Few topics in psychology are more controversial than sex differences [1]. Debates can be classified into two main types: (a) The description of sex differences, including both the size and variability of sex differences across a multitude of physical and psychological traits, and (b) The origins and development of sex differences, including the complex interplay between social, cultural, genetic, and biological factors that influence sex differences.

These lines often get blurred. Researchers who emphasize sociocultural factors in their research tend to conceptualize sex differences as small and worry that if we exaggerate the differences, then all hell will break loose in society. On the other side, those who emphasize biological influences tend to emphasize how differences in personality and behavior can be quite large.

I believe that this blurring between the descriptive and the explanatory levels of analysis has stunted the field and distorted public debates over these complex and sensitive issues. In order to make real long-lasting changes that actually have an effect on desired outcomes, our knowledge of the truth needs to be as clear as possible.

In this article I will focus on the personality domain, which has made some truly fascinating advances in only the past few years. I will argue that while the science still has a long way to go to fully flesh out the complex interplay of nature and nurture in creating these differences, it's nevertheless time to take sex differences in personality seriously.

Male and Female Personalities

A large number of well done studies have painted a rather consistent picture of sex differences in personality that are strikingly consistent across cultures (see here, here, and here). It turns out that the most pervasive sex differences are seen at the "narrow" level of personality traits, not the "broad" level (see here for a great example of this basic pattern).

At the broad level, we have traits such as extraversion, neuroticism, and agreeableness. But when you look at the specific facets of each of these broad factors, you realize that there are some traits that males score higher on (on average), and some traits that females score higher on (on average), so the differences cancel each other out. This canceling out gives the appearance that sex differences in personality don't exist when in reality they very much do exist.

For instance, males and females on average don't differ much on extraversion. However, at the narrow level, you can see that males on average are more assertive (an aspect of extraversion) whereas females on average are more sociable and friendly (another aspect of extraversion). So what does the overall picture look like for males and females on average when going deeper than the broad level of personality?

On average, males tend to be more dominant, assertive, risk-prone, thrill-seeking, tough-minded, emotionally stable, utilitarian, and open to abstract ideas. Males also tend to score higher on self-estimates of intelligence, even though sex differences in general intelligence measured as an ability are negligible [2]. Men also tend to form larger, competitive groups in which hierarchies tend to be stable and in which individual relationships tend to require little emotional investment. In terms of communication style, males tend to use more assertive speech and are more likely to interrupt people (both men and women) more often-- especially intrusive interruptions-- which can be interpreted as a form of dominant behavior.

Of course, there are many men who don't display high levels of all of these traits. But that fact doesn't contradict the broader pattern. For instance, I can recognize that I am a man who has quite a mix of extremely masculine and extremely feminine personality traits and also recognize that my own personal experience doesn't invalidate the generalizable findings. Which is why I will keep italicizing on average to emphasize that point.

In contrast, females, on average, tend to be more sociable, sensitive, warm, compassionate, polite, anxious, self-doubting, and more open to aesthetics. On average, women are more interested in intimate, cooperative dyadic relationships that are more emotion-focused and characterized by unstable hierarchies and strong egalitarian norms. Where aggression does arise, it tends to be more indirect and less openly confrontational. Females also tend to display better communication skills, displaying higher verbal ability and the ability to decode other people's nonverbal behavior. Women also tend to use more affiliative and tentative speech in their language, and tend to be more expressive in both their facial expressions and bodily language (although men tend to adopt a more expansive, open posture). On average, women also tend to smile and cry more frequently than men, although these effects are very contextual and the differences are substantially larger when males and females believe they are being observed than when they believe they are alone.

Contrary to what one might expect, for all of these personality effects the sex differences tend to be larger-- not smaller-- in more individualistic, gender-egalitarian countries. One could make the point that many of these differences aren't huge, and they'd be mostly right if we just stopped our analysis here [3]. However, in recent years it's becoming increasingly clear that when you take a look at the overall gestalt of personality-- taking into account the correlation between the traits-- the differences between the sexes become all the more striking.

