The Flynn Effect is the weird tendency of raw scores on IQ tests to go up steadily around the world, necessitating test publishers to toughen scoring periodically.

But it seems implausible that IQ scores could go up forever. James R. Flynn points out that the Flynn Effect seems to be over (or perhaps just on pause) in a number of countries (but not in the U.S.):

Intelligence

Available online 8 December 2017 IQ decline and Piaget: Does the rot start at the top? James R. Flynn, , Michael Shayer Important national differences, particularly the contrast between Scandinavia and elsewhere. Dutch trends show that IQ gains vary by age which is indicative of the strength of various causal factors. Piagetian trends provide information conventional tests do not: that the largest losses may be at the top of the curve. Abstract

The IQ gains of the 20th century have faltered. Losses in Nordic nations after 1995 average at 6.85 IQ points when projected over thirty years.

Or a decline of 2.3 points per decade.

On Piagetian tests, Britain shows decimation among high scorers on three tests and overall losses on one. The US sustained its historic gain (0.3 points per year) through 2014. The Netherlands shows no change in preschoolers, mild losses at high school, and possible gains by adults. Australia and France offer weak evidence of losses at school and by adults respectively. German speakers show verbal gains and spatial losses among adults. South Korea, a latecomer to industrialization, is gaining at twice the historic US rate.

South Korea also saw a massive increase in average height. It would be interesting to compare height and IQ changes over time by country.