From Psychological Science:
The Nature and Nurture of High IQ
An Extended Sensitive Period for Intellectual Development
“IQ predicts many measures of life success, as well as trajectories of brain development. Prolonged cortical thickening observed in individuals with high IQ might reflect an extended period of synaptogenesis and high environmental sensitivity or plasticity. We tested this hypothesis by examining the timing of changes in the magnitude of genetic and environmental influences on IQ as a function of IQ score. We found that individuals with high IQ show high environmental influence on IQ into adolescence (resembling younger children), whereas individuals with low IQ show high heritability of IQ in adolescence (resembling adults), a pattern consistent with an extended sensitive period for intellectual development in more-intelligent individuals. The pattern held across a cross-sectional sample of almost 11,000 twin pairs and a longitudinal sample of twins, biological siblings, and adoptive siblings.”
It's usually okay to talk about IQ, as long as, IQ is not equated to intelligence itself. That seems to be the pitfall many fall into when mentioning IQ.
ReplyDeleteWhat kind of name is Turkheimer, anyways? It sounds confused.
Does the "cross-sectional sample" include all races, or only whites? If only whites, authors can probably get away with it. Article is behind pay wall.
ReplyDeleteRobert Hume
http://www.youtube.com/watch?v=_BMT6BfxR7w
ReplyDeleteAmy Chua as kid argue with Obama as kid.
A little off-topic, but
ReplyDelete1) Turkheimer has been a truly exceptionally rare name in the USA.
2) Just a hunch, but I keep wondering whether a family named "Turkheimer" might have played a rather prominent role behind the scenes in the old Ottoman Empire.
Or if it's just a coincidence.
> We found that individuals with high IQ show high environmental influence on IQ into adolescence (resembling younger children), whereas individuals with low IQ show high heritability of IQ in adolescence (resembling adults)...
ReplyDeleteWell, that's a clearly-stated hypothesis that immediately suggests certain observational and interventional studies. Head Start for teenagers.
More importantly, within the Cathedral's Who? Whom? Community, which factions want to know the answer? Which don't?
Can Steve or someone else explain the cryptic title of his blog post?
ReplyDeleteI don't get the Turkheimer reference. He has, among other things, argued that socioeconomic status moderates IQ heritability, but the cited study is rather orthogonal to that claim. Environmental influences may be more persistent both among the highly intelligent and the poor.
ReplyDeleteLest we forget:
ReplyDeletehttp://www.nytimes.com/2013/07/08/sports/beyond-the-finish-line.html?hp&_r=0
That's a lot of twins. How would we have studied such things were there not so many of them?
ReplyDeleteThe other side of that, I imagine, is how much more obvious the heritability would be were twins ten times as frequent?
But the study...if your cortex keeps growing longer, you get smarter. That seems hard to argue with.
Full text?
ReplyDeleteTurkheimer's result is that low SES households have lower heritability than middle-class ones. Did this study include low SES households?
ReplyDeleteThis sounds an awful lot like observations made in the late 19th century, for example this from the ninth edition of the Encyclopedia Britannica:
ReplyDeleteNearly all observers admit that the negro child is on the whole quite as intelligent as those of other human varieties, but that on arriving at puberty all further progress seems to be arrested. No one has more carefully studied this point than Filippo Manetta, who during a long residence on the plantations of the Southern States of America noted that "the negro children were sharp, intelligent, and full of vivacity, but on approaching the adult period a gradual change set in. The intellect seemed to become clouded, animation giving place to a sort of lethargy, briskness yielding to indolence. We must necessarily suppose that the development of the negro and white proceeds on different lines. While with the latter the volume of the brain grows with the expansion of the brainpan, in the former the growth of the brain is on the contrary arrested by the premature closing of cranial sutures and lateral pressure of the frontal bone."
The funny thing about the continuing debate over IQ and heritability is that IQ is all so simple.