The Gestalt of Personality

Personality is multidimensional, which has implications for calculating sex differences in personality. Relatively small differences across multiple traits can add up to substantial differences when considered as a whole profile of traits. Take the human face, for example. If you were to just take a particular feature of the face-- such as mouth width, forehead height, or eye size-- you would have difficult differentiating between a male face and a female face. You simply can't tell a male eyeball from a female eyeball, for instance. However, a look at the combination of facial features produces two very distinct clusters of male vs. female faces. In fact, observers can correctly determine sex from pictures with greater than 95% accuracy [4]. Here's an interesting question: does the same apply to the domain of personality?

Interestingly, yes. You can calculate a metric called D which is a summary of how statistically separate two groups are from each other (i.e., how good of a line you can draw between groups from a statistical point of view). This metric allows you to take into account how all of the personality traits tend to be related to each other in the general population. For instance, people who are conscientious also tend to be more emotionally stable, so if you find someone who is very conscientious and also super neurotic, that person stands out more (has a more unusual personality profile) given the overall correlational structure. With more traits, things get even more interesting. You can have a combination of traits that are less expected, and thus more informative, because they go against the trends of the correlational structure [5].

There now exists four large-scale studies that use this multivariate methodology (see here, here, here, and here). All four studies are conducted cross-culturally and report on an analysis of narrow personality traits (which, as you may recall, is where most of the action is when it comes to sex differences). Critically, all four studies converge on the same basic finding: when looking at the overall gestalt of human personality, there is a truly striking difference between the typical male and female personality profiles.

Just how striking? Well, actually, really striking. In one recent study, Tim Kaiser, Marco Del Giudice, and Tom Booth analyzed personality data from 31,637 people across a number of English-speaking countries. The size of global sex differences was D = 2.10 (it was D = 2.06 for just the United States). To put this number in context, a D= 2.10 means a classification accuracy of 85%. In other words, their data suggests that the probability that a randomly picked individual will be correctly classified as male or female based on knowledge of their global personality profile is 85% (after correcting for the unreliability of the personality tests).

Consistent with prior research, the researchers found that the following traits are most exaggerated among females when considered separately from the rest of the gestalt: sensitivity, tender-mindedness, warmth, anxiety, appreciation of beauty, and openness to change. For males, the most exaggerated traits were emotional stability, assertiveness/dominance, dutifulness, conservatism, and conformity to social hierarchy and traditional structure.

This basic pattern of findings was replicated in another recent large-scale survey of narrow personality traits conducted on nearly a million people across 50 countries. Using different personality tests, and averaging across all countries, Tim Kaiser found a D = 2.16, which is very similar to the effect size found in the other study on English-speaking countries. While there was cross-cultural variation in the effect, there was a general trend for more developed, individualistic countries with higher food availability, less pathogen prevalence, and higher gender equality to show the largest sex differences in global personality [6].

In particular, Scandinavian countries consistently showed larger-than-average sex differences in global personality, together with the US, Canada, Australia, the UK, and other Northern and Eastern European Countries. The countries with the smallest sex differences in global personality included several Southeast Asian countries. To be sure, there wasn't a perfect correlation between more developed, gender-egalitarian countries and sex differences (e.g., Russia displayed the largest sex difference with D = 2.48). But even Pakistan-- the country with the smallest sex differences in global personality in the world according to this study-- had a D = 1.49. This means that even when you look around the world for the country with the smallest sex difference in global personality, the classification accuracy of that country is still 77%!

These numbers dovetail with a number of studies showing a similar level of classification looking at whole brain data. By applying a multivariate analysis of the whole brain, researchers are now able to classify whether a brain is male or female with 77%-93% accuracy (see here, here, here, here, and here). In fact, some recent studies using the most sophisticated techniques have consistently found greater than 90% accuracy rates looking at whole brain data (see here, here, and here). While this level of prediction is definitely not perfect-- and by no means do those findings justify individual stereotyping or discrimination-- that's really high accuracy as far science goes [7].

All of this data is really hard to ignore and dismiss out of hand. But what are the implications?

Implications

All of the findings I've presented up to this point are merely descriptive; they don't prescribe any particular course of action, and they do not say anything about the complex interplay of genetic and cultural influences that may cause these differences to arise in the first place. It is very difficult finding evidence that would indicate just how much of sex differences are due to society vs. genetics (although it's most certainly a mix; more on that later). Even the brain findings discussed above don't reveal the causes of the brain development. Experience is constantly sculpting brain development.

