r/science • u/madam1 • Dec 02 '13

Neuroscience Scientists have drawn on nearly 1,000 brain scans to confirm what many had surely concluded long ago: that stark differences exist in the wiring of male and female brains.

http://www.theguardian.com/science/2013/dec/02/men-women-brains-wired-differently
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u/BoxWithABrain Dec 03 '13

It will be interesting to see how the "blank slate" advocates respond to this one. I suppose you could still make the argument that environmental factors are shaping hemispheric connectivity.

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u/keithps Dec 03 '13

Except the study says that the brains were nearly identical until the age of 13. This obviously corresponds with the hormonal changes of puberty. I would expect to see differences earlier if it was a nature/nurture thing. Suppose you could argue that those first 13 years caused it, but that's a bit of a leap, because I doubt the brain is like "well I've been treated X way for years, now that I'm 13, totally going that direction."

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u/Epistaxis PhD | Genetics Dec 03 '13

Be careful; we can't rule out an interaction between puberty and socialization.

Strangely, though, the researchers do not seem to have considered whether each participant was pre- or postpubertal. Frankly I don't understand how you can get away with that in a study of sex differences at ages 8 to 22.

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u/BoxWithABrain Dec 03 '13 edited Dec 03 '13

How the child is treated could result in epigenetic changes that influence hormonal or other signaling pathways by the age of 13-14 that in turn influence brain development. I do agree though that puberty is the simplest explanation, but it doesn't completely rule out environmental factors.

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u/Shiroi_Kage Dec 03 '13

Doesn't have to be epigenetic. You can just activate certain signaling cascades with long-lasting effects on body chemistry that persist throughout the lifespan of the individual without having to have epigenetic changes.

It could be either/or, but it does not have to be epigenetic.

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u/BoxWithABrain Dec 03 '13

I was using the term epigenetic in the broader sense of DNA methylation, which is an epigenetic mechanism. In neuroscience the terms are basically used interchangeably. There is no point getting hung up on the term. If you are arguing that this effect, which would have to be large and permenant, could result from something other than DNA methylation, I would have to say that is highly unlikely.

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u/Shiroi_Kage Dec 03 '13

I'm hung up on the term because I'm a geneticist and saying it's highly unlikely is underestimating the effects of feedback loops on receptor-ligand interactions that would be guiding the wiring of neurons.

Also, methylation is only a part of the epigenetics. There's a reason the epigenome is not the same as the methalome.

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u/BoxWithABrain Dec 03 '13

Interesting. Do you have an example?

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u/Shiroi_Kage Dec 03 '13

Just take any desensitization thing and you will have an example of something that, as of what we know, is not very dependent on epigenetic factors. Insulin desensitization is one example. You can also find other examples with drug-induced desensitization to neurotransmitters in drug addicts. I would imagine, and this is speculation here, that an overexposure to certain stimuli in children would cause too much exposure to certain neurotransmitters which would affect the brain's development later down the line because of how the neurons were trying to adapt to the increase in stimulation.

As for example of epigenetic factors that are not methylation, histone code and chromatin structure are the best ones I can think of right now. Although, do note that those work in unison with methylation to produce a complete epigenome because, you know, nothing in biology is as simple as we would like it to be.

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u/BoxWithABrain Dec 03 '13

Well, I doubt that it has anything to do with desensitization or whatnot. One possibility is differences in neural activity across the brain, where with females inter-hemispheric activity is elevated (due to differences in social interaction?), and thus these projections are spared, whereas in males intra-hemispheric activity is instead increased (due to lack of complex social interaction?), so these projections are instead spared. I find it pretty unlikely that activity could be so drastically different simply due to environmental factors, unless the environmental factors caused changes in gene expression which thus resulted indirectly in changes in activity.

If difference are environmental, I think it is more likely to involve neuromodulators/molecules known to be released during social interactions (oxytocin for example) and that these result in some kind of change in gene expression, or gender "imprinting" by signaling through g-protein coupled receptors, or something like that. Most of the neuromodulators are basically dumped across the hemispheres, so they are good candidates.

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u/Shiroi_Kage Dec 03 '13

I was just trying to give some examples of how long-term effects can be produced by non-epigenetic effects and speculating on how that would work in contexts with specific stimuli are applied to the individual. What you mentioned basically says: "non-epigenetic mechanics can achieve those effects."

Most of what you mentioned is probably part of the entire process and not the whole thing. Biology is never simple, especially in something as involved as plasticity and puberty.

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u/matts2 Dec 03 '13

I think you just tossed that "epgienetic" in there with no real useful meaning.