ReplyDeleteFor decades we had a rather silly academic discussion of what IQ tests were and what they measured. We had disputes about if the Goodenough Draw a Man test measured anything real. Then we had long the controversy about if a reading based test could be culturally fair.
But all that's in the past. Toady we have the Raven's Progressive Matrices - simple (but not easy) puzzles. Level playing field puzzles. So IQ has come to mean simply puzzle solving ability. We no longer need the obscure philosophical discussions. We all know what a puzzle is and we recognize that some people are better at solving them than others.
We also now realize that some peoples have bigger brains than others. This is very lucky. It could be that the better puzzle solvers had some subtle brain chemistry difference that gave them their advantage, but much of their superiority is that their brains are just bigger and they therefore have more puzzle solving machinery. Simple.
We are used to buying computers with bigger faster components that compute better. No one is surprised that their PC with a quad core processor works better than their old single core processor. More RAM is better too. The analogy isn't perfect but it's close.
All the debate and argumentation should now end but it still continues because some people so desperately want to impose their personal view of the nature of mankind on a reluctant reality.
Brain functioning isn't all that different from the functioning of other organs. Guys with big muscles are usually stronger than skinny little guys. Similarly guys with big brains can solve puzzles batter than smaller brained men.
Sheer size isn't the whole story just most of it. So I'm losing my taste for journal articles like this one. IQ just isn't mysterious enough anymore.
Albertosaurus
Sounds like the government needs to identify hi-IQ or gifted children young and then give them extra support throughout K-12 education, and not put too many resources into slow children. Exact opposite of what they do and will do.
ReplyDeletePuberty is associated with the onset of the neural pruning that makes adults' brains more time-efficient but far less flexible.
ReplyDeleteThese results suggest what some people have been implying for a long time: later onset of puberty is associated with greater cognitive potential.
What would happen, I can't help wondering, if puberty were turned off and the neural pruning never took place? With advances in biotech, being able to reproduce directly might be almost obsolete.
So is what you're telling me that smart people seek out intellectually stimulating experiences that make them even smarter, while dumb people avoid those same things and remain dumb?
ReplyDeleteAstonishing!
Kevin MacDonald offered a cogent explanation in Turkheimer: Is Race Science Good for the Jews?
ReplyDeletehttp://www.theblaze.com/stories/2011/07/21/rahm-emanuel-storms-out-of-interview-after-being-asked-about-kids-school/
ReplyDeleteI haven't registered with the source site for the paper so I didn't read the whole thing, but if I am interpreting correctly what the researchers are claiming, they are saying the following:
ReplyDeleteDumb people stay pretty much at the same level of dumbness regardless of how enriching their environment is. Smart people can be made even smarter or somewhat less smart depending on how enriching their environment is.
http://www.nationalreview.com/corner/352863/best-sentence-ever-spoken-immigration-rich-lowry?utm_source=twitterfeed&utm_medium=twitter
ReplyDeleteBefore anyone starts talking about brain size and intelligence I'd like for someone to explain to me how much of the brain is dedicated to running the body and performing processing related to actually moving, as opposed to abstract thought and the stuff you measure on IQ tests.
ReplyDeleteTake an outfielder. He observes a ball's path and has to decide where to go to catch it. Some kind of processing is going on, though I doubt it is much like what a digital computer does to solve the problem. Plus some part of the brain controls things like breathing and the like.
How much of the brain is dedicated to what we would call intelligence?
Just my take on things, but I've always taken these brain size arguments with a grain of salt. Might be something there, and statistics can surely tell you something.
But I wouldn't think that all brain size differences are strictly due to associated intelligence differences.
Might not a person with outstanding coordination and dexterity have to process more data and make more calculations of a sort, than a clumsy physicist?
Just because it is something related to movement doesn't mean that calories and energy isn't spent doing it.