But even if we just stay at the descriptive level, there are still a number of very important implications of the existence of large sex differences in personality. For one, the multivariate findings may help answer a question people have been puzzling about in psychology for quite awhile: Why do we have all these studies showing that male and female behaviors are so similar, yet people in everyday life continue to think as if males and females were very separable? It is possible that people in everyday life are actually closer to the truth because when we reason about personality, we rarely reason about one trait at a time.

If people do indeed create a gestalt in personality perception, then the relevant analysis is a multivariate analysis, not a univariate analysis (which has been the predominant method in the field for so long). "People might be more reasonable than you think", Marco Del Giudice, a leader in the science of sex differences, told me. "Why would you expect people to just make up differences between men and women that aren't there? One possibility is that they are not making it up. What they are considering when they are thinking about men and women is not just one trait at a time, but a combination of traits."

Another possible factor that may help further our understanding of pervasive stereotypical expectations may also have to do with recognizing the importance of the tails of the personality distribution. Even relatively small differences at the average level can lead to very large differences in the proportion of groups at the extremes. For instance, if you look at the density distribution for agreeableness, the average difference between males and females is only about .4 of a standard deviation. However, if you look closely you can see that there are way more women than men who are super-agreeable and way more men than women who are super-disagreeable. It's likely that the behaviors carried out by those tails have a huge impact on society-- on social media, in politics, in the boardroom, and even in the bedroom. 

Now, one might counter at this point: Scott, you really should stop talking openly and honestly about these findings and implications, because if the truth got out there, it could cause harm. But here's the thing: rarely do we consider the harm that could be caused by ignoring sex differences! One can think of many ways in which pretending something doesn't exist may actually cause greater harm psychologically than accepting the facts of the matter. As Del Giudice put it to me:

"People don't want to just give up on trying to understand the world. They want to make sense of the world. And so, if the right explanation is that there is some kind of difference, and you kinda close off that possible explanation because of ideological reasons it's not like people stop asking why. They will come up with a different explanation. So you will get a chain of worse and worse and worse explanations that may actually backfire in all sorts of ways."

Take heterosexual marriage. Many couples go into a marriage assuming that sex differences in personality are minimal. However, we know that on average, females in relationships want constant emotional connections whereas on average men don't tend to be equally as interested in that aspect of the relationship. An incredible amount of stress in a marriage may be due to what people are expecting about each other based on the assumption that everything has to be equal and both partners must feel the same exact way about everything. But here's the thing: we don't all have to be the same in every dimension in order to appreciate and respect each other.

Of course, couples need to work out the fit between their very special and unique personalities. I am a strong believer that individual differences are more important than sex differences. Nevertheless, sex differences are also part of the picture, and may be particularly detrimental to a relationship if all partners go into the marriage thinking that they "should not exist", instead of coming to a healthy acceptance of sex differences, even laughing about them and attempting to understand differences in interests and motivations that fall along sex-related lines. Of course, there will be so many aspects of overlap among males and females in a relationship, but there may be a few meaningful differences that on average could be truly impactful and explanatory in predicting relationship satisfaction and understanding.

Toward a Mature, Nuanced, and Sophisticated Science of Sex Differences

I believe it's time for a more mature, honest, and nuanced public discourse about these obviously sensitive yet incredibly important issues.

First and foremost, I think this requires a recognition that none of the findings I presented in this article, nor any findings that will ever come out-- justifies individual discrimination. We should treat all people as unique individuals first and foremost. No matter what the science says, if an individual shows the interest and ability to enter a field in which their sex is extremely underrepresented (e.g., women in math and science, men in nursing and education), we should absolutely be encouraging that individual to enter the field and do everything we can to help them feel a sense of belonging. I may be weird, but I don't see any contradiction whatsoever between being an advocate for equitable opportunity for all people and being an equally strong advocate for respecting scientific findings and attempting to get as close as possible to the truth about average sex differences.

