In a new paper in Personality and Individual Differences, Michael Woodley has made the first estimate of the rate of decline in intelligence which would be expected due to mutation accumulation, and puts it at about 0.8 IQ points per decade
He also calculates the rate of decline due to selection - i.e. the higher fertility of less intelligent people, and vice versa - and puts that at about 0.4 IQ points per decade.
And at the end he estimates the magnitude of decline in average intelligence due to recent migration and immigration as about 0.3 IQ points per decade.
Combining all these, leads to an overall current rate of intelligence decline of about 1.5 IQ points per decade - which is a lot!
To this must be added the effect of an ageing population (estimate pending...).
This is what we find!