally] to sexual cues, including nonpreferred cues,” clarifies the psychologist Samantha Dawson, “would be expected to be much higher for women (e.g., tears and ecchymosis leading to infertility) than for men (e.g., the loss of a single reproductive opportunity).” In further support of this hypothesis, researchers have also found genital arousal to depictions of violent sexual coercion in women who consciously find the thought of rape revolting and terrifying, hardly erotic and arousing.  (Perv:  The Sexual Deviant in All of Us)

So, once again, we have compelling evidence that these arousal patterns are well beyond conscious control and are not a function of culture or socialization (does anyone really think that women have been "conditioned" to become physically aroused by scenes of rape?"). Furthermore, the fact that the arousal patterns of men and women differ so remarkably, even at young ages, is compelling evidence these differences have a biological, genetic origin.  Like everything else in biology, these observations simply make no sense except in the light of evolution. 

It's Not Just Arousal or Disgust, It's Emotions Like Jealousy Too

Both men and women experience bouts of jealousy.  There's nothing remarkable about that.  But what is remarkable is that the reasons men and women become jealous vary appreciably:  

While men and women present the same frequency and intensity of their jealousy in romantic relationships, there are clear sex differences in what triggers jealousy. The evidence from surveys and from physiological studies conducted in different cultures indicates that men become jealous of their mates’ sexual infidelity with other men, underlying their reproductive concern for cuckoldry. In contrast, women become jealous of their mates’ emotional involvement with other women, because emotional involvement often leads to diversion of their mates’ resources from them and their children to their romantic rivals. While recent critics of evolutionary psychology have questioned these conclusions mostly on methodological grounds, both strong evolutionary logic and a preponderance of empirical evidence support the clear sex differences in romantic jealousy described above.  (Why Beautiful People Have More Daughters)

Note that evolutionary psychology predicts that men and women should become sexually jealous for very different underlying biological reasons, and therefore from different causes, and this prediction is, like so many others made by evolutionary psychology, validated by observation.  

It Starts Early

Further countering the idea that these differences between men and women are socialized, research shows conclusively that they exist even at the youngest of ages.  In one study, day old babies were given the opportunity to observe either a human face or a mechanical mobile.  Researchers (who themselves didn't even know which babies were male and which were female) videoed the children, noting how long each baby stared at the mobile versus the face.  When they later tallied the results, boys demonstrated a noted preference for viewing the mechanical mobiles and gazed upon them far longer.  Girls by far preferred the face.

It's been long established and documented that adult human males demonstrate a stronger preference for "things" over "people", and females the opposite, and this tendency was even noted in Pinker's first video above.  That these stereotypical sex differences are observed in one day old babies is compelling evidence that the stereotype isn't socialized.  

Monkey's Do It Too

If the evolutionary explanation for these sex differences is indeed correct, we should expect (predict) to see similar sex differences in other species, especially those sharing large amounts of DNA with humans.  In fact we do.  In another famous study, 44 male and 44 female monkeys were given the opportunity to play with four toys--two stereotypically male (a ball and a police car), two stereotypically female (a soft doll and a cooking pot), and two stereotypically neutral (a picture book and a stuffed dog).  Male monkeys showed a definite preference for the "masculine" toys while females definitely preferred the "feminine" ones.  Tellingly, there was no observed difference in male-female preference for the neutral toys. 

Similarly, as noted above, female sheep, like female humans, demonstrate sexual plasticity.  Male sheep (like male humans) do not.   

Conclusion (tl;dr)

1.  Distinctions between biology and psychology are artificial and contrived.  Psychology is rooted in biology.  And nothing in biology, including psychology, makes any sense except in the light of evolution. 

2. Nature (genes) determine an individual's maximum theoretical potential while nurture influences how much of that potential is realized.  

3. In any population of sufficient size, the outcome of this interplay between nature and nurture generally follows a normal distribution and can be plotted on bell curves.  

4. When comparing two bell curves (for instance one for males and another for females), slight differences in the mean and variance result in enormous disparities at the tails. For instance, the mean height of men and women isn't great, but for every 6 foot tall women there are 2000 six foot tall men.  

5. There is no evidence that the effects of evolution (as measured by these bell curves) "stop at the neck".  In light of what we know about evolution, the burden of proof should be on those who insist it does and not the other way around.

6. Mental/psychological attributes are beyond our control more or less to the same extent as our physical attributes are.  

7. That mental/physical states (like arousal, disgust or jealousy) are beyond our control and can't be socialized away is strong evidence that they were not socialized to begin with.  

8. Research shows that some differences in male/female attributes and preferences are apparent as early as one day old, suggesting that they are not a function of culture or socialization.  

9. Many of these male/female differences/preferences are also observed in primates or other animals, further confirming that they cannot be a function of culture or socialization.  

10. The vast majority of the evidence supports the contention that genes are far more influential in influencing outcomes and preferences, and even decisions, than laypersons commonly understand.  While everyone accepts the influence of genes in the physical realm, they resist drawing similar conclusions in the mental/psychological realm. This resistance is likely due to our "persistent illusion" of free will, our politics, and our ego's reluctance to be disillusioned.  

All photos by me.  Photo subject:  @steemed-open