It seems very probable that general intelligence (or 'genotypic IQ') has declined by more than one standard deviation since late Victorian times, and presumably even more since about 1800 when the Industrial Revolution began to become obvious and these changes probably began.
But what was the cause?
Using reaction time data, the decline in genotypic IQ is of-the-order of 1.5 IQ points per decade - that is about 15 points in a century - or one standard deviation.
(This rough estimate of the size and rate of decline in 'g' has been replicated by a more sophisticated and completely different - as-yet unpublished - analysis of reaction time trends that I have seen.)
In other words, the average Englishman from about 1900 would be in roughly the top 15 percent of the population in 2000 - and the difference would be even larger if we went back further towards 1800.
These numbers are not intended to be precise - indeed real precision (in the sense of exact accuracy) is not available in IQ studies for many reasons to do with the difficulties of truly random and sufficiently large population sampling; and the fact the IQ points are not on a 'ratio scale' but are derived from putting a population sample into rank order on the basis of (usually) one-off testing.
But anyway, I think that a decline of 1.5 IQ points per decade is probably too fast to be due purely to the effect on gene frequencies of differential fertility between people of different intelligence levels.
No doubt the measured decline is substantially to do with the fact that higher intelligence is correlated with lower fertility; but within this, I think there must be at least two explanations operating at the same time.
Differential fertility would lead to a decline in intelligence - let's say - by a reduction in the proportion of high IQ genes in the population.
This happens mostly because since the Industrial Revolution almost-all children that are born will survive; so reproductive success becomes almost-purely a matter of fertility; and the most intelligent sectors of the population are the least fertile, and less fertile with each generation; until eventually (i.e. for the past several decades) the most intelligent people are sub-fertile, below two offspring per woman - so that the genes which make them most intelligent will decline with each generation - first declining as a proportion of the gene pool, and then declining in absolute prevalence.
My suggestion is that the additional mechanism of decline in intelligence is the opposite of the above: an increase in the proportion of low IQ genes in the population.
There is, I suggest, a difference between high IQ and low IQ genes.
High IQ genes have (presumably) been selected for in the past because they increased intelligence, and thereby (under ancestral - especially medieval - conditions) increased reproductive success.
But low IQ genes are spontaneously occurring deleterious mutations. These were not 'selected for'; rather it was a matter that selection failed to eliminate them.
In technical terms, the mechanism for low IQ genes is mutation-selection balance.
The idea is that before the Industrial Revolution, individuals with a higher mutational load had lower-than-average reproductive success due to very high (near total) childhood mortality rates among those of lowest intelligence.
But after the Industrial Revolution got going, and mortality rates declined for the least intelligent so that even the poorest families usually raised several-to-many children, then there was a double-whammy dysgenic effect: a reduced proportion of high IQ genes with each generation (due to progressively lowering fertility among the higher IQ) and also an increasing accumulation of IQ-damaging deleterious mutations with each generation.
So that (roughly speaking) since the Industrial Revolution, individuals with the greatest mutational load (IQ-harmful genes) have been initially been above-replacement fertile (having on average more than 2 surviving children per woman, for the first time in history perhaps), and also differentially more fertile than those with the least mutational load.
So that compared with 150 years ago there are a lower proportion (and a lowering absolute amount) of IQ-enhancing genes in the gene pool of England, plus a higher proportion and accumulation of deleterious IQ-damaging mutations.
And this double-whammy effect is, I think, why the general intelligence has declined so rapidly and so much in England over the past couple of centuries.
NOTE: The focus upon accumulation of deleterious genetic mutations due to relaxation of the selection effect of childhood and early adult disease and mortality (which had previously served as a sieve of strongly fitness-reducing mutations) was something I got from Narrow Roads of Gene Land: evolution of sex, Volume Two of WD Hamilton's collected papers. One aspect is that such relaxation of selection is probably unprecedented in human history - indeed, it is possible that recent post-Industrial Revolution conditions may have generated a positive selection in favour of deleterious mutations (amplifying their frequency, at least up to the point when they become fatal or induce sterility). At any rate, the quantitative effect of this process of accumulating deleterious mutations in a population is only imprecisely measurable (I believe); whereas the quantitative effect of differential fertility on differential IQ is pretty well understood, and fairly precise measurements of the effect size are possible.