Friday, November 14, 2014

Evolutionary Sins: The Gender Gap In Spatial Cognition And Navigation

Layne Vashro and wife Chelsea with Chief Koyo and family.
Yesterday, Science Daily reported on a study conducted by anthropologists from University of Utah entitled Spatial cognition, mobility, and reproductive success in northwestern Namibia. The test subjects were dozens of members from indigenous tribes, the Twe (Twa) and Tjimba (Cimba) people of northwestern Namibia, whom lead study author Layne Vashro has researched extensively and produced a nice amount of literature therefrom. This is likely related to the Spatial Cognition and Navigation (SCAN) Project -- a project which I had the pleasure and benefit of analyzing for an essay in one of my anthropology courses. It's very prospective research, but this particular study is still in press corrected proof, and having read the study, I think there are a few hasty conclusions drawn from their findings, namely that spatial ability and navigation are a result of evolutionary selection, and that this difference explains the somewhat more ubiquitous phenomenon of greater spacial ability in men than women. In this post I'll be examining the data from this study, and then using the products of other ethnographic research to explain why Vashro & Cashdan's findings do not necessarily show that navigational ability is a product of evolution.

This study conducted several tests of spatial ability on the subjects, including pointing to different directions in the Kunene region and stating whether a rotated hand on a screen was a left or a right hand after 7.5 seconds. It was found on the tests, after excluding individuals who didn't understand them, that men did significantly better than women. These findings, the research authors suggest, support the hypothesis that spatial ability has evolved in men for reproductive success. The reason for this, they continue, is because men who were better able to navigate long distances were also better able to find several mates with whom they could have children with; and since marriage does not restrict a man's sexual behaviours in these tribes, they were best to be studied, as they were less likely to be constrained by their marriages.

"Some people think it is culturally constructed, but that doesn't explain why the pattern is shared so broadly across human societies and even in some other species," explained Elizabeth Cashdan. At an initial glance, this seems like a pretty rational conviction with a cultural hypothesis: if this is contingent upon culture, then why is it that multiple cultures, and even multiple species, have these same abilities?

The novel debate.
The problem with this conclusion, first of all is that it's an evolutionary misconception. This is something that I touched up on in my post of a similar title: the idea that because something is, it therefore evolved to be this way. This is fallacious -- traits which have evolved are not always used in ways they developed for, and just because something is the way it is now does not mean it was supposed to be this way. In that post, I used the example of bird feathers, where it was not developed for flight, but for temperature regulation. Similarly, we can say that spatial abilities evolved, but just because this trait is exhibited in certain ways and has certain benefits now does not mean it evolved for that reason.

Second of all, the hypothesis in question has little support from research which directly tests its viability. If spatial ability is a dimorphic trait which evolved in men over women to travel over wide ranges and navigate to find more mates and produce more offspring, then we should expect the differential to be constant across species with such wide ranges, and for the gap to decrease when ranges decrease. In fact, this is not the case. The comparative analysis by Clint et al. (2012) shows that across 35 studies of 11 species, while 8 out of 11 of the species do display this gap in spatial navigation, this tendency occurred regardless of the size of territories or the extent to which male ranges spanned further than female ranges. The authors suggest an alternative hypothesis, that perhaps the differential is a side effect of sex differences in testosterone, but there is little evidence to support this; certainly, however, there seems to not be much evidence, if any at all, for the evolutionary hypothesis.

Of course, I should be fair and state ahead of time that the authors do not suggest definitively that the gender gap in spatial ability has evolved for this reason, but merely that it's a hypothesis which is supported by the evidence. No, the idea that this is definitively the answer is more the fault of science reporting and the articles that have appeared in response to this article. Ignoring the reporting and prior studies, if we go by the authors' interpretations, they're not wrong: this could suggest that the spatial gap is a result of evolutionary differences, but it could also suggest cultural differences as well. But what reason do we have to believe one over the other?

Well, one reason which might suggest that spatial ability is the product of culture is that we know this to be the case based on already existing evidence; and in fact, the discrepancy between cultures (i.e. the lack of total universality) is what supports this.

To begin, consider the way direction is analyzed in the west; that is, in America, Canada, Europe, and so on -- English speaking countries. In common usage, direction is usually told respective to ourselves: left, right, front (or forward) and back (or behind). This is how we tell stories, how we give directions, and how we map the world around us, and in fact this has been fairly unproblematic in our daily lives. It's natural -- second nature to us, and because we can all understand it, we can all interact via these methods.