Speaking of outfielders, think of Andruw Jones, who was the best centerfielder in National League for many years, even though he wasn't exceptionally fast. He just got the best jump on where flyballs were going. Yet, he sure didn't seem too bright in an IQ test-sense. Similarly, Dennis Rodman was the best rebounder in the NBA because he could watch a shot's arc and anticipate better than anybody else where it would bounce off to. Yet, you wouldn't hire Dennis Rodman to do your estate planning.
ReplyDeleteI kind of seem how this result might contradict Turkheimer's claims, but it takes a little teasing out.
ReplyDeleteSuppose we take Turkheimer's claim to be that low SES children exhibit low heritability of IQ, and higher SES children exhibit high heritability of IQ.
Start out with the assumption that there are more low IQ children among low SES children, or, equivalently for these purposes, more high IQ children among the others.
Then, applying the finding of this study, the group of low SES children should have more of the high heritability children, since the group contains more low IQ children. Likewise, the group of higher SES children should have more of the lower heritability children, because it is richer in high IQ children.
Now of course how all that really works out depends on how the groups get defined in both studies, the actual numbers for the groups and their heritabilities, etc.
But the result certainly seems at first blush to go in the exact opposite direction from the claims of Turkheimer, since it is the higher SES children who would seem to exhibit the lower heritabilities, not the other way around.
Re: "So is what you're telling me that smart people seek out intellectually stimulating experiences that make them even smarter, while dumb people avoid those same things and remain dumb?"
ReplyDeleteYou generally don't see dumbasses looking into brain fitness or other kinds of cognitive maintenance/enhancement techniques. By contrast, people on the right side of the Bell Curve, notably transhumanists, seem to care more than average about these matters. Apparently you need to cross a threshold level of the general intelligence factor before you can understand that cognitive abilities have value.
How much of the brain is dedicated to what we would call intelligence? That's pretty much impossible to answer, since memory storage occurs all across the brain, and the neural efficiency associated with high-IQ individuals isn't a localized phenomenon.
ReplyDeleteIf you want to be more specific, the frontal lobes are profoundly associated with impulse control, synchronization of cognitive processes, and abstract learning: the so-called 'executive functions'. They're compared with symphony conductors for a reason: an orchestra can play a simple piece of music without a conductor, but as the complexity of the music increases the musicians can't integrate their performances. The frontal lobes are the brain equivalent.
Might not a person with outstanding coordination and dexterity have to process more data and make more calculations of a sort, than a clumsy physicist? As has been famously noted, given the complexity of visual processing and the size of the brain areas devoted to it, you use more of your brain watching COPS than listening to Mozart.
Apparently you need to cross a threshold level of the general intelligence factor before you can understand that cognitive abilities have value.
ReplyDeleteOr you need to possess enough talent in an area to set you apart from the herd before you become interested in developing it.
If you're one of the brighter kids in a class, you're motivated to develop your intellect. If you're the twentieth-brightest, you're facing a whole lot of competition. Better to focus on your strengths than your weaknesses.
Similarly, Dennis Rodman was the best rebounder in the NBA because he could watch a shot's arc and anticipate better than anybody else where it would bounce off to.
ReplyDeleteNah, Dennis Rodman just cared about rebounding more than other good rebounders. He just wanted the ball more and was willing to devote all of his energies to ball collection, rather than scoring. Thus, he was always in position.
Allegedly, Dennis Rodman also has a 7-3 wingspan even though he is often listed at a mere 6-7.
This study's findings actually seem quite ambiguous from the vantage point of those seeking to tease out any useful correlation between nurture vs nature.
ReplyDeleteThe study leaves us data and a hypothesis but is reluctant to pitch in to this HBD fray. The hypothesis is neutral and leaves it up to us, the reader, to supply the "why." The "why" is the elemental argument of nature vs nurture.
Essentially, what I gleaned from the summary is that "high IQs" are the product of a prolonged period of childhood cerebral development. This is the nuts and bolts. This has absolutely no bearing on whether we are "born" or "made" smart. We are simply told that the data demonstrates that very smart people's intellectual evolution continues as opposed to low-IQ'd people whose intelligence springs entirely from genetic origins for a small window of time.
