Numerous psychological differences have been found between males and females, such as differences in cognitive abilities, personality, and rates of certain psychiatric disorders. However, do these differences tell us about evolutionary differences between biological males and females (nature), or just gender differences between men and women, girls and boys (nurture)? Are the brains of males and females built differently, determining differences in behavior (nature), or do we just interact with boys and girls differently from birth, essentially molding the same brain into two gendered brains (nurture)?

Humans differ from other species in that there are not simply two distinct categories, male and female. Instead, this dichotomy is complicated by the fact that humans have a sense of self, and a gendered one at that. Humans can feel distinctly male or female, influenced by both biological assignment and societal perception. This makes it particularly difficult to disentangle sex differences, or those that are inherent because an individual is biologically male or female, and gender differences, or those that come about because of the environment and society in which one lives.

For the purposes of this post, I will only be considering research on cisgender individuals, or those whose gender identity matches their biological sex. However, it is important to note that there are transgender, transsexual, and intersex individuals, and this is an area requiring much more in-depth research in order to help these minorities who far too often face discrimination. This post will unfortunately not be discussing these important groups of people, because there simply isn’t yet the evidence necessary to substantiate scientific claims; this will hopefully change in the near future. If you’re interested, here’s an article from the Scientific American about the possible biological basis of gender dysphoria.

In psychological research, a multitude of differences between males and females have popped up, including, but not limited to, differences in certain cognitive skills like memory or intelligence test scores, personality traits (read Nazia’s post for more!), and rates of psychiatric disorders. Across the studies, results are highly mixed. But even in the case of more standard findings, such as males being better at mental rotation (turning objects in your mind’s eye) and spatial navigation, while females excel at object location and spatial memory, it’s difficult to see from where these differences come. Are males really “pre-programmed” to be better at mental rotation, or do they just play more soccer, er…football? Can females really remember more words, or do they just play more word games? Neuroscientists and psychologists should be careful about drawing conclusions about why certain differences between groups (i.e., males vs. females) exist.

Another well established difference between males and females is the rate of psychiatric disorders. In some countries, the rate of depression is almost doubled in females compared to males, though this is also culturally dependent, and anxiety disorders also affect females at such a rate. Attention deficit hyperactivity disorder (ADHD) and autism, on the other hand, disproportionately affect males, as well as schizophrenia. But why would such disorders affect males and females at different rates? Again, it’s a question of whether males and females really have different predispositions toward certain disorders, so that no matter what they did, males would always have ADHD at a higher rate than females, or if societal and cultural norms push males and females in different directions of pathology.

What even decides if you’re going to be male or female?

Before I get into the differences between male and female brains, it’s interesting to look at if/why there are even differences in the first place. Beside all the philosophical debates on why someone is born the way they are (cue Lady Gaga), it usually just comes down to your genes, plain and simple.

The classical theory of sex differentiation of the brain posits that sex differentiation begins in vitro with the development of testes if the Sry gene in the male Y chromosome is present. The fetus’s testes then produce testosterone, influencing the brain to develop into a “masculinized brain.” Thus, testosterone is seen as responsible for the “masculinization” and “de-feminization” of the brain, while the absence of the sex hormone results in the “non-masculinized,” female brain. After birth, this process continues, as well the female brain starts to “feminize” due to estrogens secreted by the ovaries. This is the first step in the neuroendocrine journey that is sex hormone chemistry, but that’s a long and complicated story that we won’t get into too much. Suffice it to say that this differentiation in the beginning sets off a long chain of events, eventually leading, in most cases, to a male or female.

Can you even tell if a brain is from a male or female?

Now the question is, does this differentiation even lead to any physical differences?

The short answer is: no dramatic differences have been found yet. While there is quite a bit of evidence for differences in cognitive performance, personality, and other behavioral differences between males and females, the differences in the physical brain are very limited.

One rather interesting finding by Ingalhalikar and colleagues (see abstract here) was a large distinction in connectivity, or how different areas of the brain are connected to and communicate with other areas. In males, connections tended to be intra-hemispheric, which means they stayed within one hemisphere and ran front and back. In females, however, connections tended to be inter-hemispheric, which means they went between the hemispheres and ran left and right.

In this image from Ingalhalikar’s paper, the intra-hemispheric connections are shown in blue, and the inter-hemispheric connections are shown in orange. The upper brain represents the averaged results from the males and the lower brain represents females.

The direction of the connections in the average male’s brain could explain why males are better at “perception and coordinated action.” These connections from the back of the brain (areas associated with perception) to the front of the brain (areas associated with action) facilitate performance in hand-eye coordination and remembering shapes or the layouts of places.

For the average female, the connections running across the hemispheres from left to right and vice versa may help with social skills, and remembering words and faces. These connections enhance communication between areas associated with analytical and sequential reasoning (the reasoning you need for logic problems) in the left hemisphere, and areas associated with intuitive processing (quick judgments, often involving emotions) in the right.

There was also variance between males and females as to when these differential patterns in connectivity develops. This study was conducted with children, adolescents, and young adults, and between males and females, and researchers found that in males, stronger intra-hemispheric connectivity is evident earlier on in the course of development and preserved until adulthood, whereas in females, stronger inter-hemispheric connectivity is concentrated in the frontal lobes during adolescence, but seems to disperse by adulthood.

Brain differences = Behavior differences?

So far, the differences in connectivity discussed here, and also minor structural differences, have not been convincingly associated with differences in behavior or abilities. As is often the case in neuroscience and psychology, there is no 1 + 1 = 2. Instead, there is a complicated interaction system within the body, which involves both predispositions toward certain behaviors and developmental patterns, and the environment and culture in which one grows up.

But with all this neuroanatomical evidence (see more differences at the Thought Catalog), it might be easy to jump to the conclusion that, “This person’s brain looks like that BECAUSE they’re biologically male/female!” But be careful, because you never know how much an environment can influence a brain to look more typically “male” or “female.” It’s difficult to say exactly how much impact the social environment will have on an individual’s behaviors, or how hormone exposure during pregnancy will make one’s brain have certain structural or connective properties.

In general, we as neuroscientists need to be very careful to avoid jumping to conclusions about differences between groups, whether it’s genders, racial groups, or other categorizations. There’s usually (read: pretty much always) not a one-directional influence of biological predispositions on behavior. A couple years ago, Cordelia Fine wrote a piece titled, “His brain, her brain?” in which she urged neuroscientists to “overcome ‘nerurosexist’ interpretations,” and, as scientists trying to find empirical truths, this is a message that shouldn’t be taken lightly.

Some things to consider

Given the evidence that “sex differences” in neuroanatomy might not even amount to much in terms of psychology, do you think it’s still worth it to investigate the mystery of male vs. female brains? Or do you think that scientists should spend their time answering more worthwhile questions, such as how social surroundings influence men’s and women’s brains and hormones? Or do you think that interdisciplinary research is always the best to answer such complex questions such as male vs. female differences?