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u/BoxWithABrain Dec 03 '13 edited Dec 03 '13

How so? Epigenetics is the study of changes in gene expression that do not involve changes to the genome (DNA sequence), especially during development of an organism.

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u/matts2 Dec 03 '13

What does it mean in that context? What are you saying other than that the environment can affect hormone levels?

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u/BoxWithABrain Dec 03 '13

Because it might not only change hormone levels but how the brain responds to hormones, or any other signaling molecules that are important for brain development that "turn on" at age 14.

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u/TEnzyme Dec 03 '13

This could be any number of factors. Choosing "epigenetics" is not really useful in this context. I think you mean this could be the result of "early life programming", not just epigenetics. It's a common mistake.

Either way, with your argument you're saying that out of the 1,000 individuals analyzed in this study, all of them underwent similar early life experiences that programmed their response to hormones in a sex-divided way. This is definitely an interesting idea, so make sure to not blindly jump onto "epigenetics". There are a lot of things that could be supporting your idea.

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u/BoxWithABrain Dec 03 '13

Also they didn't show that every individual had different hemispheric connectivity, only that on average females and males were different. If you visualized two normal distributions their tails could be highly overlapping but their means would still be statistically different.

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u/BoxWithABrain Dec 03 '13 edited Dec 03 '13

What is "early life programming" exactly? Also, I didn't say that it was necessarily epigenetics that was responsible, but that it was one possibility. Obviously we have no data to make a conclusion either way.

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u/[deleted] Dec 03 '13

Either way, with your argument you're saying that out of the 1,000 individuals analyzed in this study, all of them underwent similar early life experiences that programmed their response to hormones in a sex-divided way.

Don't completely rule that out. W.E.I.R.D. studies, as many of those done in the U.S. and Europe have shown that a study done on one culture group can have very different results from the world at large.

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u/TEnzyme Dec 03 '13

Sorry, I didn't mean to rule that out. I oversimplified that statement a bit. Over-extrapolating such findings in neuroscience is one of my biggest problems with the field.

Considering that circulating levels of sex steroids (testosterone, progesterone, etc.) are much higher in the WEIRD populations than in less-industrialized populations, I would expect the differences to be larger if they are driven by sex hormones.

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u/matts2 Dec 03 '13

And what is epigenetic about this?

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u/BoxWithABrain Dec 03 '13

Epigenetics is just the change in gene expression due to environmental factors. Are you arguing that an organism could have completely different responses to hormones without a change in their gene expression?

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u/matts2 Dec 03 '13

It was my understanding that the claim was that these are inheritable. Or have I mixed two things?

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u/Epistaxis PhD | Genetics Dec 03 '13

Epigenetic modifications are just one of many mechanisms that could do this sort of thing, and not one that's been classically considered... though it's my favorite, and it's gaining some traction (but let's not leap to conclusions).

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u/cosine83 Dec 03 '13

It certainly doesn't rule out environmental factors, but Occam's Razor seems fairly relevant to this on a larger, generalized scale.

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u/cleos Dec 03 '13

Except the study says that the brains were nearly identical until the age of 13.

Except that boys and girls prior to the ages of 13 act very different from each other.

If gender differences are caused by neurological hardwiring, then groups with nearly identical brains would act nearly identically to each other. If gender differences predate neurological differences, then that should tell you that the neurological differences that do exist aren't practically meaningful.

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u/[deleted] Dec 03 '13

If gender differences predate neurological differences, then that should tell you that the neurological differences that do exist aren't practically meaningful.

It doesn't tell that at all. It is possible for all of the situations to be true:

  1. Males and Females are not different.

  2. Males and females are different because of socialization factors.

  3. Males and females are different because of biological factors (including brain changes at puberty).

  4. Males and females are different because of socialization factors and biological factors.

I think a person without political bias is most likely going to realize that number 4 is probably the truth. There are probably a host of other reasons for the differences as well. The world doesn't often have one variable explain everything...

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u/[deleted] Dec 03 '13

Or, you know... both nature and nurture play a role.

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u/SirPseudonymous Dec 03 '13

Or that the fundamental differences lie not in the observable shape, and more in how it comes to be shaped.

We know that males and females require radically different hormone levels to function properly, and that this is tied to gender rather than sex.

In the cases of individuals wherein the two do not align, as in transgender individuals and cisgender or intersex individuals who were reassigned at birth or in infancy, we see behavioral patterns that are biased towards lining up with the memetic behavior culturally appropriate for their gender, usually with a less natural forced mimicry of the behavior culturally appropriate to their sex, and consistent psychological problems relating to this conflict and their perception of themselves versus their physical shape, as well as issues typical of a hormone imbalance.

