This meta analysis in 2013 was made to answer the question
Were the Victorians cleverer than us?
The decline in general intelligence estimated from a meta-analysis of the slowing of simple reaction time
They were
>It should also be emphasized that whilst our value of a −14.1 IQ point decline is an estimate based on the best meta-analytical data available, a simple inspection of our figure shows there is a non-negligible amount of scatter around the regression line.
>The real magnitude of the effect might therefore be several IQ points lower or even higher.
>In conclusion however these findings do indicate that with respect to ‘genetic g’ the Victorians were indeed substantially cleverer than modern populations.