I personally believe this only tells us that EXPRESSION of human intelligence (IQ) is the result of an auspicious brew of environment and genes, hardly a revolutionary idea.
The question is not if nature, or nurture, are more important. The real question is which one depends less on the other to flourish.
"Can Steve or someone else explain the cryptic title of his blog post?"
ReplyDeleteThere is a 'classic' TV show called "I Love Lucy." It involves the exploits of the titular Lucy, a housewife who gets into all sort of mundane trouble, e.g., getting a job at a chocolate factory and comically messing up the chocolate-making line via her incompetence.
The husband on the show was named Ricky Ricardo. He was played by Desi Arnaz, a Cuban-American musician of some renown who was then married to Lucille Ball, who played Lucy. In the show, he essentially played himself: a Cuban-American musician of some renown married to Lucille Ball. It was one of--if not the--most popular shows of the day, and he was a star.
One of Ricky's catchphrases was: "Lou-see! You've got some splainin' to do!" He would utter this catchphrase when he caught on to Lucy's shenanigans.
The title of the post is a play on this catchphrase. It posits that Turkheimer, the author of the article under discussion, has strayed too close to treating IQ as a real thing, and is therefore in violation of society's taboo against IQ-discussion. By phrasing the post this way, Steve is also subtly reminding his readership that affirmative action for Hispanics is BS, because there is no history of institutional discrimination against Hispanics -- after all, Desi Arnaz was a huge star back in the day, _and_ married to lily-white Lucille Ball, all with no repercussions.
"individuals with high IQ show high environmental influence on IQ into adolescence ... whereas individuals with low IQ ..." remain dim whatever you do with them.
ReplyDeleteYou'd almost think there might be a case for identifying the bright children at age 11 or so and shepherding them into schools with other bright children, where bright teachers teach difficult stuff at a briskly stimulating pace. Why has nobody ever thought of that?
It was practiced extensively in prince George's county , MD during the 50's and 60's. It was stopped w the advent of bussing. The school system then steadily devolved from one of the highest rated in the country to what it is today. I was actively involved from 1953-1971, so am speaking from first hand knowledge.
Delete@ 1:09 pm anon:
ReplyDeleteThis was a very entertaining explanation. I hope you direct documentaries.
Here's an example of how dumb people reason, from a 1995 NYT article on new federal 3-strikes law:
ReplyDeleteTommy Farmer was first convicted as an adult in 1970, shortly after he turned 18. According to court documents, two young men were in a Sioux City tavern when a Nebraska veterinarian entered the bar looking for a prostitute. The two men asked Mr. Farmer to come with them, and the three led the veterinarian to a nearby playground, where he was stomped and robbed of $10. Mr. Farmer admitted hitting the victim, and though he was not charged with inflicting the fatal blow, he was convicted of second-degree murder and sentenced to 20 years in prison.
"I hit that guy and left," Mr. Farmer said in a telephone interview. "Later, I heard the guy was killed. I am not a killer. I ain't never killed nobody."
Good outfielders only have to be smart enough to know that their ability to go get fly balls is instinctive.
ReplyDeleteOne of the oldest sayings in baseball is "Stop thinking, you're hurting the team!".
Candid_Observer says:
ReplyDelete"But the result certainly seems at first blush to go in the exact opposite direction from the claims of Turkheimer, since it is the higher SES children who would seem to exhibit the lower heritabilities, not the other way around."
Right. It's hardly definitive, just that the heritability effect is in the opposite direction of what you'd expect from Turkheimer's much celebrated study.
But the result certainly seems at first blush to go in the exact opposite direction from the claims of Turkheimer, since it is the higher SES children who would seem to exhibit the lower heritabilities, not the other way around.
