IQ tests are administration-sensitive and have changed dramatically since the beginning of such a Flynn effect study. The population makeup of many countries has changed in recent years to include many immigrants for whom the study would make exceedingly little sense to include. IQ tests do not cover and do not claim to cover a comprehensive view of human intelligence, famously lacking verbal and social components entirely. It is possible past IQ tests were simply overtuned and we’re now seeing the natural correction.