However, this method of navigation is not consistent between cultures. The most famous example of this is the Australian Aboriginal tribe, the Guugu Yimithirr. Their navigational abilities, unlike ours, are based on cardinal directions: North, South, East and West. Like us, this is also their practical use of direction, i.e. the one they use when speaking to other people, giving directions, telling stories, etc. When telling stories, for example, about when a fishing/hunting boat capsized, they would say the people jumped off the East side of the boat. When comparing the same story told by multiple individuals, this direction is remarkably consistent. This use of direction isn't biological, however, but is taught from a young age, as early as two.

Guugu Yimithirr warriors.
Yet, the Guugu Yimithirr have problems with their method. When introduced to a region they aren't familiar with, where they can't orient themselves to familiar landmarks, they can no longer tell directions. This is very different from the system used by English speakers, because we can use our methodology in almost all situations, so long as we can see. Thus we can see that while the Guugu Yimithirr have developed remarkable spatial ability compared to that of English speakers, it would only confer success in their specific region. This shows not only cultural boundaries, but evolutionary boundaries as well.

These are only two examples of how culture constructs our navigational abilities, and thus potentially explain the differences between cultures and even between sexes/genders in a particular culture. This analysis examines the differences between how cultures utilize direction without regarding gender. There is evidence, however, which examines the gender gap as well, and provides strong support for the cultural explanation, namely by displaying how difference in kinship and marriage systems might explain the discrepancy.

Consider the Khasi and Karbi tribes of northern India. The Karbi tribe practices a patrilineal kinship system, which (very briefly) means that inheritance and descent are passed down through men to their offspring. Characteristic of this type of kinship system is very strong, hard-to-break marriages as well as strict gender roles and restriction of women's sexuality. The Khasi tribe, on the other hand, is matrilineal, which means descent is passed down by women, by inheritance is passed by men -- but the assets which men own are often reliant on marrying women. Cultures which have this type of kinship system more often exhibit egalitarian characteristics, with women not being restricted in their sexuality, with relationships between siblings being strong, weaker marriages, etc.

The Khasi people at a dancing festival in 2005.
It should be no surprise, then, that despite these two tribes being very genetically and geographically similar to each other, they show differences in gendered spatial abilities. In the Karbi tribe, men completed spatial ability tasks 36% faster than women; but in the Khasi tribe, they showed no significant differences. Similarly, they controlled for socioeconomic status as well: since there are some women in the Karbi tribe who do own land and wealth when there are no sons, they tested individual households within the tribe. Then, they found that in households with higher status women, the gap in spatial ability closes by about a third. In addition, controlling for education shrank the gap by a third as well.

Admittedly, this doesn't explain why or how culture influences these factors, but lends very strong credence to the cultural hypothesis, and reconciles for factors which may not be accounted for by Vashro & Cashdan (2014). There are problems with the Hoffman study, however: their test of spatial abilities (an Object Assembly task) may or may not accurately represent the cognitive faculties in question. It should be noted however that their sample was a rare one, and was intended to promote future prospective research, but to my knowledge no such research has been conducted, likely due to the difficulties Hoffman et al. (2011) faced when attempting to teach their sample subjects how to conduct higher-level tasks of spatial abilities.

Nonetheless, this study remains significant for the subject at hand: whether or not Vashro & Cashdan's evolutionary hypothesis of sexually dimorphic expression for spatial cognition holds water. Likewise, the very fact that there exists such discrepancies in spatial abilities between cultures, and even within cultures, further supports the cultural hypothesis. Finally, while I won't discuss it in full detail here, we also have the pioneering work of anthropologist Dianna Shandy and economist Karine Moe to explain the cultural pressures which cause gender gaps in and different responses to certain tasks, including navigation.

It's honestly hard to believe that this recent research is being used to support the idea that spatial memory has evolved higher in men than women everywhere, even though there are clear cultural differences in this cognitive gap. To accept the evolutionary hypothesis, the trait should be much more universal than is the gap between genders in spatial abilities; nonetheless, this is a good example of western reductionism and how we find it very easy to find simple explanations for complex traits. This is much less a criticism of Vashro & Cashdan, but of the reports of their study. It's important to suggest alternatives in the scientific literature, as the two authors have done, but there are no definitive answers to be found here. We need to be careful that when we suggest an evolutionary explanation for observed traits, that the evidence is robust enough. While it's clear that there is cultural influence on spatial ability, there may be biological influences as well; however, the evidence at present, in my opinion, is not clear to support that suggestion.

