Isn't IQ a comparative representation of one's standing within their contemporary age group?
My understanding is that within a given group the median should be 100. So you won't see it change between groups. This is highly relevant when you're talking about different groups being exposed to different concentrations of lead.
As an extreme example to make the point, if people born in 1970 were all exposed to high concentrations of lead and were all morons as a result, their median IQ is still 100.
Then people born in 2000 are exposed to far less lead and are super smart, but their median IQ would still be 100.
Points above or below 100 are merely a specification of how many fractions of a standard deviation above or below that median within the given age group a person's performance is measured to be.
That said, even within a group, 2.5 - 3 points seems largely insignificant as an individual's score might vary more than this depending on which day of the week they took the test. It seems a big stretch to draw any scientific conclusion from such a small variance.
You're right about how IQ is measured with 100 being the average and, generally, 15 points being a standard deviation. But numerous tests also record raw scores and these can be compared between generations. Scandiland and other places with compulsory conscription + IQ testing are the goldmine here. This is what led to the observation of the 'Flynn Effect' - the observation that IQ between generations was increasing, and later to its apparent reversal that generally started sometime around 1990 in the developed world. [1] That paper reports a later date, because it's about America in which studies on this topic lagged substantially behind Europe.
It's not easily explained by things like immigration since it is also present (though less pronounced) even within families. The hypothesis I find most compelling is that IQ levels have "naturally" been declining for decades, but improvements in nutrition, education, etc were helping to offset, and even rise beyond, these declines. But as nutrition, education, etc reach the point of diminishing returns, the declines dominate.
> The hypothesis I find most compelling is that IQ levels have "naturally" been declining for decades
You find compelling the idea that organisms naturally get dumber with each successive generation? What possible mechanism would cause IQ levels to "naturally" decline? Have they been naturally declining since the dawn of civilization? Since LUCA? Shouldn't the expectation be that without other causes, IQ levels would remain roughly constant throughout generations?
I think there are two distinct branches of reasons. The first is probably what you're asking about - what environmental factor(s) could be lowering IQ. And I think there are countless possibilities there, but many are quite subjective or at least unproven. The trendy one in academic circles is air pollution as a cause (seems uncompelling to me, unless China suddenly sees a dramatic decline in urban IQ). I'd also add endless entertainment, urbanization/industrialization creating low trust societies, labor swapping from widely skilled self employment to narrowly skilled employment, chemicals/microplastics/etc that we consume in food/water, and so on.
But there's also the most objective and straightforward reason as well - evolution. As far as evolution is concerned IQ isn't good or bad, it's just another highly heritable trait. When it correlates against fertility (as it currently does), IQs will decrease over time. When it correlates with fertility (as it likely did when life was more difficult), they will increase over time.
The problem with IQ is that it measures the cerebellum. People who lack abstract thinking will learn to rely on the patterns and statistics, and intuitively score high on the tests.
The second problem is that people who don't have abstract thinking don't comprehend abstract thinking, and so they think that people with abstract thinking are dumb.
The explanation is that IQ measures the cerebellum, and the declining nutrition and increasing brain damage forces people to rely more on the cerebellum, the statistical engine, instead of abstract thinking.
Hmmm, I could imagine microplastics, PFAS, artificial dies, excessive and unhealthy amounts of sugar would all have a deleterious effect on IQ. Perhaps even specifically pre frontal cortex, etc.
Though that goes against Europe hitting the decline earlier than the US which leads in sugars and unhealthy foods.
There is nothing wrong with sugar, people can't burn it, because the body needs lead to break it into pyruvate. Then you need arsenic to input that into the krebs cycle. The mitochondria need a wide assortment of metals (likely at least copper, arsenic, selenium, mercury, cadmium and possibly chromium) when you don't get those, your mitochondia begin to fail, and the tissues where they do turn into "fat tissue".
I suspect they don't do the research for real, they do some kind of simulation, and write it was in mice. They do know the metal changes the protein, but they incorrectly claim that the version without it works, but in reality the one with it does. They could'n make such an error if they actually did the research for real.
You're basically right about the relative nature of IQ scores, but you're wrong about the comparison being drawn.
To impute the effect of lead, you look at a bunch of people, measure the amount of lead in their blood, measure their IQs, and see how much of a difference there is between people with a lot of lead and people with less or none.
Modern poor people who live in crummy areas where there's still a little bit of lead are about as stupid, relative to the leadless elite, as poor people from decades past who lived in crummy areas which, at the time, had a lot more lead than they do now.
It seems like a safe assumption that the effect of lead on people with negligible lead levels has stayed constant over the decades at indistinguishable-from-zero.
But for lead to explain the gap between the lead-haves and the lead-have-nots, its effect must have increased dramatically over that same period. That gap hasn't changed. But lead levels have plummeted.
Isn't IQ a comparative representation of one's standing within their contemporary age group?
My understanding is that within a given group the median should be 100. So you won't see it change between groups. This is highly relevant when you're talking about different groups being exposed to different concentrations of lead.
As an extreme example to make the point, if people born in 1970 were all exposed to high concentrations of lead and were all morons as a result, their median IQ is still 100.
Then people born in 2000 are exposed to far less lead and are super smart, but their median IQ would still be 100.
Points above or below 100 are merely a specification of how many fractions of a standard deviation above or below that median within the given age group a person's performance is measured to be.
That said, even within a group, 2.5 - 3 points seems largely insignificant as an individual's score might vary more than this depending on which day of the week they took the test. It seems a big stretch to draw any scientific conclusion from such a small variance.