ReplyDeleteHowever, surely that is because among those with low a SES you can actually find a reasonably complete range of parental IQs, while among those with a high SES, they pretty much only have high IQ parents.
Restriction of range, surely, among the high SES individuals.
Sheer size isn't the whole story just most of it.
ReplyDeleteThe correlation between intelligence and brain size is around 0.4. That doesn't seem to fit your assertion.
In the show, he essentially played himself: a Cuban-American musician of some renown married to Lucille Ball....
ReplyDeleteLatin American pop music had some popularity in the 1940's through mid '50's.
Here's Esy Morales and his Rumba band in the very good 1948 film noir "Criss Cross."
Just goes to show that they all hated Hispanic stuff back then.
Baby Boomer Bonus points if you can identify what TV comedy the dancing girl had a role in in the late 1960's. ( Note that this movie scene came before the Twistin' Time innovation of the early '60's.)
"...later onset of puberty is associated with greater cognitive potential."
ReplyDeleteSo, speculating wildly, could the Victorians actually have been more intelligent than we are today because society managed to somehow retard puberty to some degree, in some fashion, perhaps all those sex-segregated boarding schools, late marriage, taboos about obvious sex stuff, etc.? Or is that ridiculous? Is retarding puberty even possible?
(Weren't marriage times in England almost always pretty late by world standards?)
Wasn't there a suggestion recently that the Victorians might have been (probably insufficient data) as much as 20 IQ points higher than we are today?
Read any of the many letters of civil war soldiers available for study and compare to the capabilities of today's college graduate..... you won't need an IQ test to make assessments about intelligence levels then and now.
DeleteYou'd almost think there might be a case for identifying the bright children at age 11 or so and shepherding them into schools with other bright children, where bright teachers teach difficult stuff at a briskly stimulating pace. Why has nobody ever thought of that?
ReplyDeleteBecause that would be ELITIST!
So, speculating wildly, could the Victorians actually have been more intelligent
ReplyDeleteWhich "Victorians" do you mean? The royals? The nobles? The longshoremen and garbage collectors and railroad porters?
than we are today because society managed to somehow retard puberty to some degree, in some fashion, perhaps all those sex-segregated boarding schools,
These were intended not to make kids smarter but "manlier" and "womanlier". They were also intended to stamp out homosexuality, strange as it sounds.
late marriage, taboos about obvious sex stuff, etc.? Or is that ridiculous?
All the Victorian cultural baggage applied to the middle-class, and the newly-mobilized lower middle class most of all. The aristo's were freewheeling dudes, when they kept to themselves and had not need to set any examples. And the lower classes were even less restrained than anyone else.
Despite what you may think, the Victorian era was actually worse than the modern when it came to crime, violence, booze, drugs, prostitution, and "vice".
Is retarding puberty even possible?
With the right drugs, or lack of the wrong drugs (such as fluoride), it may be.
Dennis Dale:
ReplyDeleteLily Munster.
This suggests that the very low ability kids who need help the most are the ones least able to benefit from interventions designed to boost cognitive capacity, since their brains are less plastic. Meanwhile kids who already have high ability will benefit - a classic "Matthew Effect": "For unto every one that hath shall be given, and he shall have abundance: but from him that hath not shall be taken away even that which he hath".
ReplyDeleteYou can access the paper here. The interesting part of it :
ReplyDeleteAn alternative possibility is that having a higher IQ prolongs sensitivity to the environment. For example, heightened levels of attention and arousal, as one may find in individuals of higher IQ, may allow plasticity to occur later into development (Knudsen, 2004). A related idea is that individuals of higher IQ may be more open to experience, more likely to try things and change in response to experience, whereas lower-IQ individuals may be less motivated to engage with IQ-promoting environments, as they do not get as much positive feedback from learning experiences. However, this explanation also faces some challenges. The increase in genetic influence over development comes from both an increase in the importance of existing genetic influences and the addition of new genetic influences (Brant et al., 2009). If the extension of the sensitive period is a feedback process from increased cognitive ability, it is unclear how this feedback process would lead to a delay in the introduction of new genetic influences.
