I happen to be an identical twin, with a sibling born a few minutes after myself. While we are quite similar through facial features and such, we have quite different personalities, skills, even things like allergies and sexualities which one would expect to be more biological in origin. We’ve been involved in a few studies regarding these differences but the results are yet to be published as the sample is populated. If you are a twin, I’d encourage you to get involved in research like that if you have the time.
> even things like allergies and sexualities which one would expect to be more biological in origin.
It doesn't seem to be well-known, but concordance for homosexuality in identical twins is very low (if memory serves, around 40%) compared to concordance for virtually anything else you can measure.
This is pretty unsurprising looking at the biological fundamentals -- a quick naive approach would be to predict "genes will never code for homosexuality, because it is such a crippling failure for the organism" -- but it does tend to undermine the idea that homosexuality is biological in origin.
> but it does tend to undermine the idea that homosexuality is biological in origin.
Not genetic and not biological are two different things. E.g. the differentiator might be hormonal influence during the fetal stage. See https://en.wikipedia.org/wiki/Epigenetics for a long list of things other than DNA that can affect how organisms develop.
Just because something isn’t 100% determined by genetics doesn’t mean genetics doesn’t play a heavy role still. A concordance of 40% is still really high compared to random chance by sampling from the general population, so it arguably supports biological causes for homosexuality.
But we can see that the role of genetics is miniscule compared to its role in any other trait, since identical twin concordance for homosexuality is so low compared to concordance for anything else.
Genetics affects everything including the environmental influences you are subject to. But your genes are trying as hard as they can to ensure you are heterosexual.
How? If we dont have evidence for it should we just conclude that it exists because our maths does not say it doesn't.
Lets make witchcraft a part of medicine then. A statistically significant part of the human population belives in it. Lets get that into a 4 semester course in a 6 year medical degree.
25% of the variance in sexuality is genetic or heritable. Compared to approx. 80% for intelligence. A small genetic component, but it seems as if experiences are the major deciding factor for sexuality.
80%+ is what the current corpus of scientific knowledge says. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4270739/ Here's a somewhat good primer on the topic, but the wikipedia page on heritability of intelligence (and it's sources) is also somewhat good.
Presumably the only good data can come from siblings who were raised in environments dramatically different from each other and the ended up with similar IQs? It must be extremely hard to know what factors in the environment might contribute to IQ... saying the data is great sure, but I need examples of how environmental factors are removed.
I looked into it and the actual answer as with everything it’s extremely complicated and you’re original pithy 80% of IQ is heritable seems to be dependent on how much culture is involved in the testing methodology... https://blogs.scientificamerican.com/beautiful-minds/the-her...
I think the idea that we can control properly is extremely unlikely and IQ will turn out to be a lot difficult to extract from learning/society/environment than you might think, especially as findings with epigenetics and intelligence start to be shown. https://www.europeanscientist.com/en/research/epigenetics-an...
Your first link is a blogpost about someone's doctoral dissertation, and your second link is even more of a stretch (the title of the paper ends with a question mark). That is fringe research, and not the established position. You get what you search for.
As for cultural loading in intelligence tests, maybe you could expand on that? How do you measure how much culture was involved in the methodology, and how it affected the results of the tests?
There is no wikipedia page on heritability of intelligence, only heritability of IQ. It makes me consider whether the rest of what you've said is actually incorrect since you made such a conflation.
Intelligence is a notoriously hard thing to define and measure, and I think it's more easily predicted by environmental factors than genetics. The idea that smart people breed and have smart children and that's why some people are smart and others are dumb, is more of a cop-out, since predictors of success that tests like IQ try to explain are more accurately explained by socio-economic factors.
Basically this has no real world implications. I realise this isn't the argument you were trying to make necessarily, you were just pointing out a perceived 'fact', but it lacks a lot of nuance that could lead to dangerous assumptions about the real world, especially since you related it to sexuality.
If people demanded as much rigor for other subfields of psychology as they require for psychometrics of intelligence, the whole word would simply get rid of all psychology departments.
People have no clue how much more rigorous these studies than anything else in the field and adjacent fields.
Of course I demand rigor in a subfield that exists almost entirely to inform policies like segregation, eugenics, welfare, education, etc, and I find studies based on IQ tests and the g factor to be very much inadequate at being socially relevant.