Thank you all very much for reading.

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ResearchBlogging.orgHaviland, J. (1998). Guugu Yimithirr Cardinal Directions. Ethos, 26 (1), 25-47 DOI: 10.1525/eth.1998.26.1.25
Hoffman, M., Gneezy, U., & List, J. (2011). Nurture affects gender differences in spatial abilities. Proceedings of the National Academy of Sciences, 108 (36), 14786-14788 DOI: 10.1073/pnas.1015182108

Vashro, L., & Cashdan, E. (2014). Spatial cognition, mobility, and reproductive success in northwestern Namibia. Evolution and Human Behavior, DOI: 10.1016/j.evolhumbehav.2014.09.009


  1. It's curious to think that an evolutionary explanation somehow arose from this research, I agree. To me it doesn't intuitively lend to one hypothesis over another, but instead is just another piece of data for us to interpret in the long run. As you alluded to in your article, Alexis, the pressures behind the differences in spatial cognition between genders has been very strongly supposed to be sociocultural, not biological, at least in anthropology, since as early as 2011. We have to keep in mind however that hypotheses always arise out of the biases that individuals hold when observing the data. Might Vashro & Cashdan's experiences with psychology and behavioral sciences have influenced their hypothesis? I'm curious for your input.

    I have to say that I appreciate your return to the topic of anthropology. It's good to hear from young students who are still interested in this topic to clear the water of some pervasive misconceptions and queer interpretations. Might you be returning to the topic of human race any time soon? If so, might I suggest an analysis of Richard Lynn's cold winters "theory" of the evolution of general intelligence? It seems to somewhat coincide with this topic, since spatial cognition is nonetheless a part of modern intelligence testing.

    Thank you for this lovely article. I look forward to more in the future.

    1. Thank you for your comment, Anon. As I said in my post, I think this is much less an issue of the research conducted by the study authors but more so a question of how science magazines and articles interpret the research. The site I linked to, Science Daily, is much better than most other websites, which all touted the title "Men developed navigation abilities to have more sex." This is typical of the "Science News Cycle" which many researchers are familiar with, but is nonetheless still problematic for discussing the actual scientific merits of the research available, since people are ready to scream "YES!" when any single journal article comes out and says "MAYBE!"

      I don't known that Vashro & Cashdan (2014) were influenced by their background in psychology. Certainly between my anthropological studies and my psychological studies, the latter overwhelms my perspective, and yet I didn't approach this with an evolutionary bias. The study authors were much more likely trying to produce data which would challenge people's perspectives, and that's exactly what science should be about -- it's like an art. They raised important questions, and here I provide only one response of likely many.

      I'm glad you enjoy my work so much. I've actually been trying to steer clear of the topic of race for a while and build up my blog content on other, possibly less controversial subjects that still garner a lot of interest from my readers. Likewise this is what I told spawktalk when he commented on my post "Lewontin's Fallacy and Race." Perhaps I'll write on Cold Winters in the future; however, for now, you can find a brief take on that topic on my invisible page "LaughingMan0X vs Sam Owl." I'll link to both of these at the bottom of this comment.

      I look forward to your continued input in future posts!


    2. Thank you for the reading material! I wasn't aware of the invisible page on your blog. I'll have to read that and the comments, as I see there are a lot of them. I understand your desire to stay away from the topic of race for a while, however I encourage you to consider it once you feel the air has thinned a little.

      I agree that accuracy is a prominent issue in science writing and journalism, and that this issue is most likely a reflection of media, not of the authors' intentions. Thank you for raising that important point. For my point on psychology, I meant to say that in my experiences, those with backgrounds in psychology tend to look at traits from a much less cultural/social standpoint but from an evolutionary perspective, and see biology as the base for psychology and culture/society as being a result of its effects. Maybe this was a bit of a tetchy suggestion, and so I apologize if I offended you having not known you studied psychology as well.

      I do have one question. Is the rotated hand test, for example, a subtest on a typical IQ test? If not, how do the authors figure this is a good measure for spatial cognition?

    3. Yeah, sorry about not having it in plain view, but I don't want debate topics to be readily associated with my blog content. I'll often write on controversial topics, and then I'll argue with people in the comments, but I want my blog to be a source of information that is discussed academically and informatively, not in the way that debates are held. Nonetheless I still link to it in some posts/comments because I don't want people to be unaware of its existence.

