I would be very surprised if high intelligence was anything other than the extreme edge of a normal distribution of the human population. For it to be anything other than this it would require people of high intelligence to be a sub-population that did not breed with the rest of humanity.
'would require people of high intelligence to be a sub-population that did not breed with the rest of humanity' - which is increasingly what is happening with people of high ability & potential income of both sexes attending the same universities and mating. This polarization has been documented.
There are a lot of highly intelligent people at all levels of income distribution, so while there might be a polarization effect, it's not to the point of being segregation.
Intelligence can help you gain wealth, but if you're never given an opportunity or don't have a lot of drive you'll struggle perpetually.
Skimmed through that one a bit.. I think he misses that having a patronym surname (ending with son) is very much out of fashion in Sweden today (and has been for quite some time), so people change it. The people most likely to change it are probably found in the more "elite" professions the author lists. If you're a blue-collar it may feel a bit presumptuous to change your surname, you will be ridiculed by your coworkers. OTOH if you're an attorney, it's almost mandated, "no one" will trust you if you have a patronym surname.
> One thing we have to be wary of in this calculation of persistence is surname
changing. If people going to the university born with the surname Anderson were
changing this to Wigonius, then there would appear more persistence than there really
was. The biographical sources for some of the student nations at Lund and Uppsala,
Blekingska, Göteborgs, Skånska, Smålands, and Vermlands at Lund, and Östgöta at
Uppsala, allow us to estimate the fraction of Latinized surnames which were newly
adopted in each generation at the universities, since it gives fathers’ and mothers’
surnames for most students also. Figure 19 shows what fraction of students in each
generation inherited rather than adopted a Latinized surname.18 For the earlier
generations, 1730-1819, 96% of students acquired the name by inheritance from
their father. However, 1820-1909 that proportion fell to 88%, even though by
design these are all surnames that first existed before 1730.19 This will bias upwards
my estimate of b, but can be corrected for by calculating for each period a b based
just on the relative representation of the surname among the inheritors in that
period.
It would take quite a while for asortive mating to create a new human sub-population, but given the low fertility (through choice) of people with high intelligence such a sub-population can't form.
> For it to be anything other than this it would require people of high intelligence to be a sub-population that did not breed with the rest of humanity.
It is: we call it "social classes". Because intelligence correlates positively with success and because people tend to have children with other people from a similar social stratum, the exact situation you described emerges as a pattern.
My experience doesn't bear this out. Though the few extremely wealthy people I know seem very intelligent, I don't think intelligence correlates with income for the $15K-$200K annual income range. (This is the best proxy I know of for social stratum here in the Bay Area.)
I know very intelligent and very stupid people all over this income range, with no obvious pattern.
I don't think the question is whether "intelligence correlates positively with success". That may be true but doesn't reveal much about society, IMO.
The more interesting question is which correlations are strongest. I can't find a link, but I read a study that came out a few years ago that showed the biggest correlation with financial success was the parents' income levels. That is, controlling for the intelligence of the parents, an average-intelligence person born to wealthy parents was more likely to be financially successful than an above-average-intelligence person born to poor parents.
That may also be obvious, but it has more effect on the veracity of the common view of our society being a meritocracy.
> an average-intelligence person born to wealthy parents was more likely to be financially successful than an above-average-intelligence person born to poor parents.
> That may also be obvious, but it has more effect on the veracity of the common view of our society being a meritocracy.
Why? There's nothing about that that speaks to how much of a meritocracy we are. Parents' financial success directly measures everything that contributes to financial success (rather than, say, a subcomponent); you'd expect it to be highly informative.
The result you cite still holds when both children are adopted away. Is that support for the view of society as a meritocracy?
In a meritocracy, as I understand the idea, one's success is based on ability and talent rather than on class privilege or wealth.
If the biggest factor in your success is your parents' wealth, doesn't that suggest we might not have a pure meritocracy?
Do you have a link to the study? I don't remember the part about it still holding true when children are adopted away. That is an interesting finding. It would support your point (I think the point you're making, at least) that it's not necessarily parents using their class/wealth to bolster their children, but instead the parents have the proper mix of traits (intelligence, grit, ability to delay gratification, etc.) that results in financial success.