My point is that you seem to not understand how rigorous is the field compared to academia as a whole and especially those fields that shape policy nowadays. In fact, intelligence research was always on the forefront of using cutting edge methodology and some tools that are used in psychology were invented specifically to research intelligence. Spearman invented factor analysis to measure cognitive performance, to this day this test is still used in behavior research.
If you think that IQ studies are inadequate, don’t even look at other fields that study humans (aside maybe from double blind trials in medicine), you are going to have a heart attack.
By your standards we need to ignore 90% of academia and solely rely on physics research to guide our policy.
I have completed over 30 ECTS credits studying econometrics in graduate school, I think I have enough grasp of statistics and factor analysis to not get schooled by some breadtuber who produces infotainment content for teenagers.
> Neither of your responses even dispute my points.
First you try to insinuate that IQ is not related to intelligence and even if it were intelligence has not practical application in the "real world".
Your first point was disputed by the comments related to how rigorously this subject is studied and documented, due to it being a third-rail area of academia.
I am going to dispute your second point myself.
That rigorous study and documentation associates IQ with intelligence and intelligence with 'g' which, although not precisely defined, is associated with (at least) higher problem-solving ability.
So now your argument reduces to "higher problem-solving ability has no practical application in the 'real world'".
And so I will just dispute that statement out of hand because problem-solving ability is what has allowed humans to become the dominant animal on Earth.
You're right to be suspicious. The studies that this figure are drawn from are generally twin studies based in wealthy Western nations. Heritability tends to increase with wealth, economic equality, and ethnic homogeneity, all of which are unfortunately selected for in the formation of these trials. This paradigm is thought to be a gold standard, as it reduces independent variables, but it fails to capture the effects of intergroup environmental disparities in a profound way.
The range of SAT scores at a US prep academy populated with the progeny of relatively closely related families will be substantially determined by heritable differences between individuals. Outside of that, it's disingenuous and irresponsible to throw out that 80% figure as a general measure of intelligence heritability. In fact, not even the studies say that: 80% is the measure for middle-aged and senior adults. For everyone else, even their measures are substantially lower.
>The range of SAT scores at a US prep academy populated with the progeny of relatively closely related families will be substantially determined by heritable differences between individuals.
Can you explain that more in depth? To me that is counterintuitive. If there is a group of closely related people, I would expect their differences to be due to environment, rather than genetics, because they have similar genetics.
The opposite. They're closely related compared to the rest of the world, but still come from different families, while living in substantially similar environments, which in many way have "maxed out" advantageous factors (health, stability, stimulation, etc.).
Remember that the heritable component of intelligence is polygenetic; it'd not determined by the presence of a single SNP, and two people could have similar intelligence originated in very different genetic profiles. People who are relatively closely related will have similar profiles that interact with the similar or shared environments in similar ways, meaning that the effect of small genetic differences will be more pronounced. Someone with the same "potential" for IQ, but from a different set of intelligence-related genes, interacting with the same environment might be said to have had the heritable component of their IQ become less efficacious.
For example, nutrition is a component in IQ. Everyone at this school is able to drink milk and benefit from the nutrition value-add. Someone less related to the other students also attends the school; they're lactose-intolerant, but benefit from eating certain fruits native to their homeland. The school doesn't serve such fruits, but it does serve milk. The student subsequently tests at a lower IQ. I would say at this point that variation between the LP students' intelligence is likely to be more related to genetics than is the variation between their IQ and this lone student's, which has a more substantial environmental component.
There are theories for how a low percentage of homosexuality might be selected for.
I have heard at least of the idea that a non reproducing sibling can help you raise your own offspring, who will have mostly the same genes. This is corroborated if I recall correctly by findings that each new child in a family has more chance to be homosexual than their already born siblings.
There is also the idea that there might be genes that are most of the time favorable to reproduction, but once in a while (through crossing or mutations) result in homosexuality.
Why would subsequent children get different gene mutations? I don't think it's a thing, it would be like saying that every new child tends to have thicker eyebrows or something.
