Before going any further, we've got to look at the original reason why IQ scores climbed steadily for decades, and the specifics of the Flynn effect. As James Flynn describes in his TED Talk and sites like ScienceDirect summarize, increases in IQ scores are due to an overall increase in fluid intelligence or creative problem-solving abilities applied to novel or abstract situations. The opposite would be crystallized intelligence, the skilled use of learned information over time, as Verywell Mind explains. 

To illustrate, Flynn describes a doctor of 100 years ago versus a doctor of today. Modern professions have to incorporate more and more complex information. Society, on a whole, also depends on a greater degree of challenging intellectual tasks like those performed by engineers, technicians, programmers, teachers, lawyers, etc. To demonstrate, Flynn states that in 1900, only 3% of Americans practiced intellectually demanding work. Nowadays it's 35%. 

This might make it seem like crystallized intelligence — intelligence based on quantities of learned information — ought to be on the rise, but no. People aren't remembering more information. In fact, Flynn calls younger people an "ahistorical" generation. Rather, folks have gotten better at particular pieces of IQ tests, like Raven's Progressive Matrices test and the similarity subtest of the Wechsler Adult Intelligence Scale. This indicates an improvement in abstract thinking, hypothetical reasoning, and cognitive flexibility. This is why overall IQ scores have risen, a trend defined as the Flynn effect.

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