Honestly, I don't know. I haven't given the abstract concept enough thought or done enough research to have a well-reasoned opinion.
However, I do think it is "bad thing" for members of the elite to believe that we live in a meritocracy – when we do not, IMO. Skimming that Guardian article, I think my feeling is similar to the criticism put forth there. The notion that we live in a meritocracy creates a sense of entitlement and superiority in the elite. That is, if you believe we live in a true meritocracy, then those on top are there because they're smarter and harder working, and those on the bottom are there because they're less intelligent and/or aren't working as hard. Said more simply, meritocracy results in the feeling that you deserve your riches, and they deserve their poverty.
Whereas, in reality as I see it, your financial success is dependent on many factors outside of your control. Sure, most successful people are fairly smart and work hard, but many less successful people also have those qualities. The other factors contributing to success are things largely summed up by Warren Buffett's notion of the "Ovarian Lottery".
I'm not advocating a meritocracy. I'm advocating an honest assessment of society and the factors contributing to financial success.
I notice in that page it compares the number of "full" Ashkenazi Jews in the population, to the number of people who won various academic prizes who had "full or partial" Ashkenazi ancenstry, which seems like a very blatant distortion.
On reading slightly further I see someone else has already spotted this and added that disclaimer to the text.
Seems like it would be relatively easy to compare like with like here, which makes me suspicious of why it isn't done. Does that mean that the effect disappears when you do that comparison? It's certainly going to be reduced.
Even if it were true that Ashkenazi Jews as a group had above average intelligence, there just are not enough Ashkenazi Jews in the world to have a significant influence on the distribution of human intelligence.
Your second sentence is not logically implied by the first sentence.
It is not uncommon to have phenotypes that are expressed only rarely although the genes that code for the phenotype are widely dispersed. Many well known genetic diseases fall into this category.
Intelligence is like height in that it is multi-genetic (on the order of thousands of genes). Any trait that is determined by thousands of genes is going to show a normal distribution.
I understand the reasoning behind that... however the whole point of this study was to put this logical assumption under the rigors of actual science...
because this is an 'assumption' not a logical 'conclusion'.
I just said I would have been surprised if the result was anything other than they found, not that the study was not worth doing. For something as important as intelligence it is worth checking all of our assumptions.
This study does support what previous studies have found from when people looked for genes that had a large positive effect on intelligence and failed to find any.
I also don't think you can separate genetic influences from environmental influences, i.e. one allele might make you do well in one environment and poorly in a different environment. This appears to the case for the 7R allele of the DRD4 gene: http://www.northwestern.edu/newscenter/stories/2008/06/ariaa...
I also don't think any genes actually code for intelligence as it's commonly understood. Intelligence is, basically, knowledge (including knowledge about how to acquire knowledge). Genes affect brain chemistry, which influences intelligence in different ways, within a given environment.
The theory of multiple intelligence you link to is in strong disagreement with the mainstream position of academia.
> Intelligence tests and psychometrics have generally found high correlations between different aspects of intelligence, rather than the low correlations which Gardner's theory predicts, supporting the prevailing theory of general intelligence rather than multiple intelligences (MI).[19] The theory has been widely criticized by mainstream psychology for its lack of empirical evidence, and its dependence on subjective judgement.[20]
IQ tests only measure a few factors, which may well be quite closely correlated. But I don't think everyone who does well in IQ tests has good social skills, is good at painting, playing a musical instrument, speaking a foreign language, playing football, or juggling (or could become good at them with sufficient practice). You could argue that these have nothing to do with intelligence, but they're clearly cognitive skills.
Playing a musical instrument, speaking a foreign language, and social skills, are definitely strongly correlated with IQ-loaded tests like the SAT. Yep.
I have no idea how football fairs, but you can debate the usefulness of that as an objection to the validity of IQ with someone else.
Has anyone looked at how these studies correlate with the Big Five personality traits? It seems to me that intelligent people are more conscientious but that could just be my bias.
Which seems to provide some (weak) evidence against that:
"The hypothesis of a significant correlation between various Big Five personality traits and intelligence test scores was only partially supported. Only Conscientiousness was significantly related to psychometric intelligence, correlating with BRT scores. It is worth noting that the correlation was negative, indicating that higher conscientious
participants tended to have lower gf."