Of course it is possible that statistically each new child in a family has higher probability to be homosexual for environmental reasons, but for genetic reasons? I don't see it (could be wrong, not an expert at all)
Edit after reading: apparently it's well studied and proven that each subsequent _male_ child has more chance to be homosexual and that "the fraternal birth order effect operates through a biological mechanism during prenatal life, not during childhood or adolescence"
> This is corroborated if I recall correctly by findings that each new child in a family has more chance to be homosexual than their already born siblings.
Very radical approach, seemingly against the grain of what we observe in the wild. It's such a leap of logic, you may very well have been talking about how santa claus' reindeer achieve thrust for their impressive levitation.
I'm not sure what you mean. Humans have been social animals for a very long time, so it stands to reason that they would be genetically different from non humans, although i'd expect similar results in other social animals.
> There are theories for how a low percentage of homosexuality might be selected for.
My initial response was to the above statement which to my eyes presupposes homosexuality to be genetically expressed. Which has nothing to do with social/environmental factors.
An example of similar results in other social animals are worker/soldier ants/bees/termites. They don't reproduce, but by contributing to the survival of their hive of relatives, they still end up spreading their genes.
Are they homosexual or asexual? Let's not confuse the two. They "cant" reproduce is quite a leap away from "can produce but won't" because as far as I am aware homosexuality has nothing to do with sexual reproduction and the required 'machinery'.
Unless you have some solid papers or anecdotal experiences that say otherwise I'm willing to change my stance.
Homosexuality is potentially bad for the perpetuation of an individual's genetic line (sexuality is a spectrum, and a percentage if not a majority of homosexual men and women would be open to procreation given a suitable social framework), but for communities, has the potential to be a net positive for survival; for example, it may reduce competition for procreative mating partners, and provide childless adults who can share rearing responsibilities.
In fact, one could say that the benefits of such an alternate sexuality were so recognized that some societies sought to produce it artificially through the creation of eunuchs.
Well no. I’m stupidly in love with my male son and I assume homo-sexuality is just a behavioral generalization into sexuality of same-sex “aggression de-triggers” and “attachment triggers”. It’s underpinnings are built into successful parenting (don’t murder your pups.)
Besides, close relatives in the animal kingdom (Bonobo) use significant amounts of sexual behavior in their sociality, including same-sex interactions. Unfortunately we evolved from chimpanzees, who have strongly abusive and hierarchical social structure. Whence, nasty homophobic comments.
We didn’t evolve from chimpanzees. Bonobos, humans and chimpanzees have a common ancestor. While chimpanzees are violent psychopaths, bonobos are peace-loving hippies who have sex all day. Humans seem to have a mix of both these traits.
It’s quite astonishing how much bonobos look and act like humans when they walk upright or when they play with their babies.
Correlation but with categories rather than continuous variables, for example as used in https://en.wikipedia.org/wiki/Kendall_rank_correlation_coeff... -- so "has the same sexuality as their twin" would be concordant, or I think what the parent comment is referring to is more specifically "is homosexual and their twin is too". In the general population, you'd expect a concordance of at most 10% (a high-ish estimate of the prevalence of homosexuality in the population), though of course that's conditional on the first person in the comparison being homosexual, otherwise it'd be much higher.
I'm very certain the parent meant it in an evolutionary sense that the individual will not pass on their genes to offspring - nothing to do with homophobia.
It seems like a very individual natural selection-based idea.
I’ve personally always wondered if there could be tribe-level selection for tribes with some non-heterosexual members.
But I also have personal experience, and I’ve met others who do as well, with life experiences that make me wonder if they could somehow affect sexuality. I know it might be presumptuous of me to consider the idea, but it’s only just an idea—one I usually keep to myself.
It seems obvious to me that your childhood and especially adolescent experiences would impact what you find sexually attractive as an adult. Is this considered a radical position?
The parent could have made the claim (that you believe they meant) without saying "crippling failure of an organism", and I would still think it was mildly homophobic:
- there's literally nothing to stop a homosexual person from biologically reproducing,
- it assumes that the only value of an organism is in its Darwinian fitness (its ability to reproduce), which is pretty gross. I reject this assumption.
The parent appears to be applying this heuristic of value only so far as it implies a moral failure of homosexuality: it's not a typical measure of value. Humans value other animals more highly than this, without regard to whether they have reproduced.
