In The Idiot Brain: A Neuroscientist Explains What Your Head is Really Up To, Dean Burnett write that:
"the average IQ of a population is 100. Without exception. If someone says, “The average IQ of [country x] is only 85,” then this is wrong. It's basically the same as saying, “ The length of a meter in [country x] is only 85 cm”; this is logically impossible, and the same is true for IQ." (cf https://books.google.fr)
I think this is misleading. He is right to write that the average IQ of a population is 100 (as explained here). But it doesn't imply that it's "wrong" to say that “The average IQ of [country x] is only 85,”. If one takes the whole word population as the population then one can then calculate the average IQ of the people in a particular country.
But then if it's how countries' IQ are measured, why so few countries have an IQ above 100? There is only 15 countries with an average IQ of 100 or above (the max is 108), and so many countries are below 100, the lowest being 59. Even with China (representing about 20% of the world population) pulling the world average IQ closer to 100, the countries' IQ given on that page are not coherent with an aveage wold IQ of 100.
So how are the countries IQ measure? My question isn't about why the average IQ of a population is always 100 but about the discrepancies between countries.