I'm far from an expert though, so I can't say how good that study is.
From the Wikipedia page you link on multiple intelligences:
> Intelligence tests and psychometrics have generally found high correlations between different aspects of intelligence, rather than the low correlations which Gardner's theory predicts, supporting the prevailing theory of general intelligence rather than multiple intelligences (MI).[19] The theory has been widely criticized by mainstream psychology for its lack of empirical evidence, and its dependence on subjective judgement.[20]
As for your other points...
> I also don't think you can separate genetic influences from environmental influences, i.e. one allele might make you do well in one environment and poorly in a different environment.
That's probably the case for some genes, sure. But finding strong correlations between twins raised apart would seem to indicate that many of the genetic factors are not entirely environment-sensitive.
> I also don't think any genes actually code for intelligence as it's commonly understood. Intelligence is, basically, knowledge (including knowledge about how to acquire knowledge). Genes affect brain chemistry, which influences intelligence in different ways, within a given environment.
Twin studies seem to contradict this. From Wikipedia (heritability of IQ [1])
* Identical twins—Reared together .86
* Identical twins—Reared apart .76
So yes, environment has an impact, but there's still a high degree of correlation in intelligence between identical twins raised apart.
The identical twins in these studies would have been reared in similar environments and cultures by different adoptive parents. The case I quoted were hunter-gatherer vs. farmers. Notice the difference. I was arguing, not that there's a genetic component, and an environmental component, but that the two are interlinked: different alleles can make different individuals well adapted in different enviroments.
In an modern developed economy, where you'll be more likely to do well if your IQ is high, some people will have inherited alleles which tend to raise their IQs and make them well-adapted to their environment, others different alleles which make them poorly adapted.
And, on top of that, there's an environmental, or rather cultural component.
> I would be very surprised if high intelligence was anything other than the extreme edge of a normal distribution of the human population. For it to be anything other than this it would require people of high intelligence to be a sub-population that did not breed with the rest of humanity.
Not at all. There could be a specific suite of traits that includes high intelligence, present in some people but not in most. Those people would have high intelligence, but they wouldn't be the extreme of the natural variation of the rest of the population. They would have gotten there by "cheating".
This is known to occur right now in human height. Men are taller than women. The difference is so pronounced that the human height distribution is not normal. The tallest humans (except Yao Ming) basically are the extreme of normal variation in men. But they aren't the extreme of normal variation in humans.
I hope you'll agree that "men" cannot be characterized as a subpopulation that doesn't breed with the rest of humanity.
Men can't be a sub-population by definition since men can breed with men. In regards intelligence both men and woman share the same mean. Given men have only one X chromosome we might expect that they might show a wider distribution in intelligence than women, but the evidence for this is controversial and airing it has a habit of losing you your job.
I should have written “can’t" not “can" which does rather mess up my response.
Actually height is unlike intelligence since there is one gene on the Y chromosome that has a very large influence on height. There are no such genes for intelligence as this study and many others has found.
You could make this argument for any trait. However, some traits are the result of a single gene - e.g., sickle cell anemia and the accompanying malaria resistance. Yet some of these traits occur in large populations that are not strongly inbred.
Only single gene traits. Intelligence (however defined) is multi-genetic - there are thousands of genes that contribute to intelligence. Given this the only way that individuals with high intelligence could be anything other than edge of a normal distribution is if they were part of a human sub-population.
Edit. I should add that the humans are not completely one population because of genetic isolation and differential selection (especially over the last 10,000 years), but we are almost a single population. Like everything in genetics it gets fuzzy at the edges.
"there are thousands of genes that contribute to intelligence"
And the very definition of "intelligence" is incredibly complex and slippery, which is one of the reasons why I've always found trying to summarise such a complex property into a single numerical value such a silly exercise.
Summarizing a complex property into a single numerical value is very useful and not silly. For example, temperature is a numerical summary of huge number of molecular motions.
I didn't say that all attempts to summarise a complex system in a single number are silly - just that intelligence is such a multi-faceted and ill-understood area that, in my opinion, evaluating people based on a single number is silly.