I also believe that a truly mature, honest, and nuanced discussion of the origins of sex differences must recognize the deep influence of genetics and biology [8]. That doesn't mean that we ignore sociocultural factors, which are clearly important. But sex differences in behavior are so pervasive in nearly every other species. It's just not plausible that somehow male and female psychology evolved to be identical despite the physiological differences and different reproductive roles across human evolutionary history.

This is why biologically oriented folks draw on a wide range of explanatory concepts from biology, as well as cross-cultural, anthropological, and primatological evidence about present-day and ancient humans and their primate relatives. This doesn't mean that such theories are always right. The point is that the methodology is far richer and systematic than they are so often treated in the popular media. The best sources to counteract this misconception is Dave Geary's book "Male, Female" and Stewart-Williams' "The Ape that Understood the Universe". If you want to dive into a more academic treatise, consult this academic paper by John Archer.

I'm actually really optimistic that such discussions don't have to devolve into polarization and ad hominem name calling, with accusations of "sexism" on one side and being "anti-science" on the other side. I'm optimistic because I think a great example of a mature debate on the this topic already exists.

In February 2019, psychologists Cordelia Fine, Dapna Joel, and Gina Rippon wrote an article called "Eight Things You Need to Know About Sex, Gender, Brains, and Behavior: A Guide for Academics, Journalists, Parents, Gender Diversity Advocates, Social Justice Warriors, Tweeters, Facebookers, and Everyone Else." Based on their many years observing both the scientific and popular treatment of the topic of sex differences in brain and behavior, the authors provide an accessible guide to help everyone interpret new biological findings. They rightly point out that people unfortunately tend to unthinkingly ascribe the mere existence of sex differences to "immutable biological factors", an assumption that does not automatically follow from the data. Not only that, but it's true that there is very little biologically that's "immutable" other than the genetic sequence, a fact that is widely known among all of the psychologists that I know.

Marco Del Guidice, David Puts, David Geary, and David Schmitt then wrote eight counterpoints to their article, agreeing with some of their premises but disagreeing with other premises. They argue that Fine and colleagues assume that most sex differences are small, inconsistent, highly malleable, and for the most part socially constructed, and argue that

"minimizing the magnitude of important sex differences and discounting their biological origins can be just as damaging (for science and society at large) as exaggerating them and accepting simplistic biological explanations of sex differences at face value... An honest, sophisticated public debate on sex differences demands a broad perspective with an appreciation for nuance and full engagement with all sides of the question."

In a response to their counterpoint, Cordelia Fine, Daphna Joel, and Gina Rippon note their pleasure at Del Giudice and colleague's response but point out several points of "ghost disagreement"-- that is, places where Del Giudice and colleagues argued against views that they did not express and actually do not hold.

This back and forth was such a great example of the importance of constructive debate and giving people enough benefit of the doubt to allow them to clarify their views so that they aren't misinterpreted or their views aren't taken out of proportion. Fine and her colleagues concluded that "exchanges such as the present one, when focused on evidence and claims, are valuable-- and rarer than we would like." For anyone who wants to dive deeper into these complex debates and see a great example of how real progress can be made in furthering knowledge and understanding, I highly recommend reading this entire exchange.

In my view, a more mature, sophisticated, and nuanced understanding of sex differences in personality and behavior is possible. One important step is to take sex differences in personality seriously. Only by facing reality as clearly as possible can we even begin to make changes that will have a real positive impact on everyone.

Endnotes

[1] Due to the research that has already been conducted on this topic, I intentionally used the phrase "sex" differences in this article rather than "gender" differences-- sex defined as a collection of traits (e.g., X/Y chromosomes, gonads, hormones, and genitals) that cluster together in about 99.98% of humans (see here and here). Of course, I do not mean to suggest that the exceptions to the sex binary are unimportant, and I fully believe that all variations in gender identity and sexual orientation are amenable to scientific investigation and deserve to be studied in their full richness. Also, I think it's an interesting and open question the extent to which there are gender differences in personality, especially among the many different gender identities that people are adopting in recent years. I'd definitely be interested in seeing more research looking into that question as well.

[2] However, it should be noted that men are typically found to show more variance in general cognitive ability scores than women (see here and here).

