Among adolescents of of low ability and older age, the Flynn Effect has reversed in the US, leading to worsening IQ over time.
The Matthew Flynn Effect: Flynn Effect (IQ scores > ~3 points/decade) + Matthew Effect (to those who have more shall be given, from those who have not, even what they have will be taken away). New study finds smart getting smarter, dumb getting dumber.
With n>10,000 US adolescents, study suggests disparities in cognitive ability may be widening with a positive Flynn Effect for above avg IQ and a negative Flynn effect for below avg IQ (and gifted women "benefiting" the most):