NB I say that as someone who got a very high IQ test result - which didn't exactly convince me that IQ tests are a good idea....
Intelligence really isn't "multi-faceted" (read Gardner's own admission that his theory never panned out) and it isn't ill-understood (refer to the Nature or Nurture interview with Nancy Segal on YouTube).
There's two reasons people say that. One is, they fared badly on a test and want to dismiss it, and the other is, they fared well on a test and are bashful about it.
Also, IQ tests are meant to measure a person's intelligence, not to convince them that IQ tests are "a good idea" - for that you would have to study Psychometrics.
IQ tests are somewhat ridiculous to begin with. The first tests that eventually became IQ tests were not intended to measure intelligence. They were intended to measure school readiness of rural children whose birthdates were often not known with certainty. The lack of clear birthdates meant an age cut off could not be used and rural children faced cultural differences from kids in the big city (namely: Paris) that created inherent challenges to them fitting in and doing well in school.
Intelligence is neither well defined nor well understood. It is fairly controversial stuff.
I worked on AI research for a number of years and my strong opinion is that our understanding of general intelligence is, as some wit put it, "pre-Newtonian".
What we mean in common language by intelligence may not be a single property. It might be more analogous to smell than temperature. An inherently multi-dimensional phenomena that can't be accurately described by a scalar measure.
It's actually not that difficult to turn a multidimensional measurement into a scalar quantiry; just represent the measurement as a vector composed of the deviation from median on each axis, then find the magnitude of the vector
But you're right that this doesn't tell you much about any of the individual dimensions. Maybe adding a variance across dimensions, so two numbers, would be more useful.
Speaking of which, have you seen the _Project Nim_ documentary on https://en.wikipedia.org/wiki/Nim_Chimpsky ? I would have to say that no, he did not surprise me, and the results were exactly what I would have expected after reading de Waal's _Chimpanzee Politics_.
Of course genetics plays a role in intelligence (a very important role). In the case of chimps they are not part of the human population so they are not really relevant to any discussion about the genetics of human intelligence.
Subpopulations can be physically and temporally distributed amongst the greater population. As long as breeding is not statistically independent of intelligence, there will be a tendency for intelligence-related genes to clump together.
This is true, but to get extreme outlier sub-populations for a mulit-gene trait like intelligence would require pretty effective isolation. For any sub-population to be large enough to affect the distribution of intelligence across the human population, yet be able to maintain this isolation, is very unlikely.
For mentally challenged people, researchers have done an analysis of the distribution of certain genes that are associated with the handicap. They have shown with high likelihood that mental handicap is actually different than just being on the low end of the intelligence distribution.
This study tries to answer the question, "is the same true for high intelligence?" The two general theories are the Continuity Hypothesis and Discontinuity Hypothesis. As its name suggests, the Continuity Hypothesis predicts that the high end of the intelligence distribution is continuous; extremely intelligence individuals don't violate the intelligence distribution the way mentally challenged people do. The Discontinuity Hypothesis predicts the opposite.
By analyzing the genes of twins and other close family members, the researchers found strong evidence of the Continuity Hypothesis.
BTW evolution theory suggests a priori that the Continuity Hypothesis is more likely to be correct.
Intelligence is selected for in humans. Therefore any individual gene that significantly boosts intelligence should be expected to have already spread through the population. As a result we should not expect to find any rare genes that make people super-smart. So super-smart people get there with a combination of different genes, each of which contributes very little.
The bottom end of the scale is the opposite story. Evolution says that individual genes that hurt intelligence should be selected against, and are therefore expected to be rare. (Mutation says that they should not be non-existent, but they should be rare.) Therefore there is no surprise in finding rare individual genes that significantly hurt intelligence.
This pattern is not unique to intelligence. It is predicted for any trait that has actively been selected for by evolution over a long period. The top of the range should look continuous. The bottom of the range tends to be dominated by deleterious point mutations.
Intelligence is selected for in humans. Therefore any individual gene that significantly boosts intelligence should be expected to have already spread through the population, or has a trade-off that reduces reproductive fitness.