[3] One notable exception is an interest in people vs. an interest in things. The sex differences on this dimension are actually quite large, with some large studies finding greater than 1 standard deviation of a difference between males and females on average on this dimension (see here and here).

[4] I could see someone being concerned that this finding somehow strips us of our individuality-- that essence of us that transcends our biological sex. However, I think that fear is unwarranted. After all, there now exist really sophisticated apps in which you can change the sex of your face, but even then, you still remain recognizable. I think maintaining one's individuality doesn't contradict the generalizable findings regarding the high classification rates of sex based on one's physical characteristics.

[5] To be sure, the multivariate approach (where you look at personality as a whole) isn't always better than a more univariate approach (where you focus on a specific variable). It's all about context and what you are trying to predict and your purposes of prediction. For instance, if what you are trying to predict is clearly based on a particular subset of traits, then just adding more traits into the model may produce an illusory effect. There are a few criticisms of the multivariate approach, however, that really do not hold water (see here). One is the criticism that a multivariate approach to personality doesn't say anything meaningful because it's not valid to aggregate traits in a multivariate analysis. This is a fair criticism for domains that include a hodgepodge of traits that don't go together in any meaningful way. But that doesn't apply to the domain of personality. There exists a plethora of research across cultures on the correlational structure of personality. Of course, if you start adding irrelevant variables such as shoe size, voting preference, or height to the personality data you will get an artificially big separation between the sexes and it wouldn't tell us much of anything meaningful. However, that's not how these studies are conducted. A second potential criticism is that the more traits you throw into a multivariate analysis, of course the effects are going to get bigger and bigger and bigger. So it's not interesting that we get these big effects. While this criticism is true-- technically speaking, the more traits you add, the more differences will grow, and will never shrink-- it's simply not true that the differences will keep growing at the same rate. Because the multivariate analysis takes into account the correlation between the traits, you will eventually start seeing less of an effect of adding in additional personality traits because additional traits will start becoming more and more redundant.

[6] Interestingly, Kaiser found that after controlling for some potential confounds relating to ecological stress, only historic pathogen prevalence, food availability and cultural individualism were still correlated with sex differences in personality (the specific correlation between the gender equality of the country and sex differences was reduced to zero after controlling for confounds). Kaiser concludes that "[previously] reported correlations between greater sex differences and outcomes of gender equality could be due to confounding by influences of ecological stress."

[7] Someone may look at these studies and say: Well, what about this NY Times Op-Ed: "Can We Finally Stop Talking About 'Male' and 'Female' Brains?" It turns out that the data that is mentioned in that study conducted by Daphna Joel and colleagues (see here) was not based on whole brain data. This matters. The researchers left it to the reader to infer that their findings also apply to whole brains by extension, but it turns out that such an extension is not warranted given the recent spade of studies that are all converging on 77%-93% classification accuracy based on whole brain data-- including a more recent study led by Daphna Joel! What's more, the method that Daphna Joel and colleagues devised for quantifying "internal consistency" in their earlier article is a straw man guaranteed to always find very low levels of consistency. By defining "consistency" as 100% uniformity, there is no way that their method will ever detect consistency as long as there is some variation within each sex. Marco Del Giudice and colleagues have shown this to be the case with artificial data, and illustrated it by showing that the method cannot even detect consistency within species (they compared the facial anatomy of different species of monkeys). More realistic than having 100% consistency, in my view, is whether the pattern is statistically robust-- whether you can distinguish between men and women with a very high degree of accuracy based on aggregate patterns of interests. And this is why their initial finding is such a red herring: Their conclusion is not based on whole brain data. To dive deeper into the critique of the Joel and colleagues study, I recommend reading this and this.

[8] I intentionally separated out "genetic" from "biological" in this sentence because it's a common misconception that "biological" equates to "genetic." The question "Are sex differences biological or cultural?" is actually a meaningless question since every sex difference is biological when it's expressed, regardless of whether its origins are cultural or genetic. Social learning processes are biological. Aspects of personality that are learned are also biological. In fact, anything that affects behavior is acting biologically on the brain. When people say traits or sex differences are "biological", they probably really mean "genetic."