      Oh, and yes, after a month or two (or maybe more) I'll likely return to the topic of race, and the prompt you suggested will be on the list. Although, I should mentioned that soon I might be expecting a response from spawktalk on The Right Stuff (TRS) (even though I really don't want to be engaged in that kind of discussion, especially on TRS, hahaha), so I may address that topic sooner than expected. Stay tuned!

      Your comment wasn't offensive, and I see your point. I agree that many psychologists, probably the majority, look at these issues from a biological rather than cultural perspective, so your question wasn't invalid. I was simply stating that I don't know if it's a productive way to approach the issue, since people who have backgrounds in psychology can look either way, and an individual who has background in both psychology and anthropology may be more likely to look at things culturally anyway (such as myself).

      I'm not entirely sure if the rotation of the hand is specifically a subtest on any standard intelligence test; however, mental rotation is commonly used as an evaluation of cognitive rate of spatial processing and is thought to be associated with general intelligence (g). That being said, the authors' tests of spatial cognition are valid and not suspect to much scrutiny, besides maybe the sample sizes employed. The differences in spatial cognition, between genders is not something that is up for much debate, as review of the available literature (via multiple meta analyses) has revealed that the gap is unequivocal. However, you might be interested at how strongly spatial cognition is affected by environment. Feng et al. (2007) showed that playing action video games reduces the gap, and as the studies mentioned in the post reveal, kinship systems, socioeconomic status and education may also greatly reduce the gap, although this association isn't very robust.

  2. Missed distinction: adaptation vs. exaptation (feathers example).

    Presumption: the brains/minds of males and females are sexually monomorphic. They are not, and, there is animal data (voles, etc.) in which there are cognitive sexual dimorphisms with respect to spatial abilities.

    Error: Wrong question. All behavior is a complex interweaving of nature and nurture. The question is whether the trait in question has its foundation in a monomorphic or dimorphic psychological adaptation. Even if it is dimorphic, environment/culture can influence, and even reverse, the overt sex behavioral sex difference.

    Don't: set up a false nature VS nurture straw man just for the purpose of batting it down with cultural determinism / social constructionism.

    1. What missed distinction? I already stated that feathers are an example of exaptation on my post on evolutionary misconceptions. The point was that just because something is, doesn't mean it was meant to be. Just because right now, higher spatial cognition in men in SOME cultures confers higher reproductive success does not mean it evolved to be that way; just as it is that even though feathers confer the ability of flight in MOST birds does not mean that it evolved to be that way.

      I didn't presume sexual monomorphism in humans. Just because there is sexual dimorphism in some traits in humans does not mean there is sexual dimorphism in this trait in humans; and just because there is sexual dimorphism in this trait in other animals does not mean there is sexual dimorphism in this trait in humans.

      The question was not "is it nature or nurture?" The question was "is this evolutionary explanation viable?" It's not so, for several reasons:

      1: The hypothesis was based on an evolutionary misconception.
      2: The data did not necessarily support the hypothesis.
      3: The discrepancy can already be well explained by culture and environment.
      4: The discrepancy is not ubiquitous enough to suggest that it has evolved in humans, as the article suggests.

      I didn't set up a false nature/nurture straw man. If you read the post, you would have seen this:

      "We need to be careful that when we suggest an evolutionary explanation for observed traits, that the evidence is robust enough. While it's clear that there is cultural influence on spatial ability, there may be biological influences as well; however, the evidence at present, in my opinion, is not clear to support that suggestion."

      I openly welcome the suggestion of other hypothesis, but merely wish them to have solid backing. I'm also open to biological explanations as well as cultural ones. Please don't misrepresent the content in my post (or perhaps comment without having read my post in full detail) just to leave a curt, pseudo-critical judgment, if only to point out to a young researcher that nature vs nurture is a false dichotomy. I'm aware of this, and never said anything to the contrary. Please reread the content of my article before replying again.

  3. Do you really think that the four piece puzzle used in the Hoffman paper is in any way comparable to the tests normally used to operationalize "spatial cognition"/spatial ability? While I understand that the task was chosen by necessity, it doesn't mean that it allows for attaching just about anything you want to the hypothetical construct of spatial cognition/ability (not a construct I am trying to defend btw).