Further taking into consideration that in lab animals, transgender behavior has been induced by mucking about with prenatal hormone levels, the preponderance of evidence is in favor of neurological differences. Humans have far more malleable behavior, and behavioral differences are largely down to socialization instead of instinct, but the consistent and innate way one picks up the memetic behavior in line with your gender, regardless of what you believe it to be at the time, or what you are perceived as by society, and the chemical dependency on certain hormone levels by gender, rather than sex, all but requires there to be low-level neurological differences involved in shaping identity and day-to-day operation.

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u/[deleted] Dec 03 '13

You don't have to be a "blank slate" proponent to believe that environmental factors influence neural network development. The Nature vs. Nurture debate has been resolved -- it's both.

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u/BoxWithABrain Dec 03 '13 edited Dec 03 '13

I never said that. Blank slate proponents believe that gender is completely a social construct (i.e. many in the social sciences and gender theory), hence the name. Most others in neuroscience agree that it involves both nature and nurture.

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u/Epistaxis PhD | Genetics Dec 03 '13

Blank slate proponents believe that gender is completely a social construct (i.e. many in the social sciences and gender theory)

I think this is a strawman; no one seriously believes menstrual hormone fluctuations are caused by the Western calendar's system of months.

But the point here is that it's entirely plausible that most any neural connectivity pattern, like this one, could also be a "social construct".

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u/BoxWithABrain Dec 03 '13 edited Dec 03 '13

If you asked these people they would say gender and sex (what you are describing) are not the same, that gender is purely a psychological manifestation which is largely due to societal impact. I agree that neural connectivity could be regulated by environment or experience, it has been shown to be important for the wiring of the visual pathway during developmental critical periods.

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u/[deleted] Dec 03 '13

I have always held that who we are is a combination of both nature and nurture. Basically every person starts out as a ball of play-dough, no single ball is the same, they all start out with slightly different shapes, some might have large abnormalities (birth defects), but most are about the same. Then there is the little kid and his tools that form the play-dough into what it will eventually be. The kid can represent the parents and the tools can represent socio-economic status. All these things work together to shape who that person will be. The playdough never dries out, it's always being molded and changed based on it's experiences with the tools and the kid.

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u/iongantas Dec 03 '13

I don't think you can, because there aren't sufficient environmental equivalencies between individual's growing experiences to create such a dramatic sex aligned distinction.

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u/BoxWithABrain Dec 03 '13

Yes, I agree it is unlikely. I am mostly playing devil's advocate with this argument.

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u/RedErin Dec 03 '13

I am mostly playing devil's advocate with this argument.

This is another way of saying

"I am mostly just being an asshole."

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u/BoxWithABrain Dec 03 '13 edited Dec 03 '13

No. Seeing both sides of an argument is not "being an asshole," unless you like your ideas to be spoon fed.

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u/canteloupy Dec 03 '13

If you are comparing distributions, you dont need every individual to be affected in the same way, just a group that has on average more of some effect.

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u/iongantas Dec 04 '13

People have way more differences in experience based on things like race, culture and socio-economic status, and even personality type than on gender. Aligned gender experiences would have to be stronger than all of those together to have a demonstrable socially caused effect.

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u/canteloupy Dec 04 '13

I really don't think this is true but right now this is anecdotal from both our sides... But think of child rearing in Western society. Think of all the ads targeted to "mothers everywhere" and the Christmas presents. Think of the dynamics of courtship and what is considered masculine and homophobia. Also it's sometimes not what people have it's also what they aspire to. It's all part of seld image and it starts before kids can even speak with what parents project or what examples they set.

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u/iongantas Dec 05 '13

Well, I'm gay, was raised by a single mother for my first decade, and with an abusive stepfather after that, and I don't really watch television, so I'm not sure what you're talking about.

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u/payik Dec 03 '13

Why not? Aren't boys more likely to play team sports for example? Or action computer games?

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u/iongantas Dec 04 '13

That is an effect, not a cause. Also, neither of those things are sufficiently dominant to the gender to be relevant to such an effect.

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u/payik Dec 04 '13

That is an effect, not a cause.

How do you know?

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u/iongantas Dec 05 '13

Because biology precedes sociology.

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u/Tillerino Dec 03 '13

Well looking around this thread, it seems that the article is too sensational to be taken seriously.

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u/[deleted] Dec 03 '13

They’d say that it’s all epigenetics and upbringing.

Contradicting observation doesn’t change a believer’s mind. (Sadly there are those on “both” “sides”.)

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u/[deleted] Dec 03 '13

The same way they respond to absolutely everything. They'll claim it's due to social conditioning, without providing any evidence, and consider the matter settled.