I too am an identical twin, and it's actually quite the opposite for us. My brother has nearly the same personality, skills, and medical ailments.
In my experience from meeting other identical twins in high school, you're either basically the same or polar opposites. There's rarely a set of twins that are in-between (although perhaps one could say the differences in the polar opposite twins become less pronounced with age).
This is pretty rare. Almost all test of twins that are in doubt are identical.
Don't have a source right now but I remember because we wondered if we should take a test as well. In retrospect (now 40+ years old and still looking very similar) it's hard to understand why we every doubted.
“I was born this way” is no longer the thinking w/r/t homosexuality. It was a short-lasting trope actually, probably more political than scientific in origin. Modern theories leave some room for genetics but nothing like what the public came to believe back in the 2000s and early 2010s.
There’s a lot more to being born a certain way than genetics though. For example the pseudorandom way neuronal cell interconnections are initially established. The dynamics of the way cells divide and specialise and how organ structures form in an embryo is a long way from being predictable or well understood in terms of the variation it might introduce.
It's certainly not the same argument it was, but there are still quite a few theories that involve genetics, epigenetics, or familial gene competition.
For instance, that certain gene combinations that may present advantages for not the individual, but the individuals mother, grandparents, or other family members.
I can believe it. My nephews are identical twins, and not at all identical. They look very similar, of course, and are nearly indistinguishable if they style their hair exactly the same and wear the same clothes, but they have very different personalities. One of them is even gay, but the other is straight. One is outgoing, the other is a quiet introvert, etc.
Colloquially the family has always called them mirror image twins, because coincidentally they had some early moles on their face that happened to be on opposite sides from each other, but IIRC the term actually means something else. They are in fact monozygotic.
Well look at that, I stand corrected. Or, correct to begin with, depending on how you look at it :). I misunderstood another definition of mirror image twins that I had read, but I see now that it does describe pretty accurately the differences between my nephews. Fascinating!
> The Swedish Twin Registry (abbreviated STR) is a twin registry based at the Karolinska Institutet in Stockholm, Sweden. Originally established in the 1960s, it is the largest twin registry in the world. It is widely used for medical research, with about thirty active research projects using data from the study as of 2019. As of 2012, it contained a total of 194,000 twins, 75,000 of whom were of a known zygosity. In principle, it contains records of every twin born in Sweden since 1886.
Piqued my interest. Didn't realise there were so many types of twins. Superfetation just seems incredible to me. I'm just thinking of the perfect combination to produce the maximum amount of offspring for a woman. A quick google says the greatest officially recorded number of children born to one mother is 69, to the wife of Feodor Vassilyev (b. 1707–c.1782), a peasant from Shuya, Russia. In 27 confinements she gave birth to 16 pairs of twins, seven sets of triplets and four sets of quadruplets.
If we involved superfetation in Feodor's nameless wife we could trivially improve the output by a huge amount.
I grew up with two identical twins from elementary school onwards, and it was always super easy to tell the difference. They were completely different people to me (and had really different personalities, which might have helped). In high school, another pair of twins arrived from one of the other middle schools, and I was never able to tell the difference between them, even after 4 years. Why is that?
I would guess is has to do with exposure and knowing them from an early age. It's weird how that works, but I imagine parents of identical twins can very easily tell the difference, so there must be some major difference. Recently, I met another pair of twins and I couldn't tell the difference between them either. It must be extremely nuanced and only obvious to people who have been around both twins for a while. I would guess that it would be easier as twins get older and diverge in physical ways (body fat, muscles, and other physical traits).
In my anecdotal experience it became harder as my twin friends grew older. They were not identical twins but many people said they look similar even though I never had an issue in recognizing them. After I returned from a one year trip abroad I met up with one of the twins. I knew which one I was meeting, but when I get into their car I was confused why the other twin was picking me up. It took me a solid minute of talking with him before I realized it actually was the twin I was supposed to meet up with. I now recognize them better again but still occasionally have a splitsecond of confusion. Their facial features, silhuetes and fashion has grown to be very similar.
