Dec 01 2015

Male and Female Brains

Is it more accurate to say that male and female brains are generally the same or categorically different? This question has been somewhat of a controversy, both scientifically and culturally. A new extensive comparison of male and female brains with fMRI scans hopes to provide a definitive answer.

First for some background, we need to address the basic question of how we even approach or address the issue of categorization. Nature is fuzzy and complex, but humans tend to prefer neat and tidy categories to simplify the task of keeping track of everything, and even to help our understanding. There is therefore frequently a conflict between our desires and reality when it comes to creating categories.

The Pluto controversy is a good example of this, one which was surprisingly heated despite the fact that there are no real social or political issues at stake. There is no objective and definitive line between planets as solar system objects and other planet-like objects. Astronomers had to come up with some rules, rules that are unambiguous to apply. Ideally, such rules of categorization will reflect some underlying phenomenon, in this case, for example, how planets form.

Categorizing life on Earth has been very challenging, and our systems have changed over time as our understanding of biology has changed. The latest system, called cladistics, classifies creatures entirely based on their evolutionary relationships. Some biologist disagree that this is the best system, arguing that morphological similarity should count also. Birds, for example, cladistically may just be one small subgroup of dinosaurs, but some would argue they are different enough to warrant their own category.

When we get to categories that have huge social and political implications, fighting over categories spreads beyond the scientific journals and meetings. The three that most readily come to mind are sex, gender, and race. I often find that such questions are approached in black and white terms – does race exist, for example. I don’t think, however, that phrasing the question that way is helpful.

The problem with trying to argue that men and women are the same, or that race is just a social construct with no biological reality, is that such absolute positions are difficult to rectify with our common experience. This can lead to rejection by some of the underlying point because it sounds like political correctness rather than a scientific conclusion.

A better approach is to ask several more specific questions. The first is, are there objective categorical differences between two or among three or more alleged groups? A categorical difference is a characteristic that is present in all of one group and none of the other, without any overlap. You can also ask, how frequent are exceptions to apparent categorical differences?

For other traits you can ask if there is a statistical difference, is there any overlap, and how likely are members of various alleged groups to have specific traits. This is where everything gets fuzzy, and it essentially becomes impossible to answer the category question with absolutes.

For example, we can look at biological sex in general. Humans mostly are sexually dimorphic, with two distinct categorical sets of genitalia. The genitalia take one of two developmental pathways, with no overlap. There are a minority of people, however, with ambiguous genitalia, usually associated with known hormonal, genetic, or developmental anomalies. Genitalia are not a continuum – there are two distinct groups with some exceptions in the middle.

If we look at a trait like height, however, we see a different picture. Men on average are taller than women, but there are tall women and short men. There is tremendous overlap along a continuum. This is not a categorical difference but a statistical difference.

Another way to look at the difference between categorical and relative difference is to ask this question: If you know someone is male, can you predict their genitalia? Can you predict their height? The answer to the former question is mostly yes, and to the latter mostly no.

The question the researchers of the current study were asking is this – are male and female brains categorically different like genitalia, or perhaps only statistically different like height, or perhaps not different at all?

They looked at four data sets of MRI scans and fMRI scans of brains, including over 1,400 samples. They looked at specific anatomical structures in the brain in which size was measured, and at connections or pathways in the brain for robustness. Lead author Daphne Joel is quoted as saying:

“We show there are differences, but brains do not come in male and female forms. The differences you see are differences between averages. Each one of us is a unique mosaic.”

They found that the differences were statistical, and not categorical. If you look at any one region or pathway in the brain, there were statistical differences between male and female. However, there was a tremendous amount of overlap. Further, as with height, knowing a person’s sex does not allow you to predict any one trait.

Further, individuals rarely had all male or all female traits. Across the four data sets they found that 0-8% had traits consistently of one sex, while 23-53% had a combination of male-end and female-end traits. Individuals are mosaics, with only statistical differences between males and females.

This does not mean that males and females are the same, or that there are no differences. It does mean that individuals are individuals. People are not mentally defined by their sex.

The paper did not address the issue of whether the statistical differences seen were due to inherent or cultural causes. I suspect the answer is both, for various traits. It seems, for example, that testosterone makes males more aggressive. However, a preference for the color pink appears to be a minor and entirely cultural difference.


What are we to make of the results of this study? To summarize my own approach, I think it is counterproductive and not scientifically accurate to deny that there are real differences between identifiable categories of people.

At the same time it is important to recognize when those differences are only statistical with large overlap. What this means is that in such cases it is not scientifically justifiable to treat individuals as members of a group. Membership in a group does not predict what traits the individual will have. It is therefore best to treat people as individuals.

From an ethical point of view this also works. The basic principle of respect for everyone’s individual dignity demands that people be generally treated as individuals. It just so happens that science supports that position also.

I think the medical profession has struck an appropriate balance. For categorical differences, like male-female biological difference, we have no problem treating people generally as members of a category. There is an entire medical specialty dedicated to female medicine (OBGyn). This is not an ethical or scientific problem.

Otherwise we treat people as individuals, but we may use statistical information to inform our decisions. This is because medicine is the practice of taking statistical group data and then applying it to individuals, knowing that group data does not always predict individual response and we have to individualize treatment as we go.

Genetic heritage, for example, is used to predict the probability of certain diseases or even the response to certain treatments. We don’t ignore race or sex in medicine, because these categories have a statistical reality that informs our very important practical decisions. But we recognize that these categories don’t always predict individual traits. Patients – and all people – still need to be treated like individuals.

18 responses so far