The most prominent theory of developmental increases in the heritability of IQ posits that across development, individuals gain more scope to shape their own environments on the basis of their genetic propensities (active gene-environment correlation), which causes an increase in genetic influence over time (Haworth et al., 2010; Plomin, DeFries, & Loehlin, 1977). Our results challenge this explanation, as they show a later increase in heritability for individuals of higher IQ. To explain our results in the context of active gene-environment correlations, one would need to posit, counterintuitively, that higher-IQ individuals seek out environments concordant with their genetic propensities later in development than do lower-IQ individuals.
The reason for developmental increases in the heritability of IQ thus remains unclear. Other possibilities include amplification of existing genetic influence by increasing population variance in cognitive ability and the simultaneous limiting of environmental influences and introduction of new genetic influences as a result of synaptic pruning processes and myelination at the end of the sensitive period (Plomin, 1986; Plomin et al., 1977; Tau & Peterson, 2010).
Candid_Observer said:
ReplyDelete"But the result certainly seems at first blush to go in the exact opposite direction from the claims of Turkheimer, since it is the higher SES children who would seem to exhibit the lower heritabilities, not the other way around."
Steve Sailer said:
"Right. It's hardly definitive, just that the heritability effect is in the opposite direction of what you'd expect from Turkheimer's much celebrated study."
Not quite. The Brant et al. study focuses on the developmental timing of the change in causal influences on cognitive ability. For higher-IQ individuals, the greatest amount of change in the magnitude of the shared family environmental vs. genetic influences on IQ occurs between adolescence (age range 13-18 in this study) and adulthood (ages 18+); for lower-IQ individuals, the greatest amount of change occurs between childhood (ages 4-12) and adolescence. Turkheimer's 2003 study examined estimates of genetic and shared environmental influences on IQ only during the childhood period--children's IQ scores were assessed at age 7 in his study.
In the Brant study, a figure presents a plot of the proportion of variance in IQ explained by genetic and shared environmental influences for low- and high-IQ groups at each developmental stage. During the childhood period, the estimate of the genetic influence on IQ appears lower for the low-ability group (h-sq.=~.43) compared to the high-ability group (h-sq.=~.57), while the estimate of shared environmental influence was similar for the two groups (c-sq.=~.22). Thus, these findings appear consistent with Turkheimer's study if we assume the higher-IQ group is higher-SES and the lower-IQ group is lower-SES. Of course, remaining differences could be due to methodological differences between the studies: Specifically, it’s not clear whether the samples used by Brant contain a large number of poor and minority children, and Brant does not examine moderation by SES.
So, speculating wildly, could the Victorians actually have been more intelligent than we are today because society managed to somehow retard puberty to some degree, in some fashion[edit]?
ReplyDeleteUnlikely. Most things that would force a delay in puberty would have pretty serious negative effects on brain development and cognition. Nutritional deficits, for example.
Now, it's known that children are hitting puberty earlier and earlier nowadays, but we don't know the reasons why. Speculations range from (our bodies try to avoid growth spurts and reproductive status until we have enough resources and we're so overweight that we're triggering everything early) to (artificial hormones given to cows transmitted in meat and milk) to (chemicals leached out of plastics disrupting endocrine balance).
Nor do we know if earlier sexual maturity is necessarily associated with earlier neural pruning, although it's certainly reasonable. There are suggestions that high-IQ people are more likely to have the pruning not work as it's "supposed to", so they're carrying around slower but more flexible neural nets that can adapt themselves more efficiently than those of normal, streamlined brains.
If high IQ is a possible effect of delayed neural pruning, AND entering puberty earlier is associated with early triggering of neural pruning...