The article mentions, nature versus nurture debate and that's complete nonsense. Any scientist should know by now, all forces factor into an equation for whatever outcome. So the phrase "nature versus nurture debate" is annoying to read and I assume it generally confuses the public. Similar to how doctors used to say someone has a chemical imbalance in their brain for the cause of whatever illness the person is suffering from. I feel like until people understand "cause & effect" an ignorance is going to continue existing and that really misleads the general populace towards incorrect assumptions or even magical thinking.
You can see this in studies that try to determine how much some traits are inherited .
Complex traits usually converge towards 50/50 because that's what you expect when the question is framed wrong. Different genes activate based on input from the environment.
> Different genes activate based on input from the environment.
Precisely. Traits feed back and feed forward, etc. You cannot simply say 30% genes and 70% environment because you cannot ignore how they interact with each other.
Did anyone really think that twins were the same right down to each individual gene? Just seems intuitive that there'd be some slight differences between identical twins.
Yes, it indeed happens on a non-chimeric person (just try comparing the DNA from a muscle in arms versus legs, or even head hair and genital hair). I don't know the point of the study (except if it is just to undeniably confirm this scientifically because some doctors still don't believe that identical twins are not quite identical).
I would assume that the point of the study is to also quantify the amount of mutations so that future twin-based studies can more accurately attribute differences that also exist between twin pairs.
Would the microbiome be similar growing up (assuming both are living and eating the same foods in the same places/family) and then diverge later when they go their separate ways?
Well, I can give one well-known example for this in genetics. Females carry two X-chromosome, against male's X and Y. There is one thing called X-inactivation, which means one of the X chromosome in the cell is inactivated randomly in cells. By this, even organs only carried the same kind of cells are mosaic in aspect to genes in X chromosome. Then think about a well-know X-chromosome linked disease, the Rett Syndrome, and the cell in body, in the brain.
Why "intuitive"? It seems far from obvious that they'd be different when they come from the same sperm and egg cells.
Does "anyone" really think that? I, who's well more versed in sciency things than the average although I never really thought about this particular subject, would have guessed that they have the same genetic code. So yes, I'd guess that a big proportion of people would think that
This podcast with Michael Blastland about his book The Hidden Half touches on the topic of twin differences and how it shows how much more there is to discover and learn. The discussion of ostensibly genetically identical Marmorkreb crawfish is especially interesting.
My best friends growing up were identical twins. They both went to the same university. They both got the same degrees. Both PhDs. They both practice psychology in the same major metro. They both married men with the same _ultra specific_ backgrounds. That's where my knowledge of them ends, I haven't spoken to either of them in a few years -- I should reach out.
I have fraternal twins. They are not at all that similar! Not in looks, or size, or hair color, or personality. They're only ~2 years old and the differences between them, both physically and mentally, is so staggering, already.
Completely insane mechanism that alters DNA of our cells in their lifetime in useless and dangerous way just because for DNA/RNA sequence to survive and spread it's sufficient that is figured out how to copy itself.
I have young identical twin sons. I discussed this with them, then pointed out that five mutations in a genome of 3 billion base pairs wasn't massively significant..
It's my impression that identical twins do actually have similar (but not identical) fingerprints. Specifically, fingerprint patterns can be broken down into a few broad categories like 'loop' or 'whorl', and I've heard identical twins will often have the same patterns on the same fingers.
Isn't cell division more or less a random process? So even if the genetic code is similar, it can result in different cell patterns. I.e the process from DNA to cells isn't fully deterministic.
For example, if you plant identical trees, will they grow their branches identically, even in a very strictly controlled environment? I guess not.
I never understood why people act like the expression of DNA is a deterministic system unaffected by external inputs, as if it were just some code being compiled. It's obviously a hugely complex emergent process that can be affected by environment and even just random chance.
Expression of genes is also driven by the external environment. Different environment (even minor differences) can result in different phenotypes despite the same genotype.
That was the theory proposed as to why identical twins have different fingerprints. Slightly different conditions in the womb result in slightly different fingerprints.
> They measured mutations that occurred during embryonic growth and found that identical twins differed by an average of 5.2 early developmental mutations. In 15% of twins, the number of diverging mutations was higher.
Though sometimes it takes awhile to see the differences. My daughter has friends who are identical twins and it took her years to be able to reliably tell them apart.