The same can be said with drugs -- any chemical that your body could have reasonably produced on its own should be expected to reduce your reproductive fitness. So brain drugs should nearly all have side-effects.
I love how you accurately summarized a 10 page paper into a few concise sentences understandable most likely even by those on the low end of the intelligence distribution.
But he didn't; he stuffed the entire meat of the paper into 'By analyzing the genes of twins and other close family members'.
To actually try to explain the design here:
we think intelligence is caused by thousands of common genes, each of which slightly helps or hurts. When you have thousands of independent genes, then they add up to a normal distribution like the one we see when we measure a lot of people's intelligence. The exception is that there are too many retarded people; if the normal distribution was the whole story, then retarded people would be as rare as geniuses, but they are much more common than that. We explain there being too many retarded people by saying that they have one or a few very rare mutations, mutations which are very harmful. But another theory might be that the retarded have very few normal intelligence genes, or that intelligence genes don't simply add up but interact in complex ways, or that they have very harmful environments. How do we decide which theory is right? Well, we look at the retarded people vs their siblings; if their parents were lacking a lot of good genes, or they were being raised in a toxic waste dump, then we would expect the siblings to also be near-retarded themselves since they also inherited few good genes or are affected by the toxic waste. But they're not; they are almost average! This is more consistent with there being one bad mutation and the retarded sibling had bad luck than the other theories. This theory has since been confirmed by finding hundreds of unique and harmful mutations in retarded people. So we conclude that the effect of intelligence genes is indeed a normal distribution, with an occasional bad mutation overriding that and making someone retarded.
This immediately raises a question. If we admit that on the low end intelligence may be controlled by a single rare mutation, why not on the high end too? Maybe there are special genius genes floating around. This would be important because it means that you can't make much progress by just looking at the genes of regular people, and it also means that SNP studies will be extremely limited in what they can find. How can we check this? We can do the same thing as on the low end: if there is a special genius gene, then geniuses will have much higher IQ than their siblings do, who will be close to average; but if their parents have lots of good genes and there is no single special gene, then the siblings will also be well above average and similar to their genius sibling.
Using a very large set of siblings and twins, OP finds that very intelligent twins/teens are similar to their siblings. So this is the opposite of the retardation findings. There are rare genes for retardation, but there are not rare genes for genius.
This is what you would expect for any complex system under multi genetic control. A race car is a good analogy; it just takes one broken part to stop a race car from being able to move, but increasing the speed of a race car requires adjusting and fitting thousands of specific parts that work well together. A genius gene would be the equivalent to replacing a single part on a race car that made it go twice as fast.
> This is what you would expect for any complex system under multi genetic control
Not necessarily. This is something you might expect if intelligence is a net fitness advantage and it is in a mutation load situation where rare variants need to be purged to keep things constant. But if intelligence is only worthwhile up to a certain point and it is controlled by frequency-dependent selection, or there is heterozygote advantage, or if intelligence is not necessarily reproductively fit at all, or other situations, then there could certainly be rare variants of large positive effect. (I would not have bet on their existence for many reasons, but not because it's impossible.)
To give an example, your claim would predict that there is no such thing as a single mutation which increases muscle mass a lot because it's a complex system affected by a lot of genes; yet nevertheless, there is a single mutation affecting myostatin which makes humans and pigs and dogs much more muscular, and it's even been edited into pigs with CRISPR this year and last year into sheep and cows. Presumably the reason that not all animals are ultra-strong thanks to the mutation is that it causes birthing difficulties and increases metabolic demands considerably, and so despite the obvious advantages of being ultra-strong, it's not actually fit.
Of course it is possible that there are alleles with a large positive effects on intelligence, but is very unlikely because of the complexity of human intelligence.
To use your myostatin mutation example, the increase in muscle mass does not lead to significantly faster animals as the supporting structures are not there to utilise this increased muscle. Human intelligence is an emergent trait like speed determined by the co-ordination of many sub-systems.
These sort of emergent traits are almost never positively controlled by single large positive alleles. The one major exception are systems that are under different selection in males and females (e.g. height, plume color in birds, etc). In these examples two different systems have emerged controlled by the sex of the individual, but you don’t tend to find single genes that contribute massively in a positive way towards variation within each sex.