It'd be an massive contributing cause to Idiocracy that's not (necessarily) genetic in origin. Put dysgenic breeding aside for a moment: the potential of kids overall in our society would be limited because a critical developmental stage would be hit earlier than before.
Being an "early bloomer" isn't always a good thing. Especially with those executive functions. People talk about how teenagers' brains aren't fully developed in the prefrontal regions, and act as though that implies they're impaired. But development of those regions takes time, and probably ceases or at least slows down significantly once those regions are done growing and have undergone pruning. If the growth was terminiated early, the kids wouldn't have stronger executive functions, they'd be weaker. Late adolescents show no difference in executive function than mature adults, even though their brains aren't "mature" yet, because maturity isn't the same thing as functionality. Make the brains mature earlier, and the development process might be truncated.
It's complicated. If high-IQ people are the ones where pruning isn't rigorous, it may not matter so much when the pruning takes place. For the majority of the population, though, the timing of the pruning would be quite important if it disrupted the development of higher mental functions. So we can conceive of a case where the cognitively elite aren't affected by scheduling shifts but the masses would be impaired.
This brain plasticity difference discovery is pretty old. But ofc useful when talking about iq
ReplyDeleteExecutive functions mentioned in comments are extraordinarily important and do not fully develop until 25. Ah, to be 24, in the glory of youth!
They are what we term wisdom rather than intelligence, but more predictive rather than experiential, and they are sourced in the frontal lobe, the newest and most advanced part of the brain, that which distinguishes ourselves from all other species
And these executive functions are the most dependent on sleep. Speaking of sleep..
Executive functions mentioned in comments are extraordinarily important and do not fully develop until 25. Ah, to be 24, in the glory of youth!
ReplyDeleteNo, that's been repeatedly shown to be false. The brain regions housing those functions don't mature until about 23-26... the functions mature significantly before then.
"So, speculating wildly, could the Victorians actually have been more intelligent...
ReplyDeleteWhich "Victorians" do you mean?"
The bit about Victorian intelligence is not a reference to Victorian culture, society, crime, schooling, aristos, etc., but a direct reference to Victorian IQ.
A few months back there was a paper that claimed existing reaction-time test data showed a sample of Victorians had higher IQs than people today. Their claim is based on data available since 1884. Of course, perhaps the sample was completely non-representative, perhaps testing equipment changed, or maybe something else is wrong.
Here's the summary from phys.org: "Researchers suggest Victorian-era people more intelligent than modern-day counterparts", phys.org, May 17, 2013, Bob Yirka.
The authors proposed a simple dysgenic explanation:
"...the team looked at RT tests given by various researchers during the period 1884 to 2004, and found that RT rates slowly increased over the entire time period. For men, the increase was found to be 183ms to 253ms; for women the increase was from 188ms to 261ms. The researchers claim this proves that people have grown "less clever" over time. They back up their claim by suggesting they know the reason for the decline in intelligence—smarter people having fewer children, while the less smart, have more."
(RT is "reaction time", which turns out to correlate with IQ in the type of tests they are talking about.)
Steve Sailor did a post about this, "Were the Victorians cleverer than us?", May 30, 2013.
Here is the actual paper online (behind a paywall, though, but at least the abstract, etc., is at the page):
"Were the Victorians cleverer than us? The decline in general intelligence estimated from a meta-analysis of the slowing of simple reaction time", Michael A. Woodley, Jan te Nijenhuisc, Raegan Murphy, Intelligence, 7 May 2013.
Here's the full paper online (pdf).
Their rough estimate is that Victorian IQ may have been about 14 points higher than today:
"...our value of a −14.1 IQ point decline is an estimate based on the best meta-analytical data available, a simple inspection of our figure shows there is a non-negligible amount of scatter..."
In their conclusions: "There are some limitations to this study. ...
In conclusion however these findings do indicate that with respect to ‘genetic g’ the Victorians were indeed substantially cleverer than modern populations."
Intriguing but "more work is required"...