    1. As I discussed above, one of the tests used by Vashro & Cashdan (2014) was a test of mental rotation, as was the test conducted by Hoffman et al. Tests of mental rotation are often used on IQ tests to test spatial memory, which is a critical component of spatial cognition. Were Vashro & Cashdan's tests superior? Probably. Was Hoffman's sample superior? Yes. We can't just eliminate data that isn't necessarily comparable in some respects, and the Hoffman paper is still relevant.

      And no, it doesn't allow for attaching anything to spatial cognition. I also find no contentions with the concept of spatial cognition. I also, however, don't see this as being problematic to the research of this cognitive function.

    2. If you want to call a four piece puzzle a mental rotation task (which is not something Hoffman et al does) that is up to you, I'd also say that even thinking that it taps into spatial cognition is a stretch, but I guess we will just have to disagree on these points.

      If you don't find any contentions with spatial cognition, what is your definition of it, and what visual tasks could be clearly regarded as not relying on spatial cognition? Most tasks in experimental psychology could be regarded as tapping into some aspect of spatial cognition, so where would you draw the line for what is a good measure?

    3. If you'd prefer, I could replace "mental rotation task" with "visual-spatial task." I've explained the reasoning behind my claims, while you have not. Surely we'll disagree, but your contentions aren't holding much weight, as you've spent the large majority of your time here critiquing my views while not substantiating your own disagreements. I don't take that lightly.

      My definition of spatial cognition is irrelevant to this discussion. One could operate off of a multiplicity of definitions provided in the academic field. The APA's definition, for example:

      "Spatial cognition is a branch of cognitive psychology that studies how people acquire and use knowledge about their environment to determine where they are, how to obtain resources, and how to find their way home."

      Another working definition:

      "Spatial Cognition is concerned with the acquisition, organization, utilization, and revision of knowledge about spatial environments."

      Depending on what definition you use, it could be said that the vast majority of tasks have some relation to spatial cognition; however, this is also irrelevant. We can say that the vast majority of human actions is related to IQ (or intelligence, if you prefer), and yet we can't just arbitrarily decide that EVERY task could be seen as a test of said cognitive capacity. No, what we would look for is a task that is consistent in testing, has a high correlation to the same factor which is highly predictive and related to cognitive capabilities, and has practical value in research.

      As I stated in my comment above, spatial memory is no doubt a key component of spatial cognition. The problem is that this is context specific, and thus I sought to compare the multifaceted approach Vashro & Cashdan (2014) took to understand spatial cognition with individual, comparative pieces of research. They were concerned with tasks such as navigation, mobility, etc. which is consistent with the APA's definition of spatial cognition; thus, I used examples which compare these factors and see if there is a more plausible explanation for why these traits might appear, or if theirs holds water. The results speak for themselves.

      But all of this aside, your contentions with "my" definition and usage of spatial cognition and its related functions are entirely missing the point of this article. We are operating under the premise that spatial cognition exists, here, for practical purposes, and picking apart its usage does nothing informative for the discussion at hand.

    4. "No, what we would look for is a task that is consistent in testing, has a high correlation to the same factor which is highly predictive and related to cognitive capabilities, and has practical value in research."

      I totally agree with this statement, but can you then explain to me how the puzzle task used by Hoffman et al stands up to these criteria? I am sure you are familiar with the critiques from Bailey et al, and Daly, which both have issues with the task.

    5. Gladly, although I will not be the one addressing your concerns. As you're aware of Bailey et al. (2012) and Daly (2012), I'm sure you're also aware of the response by Hoffman et al. (2012). I'll link to their reply at the bottom, but here are some highlights:

      "We therefore agree with the sentiments of Bailey et al. and Daly: Our study should not be viewed as the definitive study on this topic but as a proof of concept, which should propel researchers to exploit this unique sample to its fullest advantage. Further research using noncognitive measures, as well as alternative spatial measures would prove invaluable in addressing some of the shortcomings pointed out by Bailey et al. and Daly."

      This is not contradictory to my statement in the article:

      "It's important to suggest alternatives in the scientific literature, as the two authors have done, but there are no definitive answers to be found here."

      The funny thing, however, is that to my knowledge, no further studies have been conducted from Hoffman et al. (2011)'s leads. This is concerning to me, as there were multiple critiques but no effort to expand and improve on their research. What good is that?

      Problematic to the concerns brought by Bailey et al. is that they claimed Object Assembly shows hardly any sex differences (d = 0.10); they cited no evidence for this, and in fact the data I've seen suggests it to be around twice as much, although I have no source for this either. Furthermore problematic is that none of this is relevant to the purpose of this article, though I will be making changes to it in the near future so as not to lose focus again.