As an aside I remember reading a paper from long ago that suggested that high intelligence was the result of the relatively absence of mutant alleles at the various intelligence loci. When you look at the effect of null mutations at these loci the effect on intelligence is very low (less than a point for most). We all carry a large number of mutant alleles so the suggestion is that those with a high intelligence just happen by chance to have a lower frequency of negative mutant alleles. The interesting thing about this hypothesis is it would suggest that high intelligence is the default and low to normal intelligence is the result of mutational load. This is defiantly something that we can explore in the future as we get whole genome data from large numbers of people.
> To use your myostatin mutation example, the increase in muscle mass does not lead to significantly faster animals as the supporting structures are not there to utilise this increased muscle.
It leads to significantly stronger animals. That's not useless, for either prey or predators.
> Human intelligence is an emergent trait like speed determined by the co-ordination of many sub-systems.
We don't know what human intelligence is on a neurological basis. Imaging studies are linking it to a number of things, but it's still preliminary and so it's premature to baldly assert that there's no possible single mutation which might help.
> As an aside I remember reading a paper from long ago that suggested that high intelligence was the result of the relatively absence of mutant alleles at the various intelligence loci.
Mutation load hasn't been found to be strongly correlated with intelligence in studies which have looked at it directly, and the GCTAs already upper bound any such effect: "The total burden of rare, non-synonymous exome genetic variants is not associated with childhood or late-life cognitive ability", Marioni et al 2014 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3953855/
"A genome-wide analysis of putative functional and exonic variation associated with extremely high intelligence" http://www.nature.com/mp/journal/vaop/ncurrent/full/mp201510... , Spain et al 2015
Of course it is possible that there are alleles with a large positive effects on intelligence, but is very unlikely because of the complexity of human intelligence.
Some years ago, I read something that suggested otherwise. IIRC, it suggested that we had some idea of what alleles positively influenced intelligence based on the fact that Ashkenazi Jews were something like 2% of the population but had won something like 10% of Nobel Prizes and about 60% of the population had one or more alleles for serious genetic disorders known to impact neurology.
I remember this in part because, although I am not Jewish, I have one of the genetic disorders they tend to carry.
It is in principle possible for a sub-popualtion like Ashkenazi Jews to have a higher mean intelligence and hence many more individual with extremely high intelligence without there being alleles with large affects on intelligence. It would just mean that they have a higher frequency of the small positive effect alleles. More importantly, people have looked for such alleles in the Ashkenazi population and have failed to find them.
As far as I am aware there in no genetic disorder that only Ashkenazi’s suffer from that is not also present in the rest of the human population, the only difference is frequency.
It is in principle possible for a sub-popualtion like Ashkenazi Jews to have a higher mean intelligence ...
As far as I am aware there in no genetic disorder that only Ashkenazi’s suffer from that is not also present in the rest of the human population, the only difference is frequency.
Do you honestly see no possible connection between these two things?
That's a serious question, not intended as argumentative per se.
I think what you are asking is do I think there is any connection between the sorts of mutations found more frequently in the Ashkenazi population and intelligence and the answer is there may well be some, but the contribution for any one mutation is very small.
I don’t think anyone has shown that being a carrier for any of the common mutations found in the Ashkenazi population increases intelligence in a measurable way, but if anyone knows of such a study I would love to read it.
Yes Ashkenazi Jews are found disproportionally in professions where having a high intelligence increases the probability of success, however, no one really knows why. It is possible that it is genetic (I would be surprised if there was not at least some genetic component), but it could also be cultural.
Which lectures did you attend to learn about things like "net fitness advantage", "mutation load situations", "frequency-dependent selection", "heterozygote advantage". I always wanted to learn about these things. If you can refer me to an online lecture or a book I would be very thankful.
More or less the main crux is high intelligence is just a 'normal' manifestation of regular genes.
Take height as an example, most people tall or short have 'regular' genetics for their height. A bunch of different factors play together along with the environment to lead to the height they actually have. its basically like a 1000 different genes interacting that each may give you or take a way a fraction of an inch and all together they sum up to your actual height.
