My point about “crainial capacity explains everything” is about how purealpha’s argument works, not about what the data says.
Purealpha says that the correlation between cranial capacity and intelligence is 0.4. It would be good to get a reference to where that data point comes from, because I have no idea if cranial capacity is being measured between adults of different geological eras, human adults vs human children, humans vs primates or other animals, or if they gave human adults IQ tests before cracking their skulls to see how big it is inside. We don’t know how they’ve measured intelligence, whether it is IQ or how many dots they were able to engrave on a stone in 1 hour, but we do know that it must be some kind of a quantitative measure, because otherwise a correlation coefficient would be completely the wrong statistic to use.
Even if the correlation is indeed +0.4, that suggests that 0.16 or 16% of the variance in intelligence is explained by cranial capacity, which leaves another 84% explained by other factors. That seems to be a lot of “other factors” that can go into determining whether we are getting smarter or getting dumber as a genotype (there, I used a big anthropological-sounding word). So cranial capacity may have diminished a bit (we haven’t established how much).
Even if the correlation were exactly 1, it is still quite possible that the line it lines up on is IQ = CranialCapacityInLitres * 0.000000000000000001 + 100, and you would still get a correlation of 1.
So we would have to see this study you have. Since you’ve spent so much time studying and it is at the center of your argument, please give us a link so we can see if it makes any sense for you to use the results.
The challenge with evolutionary explanations is that one can casually come up with evolutionary explanations to argue just about anything, just as one can easily invent reasons for almost any correlation between variables chosen at random.
The way to be sure that your theory or explanation is right is not to sit there and say “I’ve studied it, and it makes sense to me,” but to compare the explanation with alternatives and see whether it has more evidence or than the others, or if there are things that it predicts that have not happened, but which other things do.
We all have pet theories, and yours is not implausible, but you have not presented data that gives strong support to it, and you are busy saying that other explanations about intelligence aren’t correct, because you are asserting yours is the dominant explanation for the original observation that average IQ test score has declined over the past 60 years.
If that’s true, and it is driven by a decline in cranial capacity, and the other explanations that people have been floating are truly bogus, the way you’ve been arguing, that that suggests that the % decrease in cranial capacity in the last 60 years should be approximately (% change in IQ) / CranialCapcityBeta.
For a 9% decline in intelligence, assuming a cranial beta of around 1, if your argument is driving this, we should see a decline in cranial capacity of around 9%. That should be something easily observable, and not hard to verify even if the cranial capcity has declined by half that since 1950. If the beta is less than 1, the decline in cranial capacity should be even more pronounced and visible. If beta is greater than one, then it should be noticeably obvious to the casual observer that people with bigger heads are more intelligent, the way it’s obvious that people with bigger biceps can generally lift more, and really it would have to be greater than about 2 or 3 before we might start to think that cranial capacity had declined in the last 50 years unnoticed (by doctors at least).
But this all assumes that the correlation between cranial capacity and intelligence is actually substantively meaningful, when - even if the 0.4 correlation is correct - 84% of the variation in intelligence is still determined by “other factors,” which seem much more likely to be the right explanation for what’s going on since 1950.
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Perhaps our cranial capacities are declining since the agricultural revolution. It is true that anthropologists find that the hunter-gatherer diet was more nutritious and varied than the early-agrarian diet, which had a tendency toward sesonal monocultures. The idea was that you could sustain a larger population with less overall labor and greater reliability of food sources this way, but it was a nutritional step backward. It was a trade off that may well have affected our brains somehow, but it also permitted specialization of human intellects that brought us forward and created things like writing and engineering and science and all the stuff that we actually think of as being signs of human intelligence.
It’s not that your theory is completely implausible (other than as an explanation for the decline in intelligence test scores since 1950), it’s just that there are too many other confounding factors for you to dismiss the other explanations the way you have.
But we all have our pet theories, and now we know yours.