      Thank you for your concerns.

      Hoffman et al. (2012):

    6. To the Anon, I didn't realize spatial ability was a hypothetical construct. Tests like the one Hoffman used are pretty common when given the limitations, and is totally valid. Also, the terms "spatial cognition" and "spatial memory" are used interchangeably in the study. Regardless, Lex's point has a ring of validity to it: this study was limited, yet nobody has tried to expand on the findings. There has been so much research in the male-female spatial ability differential, and yet hardly anyone looks at this remarkable sample to find the answers.

      But it's beside the point. Whether or not culture is the definitive "why" behind this differential, the evolutionary one, given the other evidence does not seem to hold much water.

  4. Thanks for this. I am getting so tired of studies that pull any semi-plausible evolutionary hypothesis out and then say case closed without even considering the possible cultural factors. It is simply antiscientific - and such studies should quite simply not be published untill they at least include the possible cultural caveats. I hate it that the gulf between evolutionary people and cultural anthropologists is too deep to secure these minimum standards of science because they simply dont read or review eachothers work for the most part.

    1. Thank you for your comment, Magnus. As I said in the article (and on Google+) I think we have to be careful who we find fault with here. There has definitely been a growing schism in the anthropological community namely between "anthropological science" and "social science anthropology," and this has been taking place probably since as early as the 1970s. The issue here, however, is that most science reporting websites have been misrepresenting what the study authors attempted to do, and that was create a lead for future research.

      Consider Hoffman et al. (2011). The study design clearly had its shortcomings and flaws, as they were limited by the capacity of their test subjects, but their attempts were not to suggest that the gap is only attributable to culture; on the contrary, it was to promote the same thing suggested by Vashro & Cashdan. From Hoffman et al. (2012), which I quoted in the comment string above:

      "We therefore agree with the sentiments of Bailey et al. and Daly: Our study should not be viewed as the definitive study on this topic but as a proof of concept, which should propel researchers to exploit this unique sample to its fullest advantage. Further research using noncognitive measures, as well as alternative spatial measures would prove invaluable in addressing some of the shortcomings pointed out by Bailey et al. and Daly."

      If we treat this as an antiscientific crime, the perpetrators are most likely websites like KSL and Fox 13.

  5. Confidence mediates the sex difference in mental rotation performance:

  6. I've noticed a distinct decline in quality in scientific research in the last few years. Studies are a lot less rigorous now, and nowhere near as transparent as they should be. Even the most shoddy studies looking for connections between our genes and behavioral traits are getting published; yet when someone attempts to critique this viewpoint, they're assaulted for every methodological flaw in their study. This is why there is usually a section on study limitations, which most of these critiques (such as those of Hoffman's paper in 2011) seem to not even read.

    Another example: I saw that it's being suggested that depression should be "reconceptualized" as an infectious disease... Really? What the fuck?

    I rest my case.

    1. I agree Nick. The quality of research has been degrading, and people are really quick to jump to conclusions. Reductionism, amirite?

      I saw that thing about depression, and I responded to Neurochallenged on his article about it. There's really little evidence to support the reconceptualization. I'll just copy/paste my response here, but I'll link to the article too:

      "There are likely multiple different "types" of depression, some of which may be caused by infectious diseases as Canli (2014) suggests; however at present the association is hypothetical, as you said, and there isn't much to go on for this particular conceptualization. I agree that this should be a realm further explored in scientific studies, but the study author's suggestion should not be taken, in my opinion, as seriously as it has been in the past week or so.

      Consider his use of the existence of inflammation biomarkers in MDD as evidence that MDD is an infectious disease; yet at the same time, the author cites energy loss as being exemplary of illness-like symptoms. The problem with this, in my view, is that energy loss can be a result of a multiplicity of things, existential crises being just one of them. Such energy loss has been suggested to impact the development of chronic low-grade inflammation (Bruunsgaard, 2005). In addition, obesity may play a role in the development of chronic low-grade inflammation (Bastard et al., 2006; Ghanim et al., 2004; Das, 2001; Trayhurn & Wood, 2004) and intuitively would play a role in energy loss as well, while obesity and depression often occur together.

      As I said, the reality is often more complex than we think, and I'm confident in the future that MDD will be further divided to more accurately address the scientific information that comes out; however, at present, I think the suggestion to reconceptualize MDD as an infectious disease is hasty, but was likely already on researchers' minds even without this new study."


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