However sometimes you have people who are dwarfs, these arent just a combination of many genes, there is a overwhelming specific set of a few genes that totally overwhelm all the other height genes and give you greatly reduced height. Similarly on the other end of the spectrum you have some forms of giantism where a few genes overwhelm all the other height genes and give you greatly increased height.
This study is more or less saying high intelligence follows a model like regular tall height, many different genes interacting to sum up to above average height. Not a model like giantism or dwarfism where a very limited number of genes overwhelm the 'normal' genetics of intelligence.
The researchers did a study of Swedish military conscripts (98%) of male population. They tested if high-intelligence is as inheritable as general-intelligence. They found strong evidence that it does as siblings of high-intelligent soldiers also tended to more intelligent than the norm.
Similar levels of intelligence have been seen even in switched-at-birth twin cases where one twin is raised by one family and another by another. The IQ between the twins was in every case more correlated than the IQ of each twin and their family-siblings. Look up the interview with Nancy Segal on YouTube about Twin Studies and IQ.
They controlled for environment by considering the difference between non-twin siblings (< 2 years apart in age), fraternal (two-egg) twins, and identical (single-egg) twins. The correlation in intelligence between non-twin siblings and fraternal twins was similar, and both are significantly lower than the correlation between identical twins. The most plausible explanation for this is the genes that identical twins share.
They wanted to know what accounts for the difference between the high-intelligence group and the average-intelligence group.
> (1) we conclude that high intelligence is familial, heritable, (2) and caused by the same genetic factors responsible for the normal distribution of intelligence.
The 1st part is simple to understand, what is confusing everyone is the 2nd part... As the combination of both parts does not make sense (due to the type of "interpretation" presented).
Here is the clearer version -
(1) They found that intelligence was mostly hereditary (inherited via genes passed on by parents).
(2) They further found that the top scorers could also be divided into their own bell-curve. That the high-intelligence group had there own distribution that followed the same pattern (which gives the bell-curve even more validity).
To understand #2 just imagine a smart kid (in high-school) getting accepted into MIT, and once in, that smart kid finding out that he is now just "average" compared to some of the others.
These conclusions are very politically-incorrect, especially for a progressive country like Sweden where you are not supposed to even acknowledge that different dog breeds have different behaviors.
Point number two above is well taken. Also there's more 'stretching' at the higher end. For example you may find a couple of individuals who can do complex del functions on a variety of topics in their head and somehow seem 'inspired' whereas others specialize on one arcane topic and spend countless weeks feeding research data into matlab or Wolfram apps while scouring onscreen .pdf tech archives for suitable algorithms they can adapt. But at this stage of the game functional genomics is in its infancy (you can't yet feed a genome's UCAG sequence into your 3D printer and expect it objectify a true object). For fuller understanding of implicit nature / nurture parameters concerning intelligence it may be more practical to select and isolate a suitably large sample from the center of the general population, where (dy/dx) = -yx is relatively flat, and insert such into a distraction-free positive artificial environment which focuses primarily on intellectual development and includes cognitive enhancement medical supplements such as nootropics or whatnot to varying degrees. Results may suggest high-intelligence is more latent than suspected, especially if sub bell curve distributions manifest.
High intelligence (general cognitive ability) is fundamental to the human capital that drives societies in the information age. Understanding the origins of this intellectual capital is important for government policy, for neuroscience, and for genetics. For genetics, a key question is whether the genetic causes of high intelligence are qualitatively or quantitatively different from the normal distribution of intelligence. We report results from a sibling and twin study of high intelligence and its links with the normal distribution. We identified 360,000 sibling pairs and 9000 twin pairs from 3 million 18-year-old males with cognitive assessments administered as part of conscription to military service in Sweden between 1968 and 2010. We found that high intelligence is familial, heritable, and caused by the same genetic and environmental factors responsible for the normal distribution of intelligence. High intelligence is a good candidate for “positive genetics” — going beyond the negative effects of DNA sequence variation on disease and disorders to consider the positive end of the distribution of genetic effects.
What if being retarded and being highly intelligent aren't ends of the same spectrum? High intelligence seems much more situational than being retarded. A retardation is fairly objective and would stand out in almost any scenario, whereas high intelligence can be invisible when the subject isn't taking an IQ test.
