Are people with bigger brains smarter (on average)?
Here I will summarize some of the biggest studies which test whether there is a correlation between intelligence and brain size (within modern humans).
Thread.
2) In modern intelligence tests, you give individuals a battery of cognitive tests, testing a wide range of cognitive abilities at once. Then a general factor of intelligence can be estimated, in short "g".
Brain size can be measured with modern brain-imaging technology.
3) To address this question, McDaniel (2005) gathered 37 samples with a total of 1530 people who had been intelligence tested and brain-scanned.
He found that, on average, the correlation between brain size and general intelligence was 0.33.
4) In a later and larger meta-analysis of 148 samples and more than 8000 individuals, Pietschnig and colleagues (2015) estimate the correlation to be slightly smaller, but still highly significant, r = 0.24.
5) Quality of tests vary (e.g. Brief tests vs longer and more comprehensive tests).
Gignac & Bates (2017) reanalyzed the studies from the previous meta-analysis and found that the correlations were higher when higher-quality intelligence tests were used.
6) In fact, this is just a statistical phenomenon. When there is a correlation between two variables, then the better measured the two variables are, the higher the correlation will be.
Poor measurements contain lots of noise which weakens the relationship between the variables.
7) Instead of meta-analyses that pool results from many samples, we can also look at individual high-quality studies.
Ritchie and colleagues (2015) look at many brain variables in a sample of N = 672, but total brain volume and "g" correlated r = 0.31.
8) Dubois and colleagues, in a large sample (N = 884), test if resting-state brain activity correlates with intelligence. It does, but they also report other brain correlates. They find that brain volume correlates r = 0.33 with general intelligence.
10) To minimize possible effects of different environments people have lived in, you can look within families. Lee and colleagues find that, of siblings raised in the same family, the sibling with the larger brain tends to be more intelligent.
11) Another source of evidence comes from "genome-wide association studies." Here you take DNA samples from many individuals and see which gene variants tend to be correlated with some chosen variable of interest.
You can then calculate the genetic overlap two traits have.
12) Genome-wide association studies have now been done for many traits, including intelligence and head circumference (a noisy proxy for brain size).
One such study finds a genetic correlation of 0.31 between intelligence and head circumference.
In studies with high-quality measurements, the correlation between brain size and intelligence is roughly r ≈ 0.3.
This is a real, but not a strong relationship. If you want to estimate intelligence, test it directly rather than measuring just brain size.
14) But brain size is just a single variable. Other brain variables also correlate with intelligence. In principle, prediction of intelligence from brain imaging could likely become much stronger if we took multiple brain variables into account at once.
15) Brain size is probably the simplest brain variable to measure, and likely that's why it has been included in many studies. What other brain variables might contribute to differences in intelligence? That's for other research to establish.
Thanks for reading.
@rexjung And yes, it must be frustrating working with expensive research with highly noisy measurements. So I can understand why people would prefer working with structural imaging.
@DumoulinBaptis2 @JamesPsychol For example, take the study in tweet 9). This is likely the most representative. They find estimate of 0.276. If we divide by 0.8 for correction for attentuation (which seems reasonable, perhaps even too generous), we get 0.276/0.8 = 0.345.
So I don't think it's higher than 0.35
@DumoulinBaptis2 @JamesPsychol Find me an individual study (not a meta-analysis) that has a large sample (say, N > 500) and a good intelligence measure that finds correlation above 0.4.
These studies should be plenty if you think the *average* correlation is around 0.4.
@DumoulinBaptis2 @JamesPsychol And this challenge is still open. I'm genuinely interested. Because I've read this literature fairly closely, and from my reading the value is closer to 0.3 than to 0.4.
New genome-wide association study of brain volume. The genetic correlation between brain volume and intelligence they find is 0.24, quite close to other estimates given in the above thread.
The weird thing about articles like this is that if they had asked a demographer, they would've known that there's a consensus that immigration cannot solve the problem of ageing populations.
More precisely, the amount of immigration necessary to keep the population age down is extreme and unprecedented. Infrastructure wouldn't even be able to keep up.
Some right-wingers will post the image of this old 2000 UN report called "Replacement Migration" without reading it. It actually concludes replacement migration cannot realistically solve population ageing.
It's usually a good idea to read what you're sharing (insofar one is trying to be honest).
The "7 times" figure did not come from the study with 25,000 couples (that's citing an old study, which doesn't replicate). The new large study finds small differences.
In a previous thread, I tried to review the research on this question. Generally, sex differences tend to be small, and are not rarely in the opposite direction of the claim.
There's a study from Finland with ~130,000 couples, reviewing multiple illnesses and conditions, which finds an effect in the *opposite* direction as claimed.
It must be a total coincidence that they don't cite such results
A comparison between fertility measures in Denmark -- or why over-relying on "total fertility rate" is a mistake
This chart shows three fertility measures: (1) the most commonly used "total fertility rate", (2) completed cohort fertility, and (3) the "tempo-adjusted" TFR.
Completed cohort fertility rate is actually the "true" fertility. I.e., it's the actual number of children women have over their lifetime.
The "total fertility rate" uses snapshot data from one given year at a time.
This snapshot of the fertility rate is imperfect because it reflects the interplay of "tempo" (timing) and "quantum" (level) effects.
As women change their timing in childbirths it creates strong fluctuations in the total fertility rate.
This one's easy to address. It was actually taken by the PhD student Raymond Gosling, who worked under supervision of Maurice Wilkins and Rosalind Franklin.
2) Watson and Crick did not steal Photo 51.
Maurice Wilkins -- who was head at the lab where Rosalind Franklin worked, and supervisor of Raymond Gosling (who took the picture) -- willingly showed the picture.
In this thread, I will talk about the issue of youth immigrant robbery in Sweden.
The reasons for doing robbery range from feeling bored, to purchase cigarettes, to feel powerful, to wanting to "fight the Swedes", to wanting to humiliate their victims.
In the 1990s, a distinct phenomenon of youth robbery became prevalent in places such as Malmö and Stockholm in Sweden. It was almost exclusively young boys targeting other young boys. The perpetrators were usually foreign-born and the victims were usually not.
A 2000 report showed that reported youth robberies had sharply increased between 1995 and 1999.
A school victimization survey indicated that about 1 in 5 young boys had been a victim of a robbery (or robbery attempt) in the last 2 years.
So I decided to look at car accident deaths over time in the United States, 1999-2022.
Is there any evidence that the female death rate suddenly drops relative to men's? No, the male and female *trends* are almost identical.
In the 2000s, there is a strong decline traffic accident deaths overall. But this is independent of sex, so clearly this isn't caused by this narrow sex-specific factor.
The homicide rate also fell from 2000 to 2010, so perhaps it's a more general decline in recklessness.