In sum: Although both intelligence and personality differences (within a population) are highly heritable - general intelligence is a highly sensitive fitness marker (perhaps the most sensitive); therefore absolute (not differential) intelligence declines with dysgenic change.
In other words average intelligence tracks population average population fitness.
But, since it is highly heritable but not sensitive to fitness, personality should be much more robust to population fitness change.
The work of Geoffrey Miller made clear that general intelligence is a sensitive fitness marker - in other words, high intelligence is a marker (or honest advertisement) of low mutational load.
(Because the brain is a large mutational target, therefore any deleterious mutation has a high probability of affecting brain function - and intelligence is the main summary measure of overall brain function.)
So as the mutational load of a population declines, we would also expect to observe a decline in intelligence - and this is indeed what we find. So average population intelligence (expressed on an absolute scale and measured - for example - by simple reaction times) has been declining for the same period as mutational accumulation has become significant - i.e. since the industrial revolution began sharply to reduce child mortality rates, and the differential mortality rates between the more-intelligent (lower mutational load) and less-intelligent (higher mutational load) went into reverse (with low intelligence becoming associated with higher reproductive success to relatively sustained fertility rates among the less intelligent, ion a context of greatly reduced/ nearly abolished mortality rates).
BUT, although personality differences is approximately as heritable as intelligence, personality is not a sensitive marker of mutational load - although eventually personality would inevitably be affected by the cumulative dysfunctionalities of increasing mutation load.
Thus, personality is relatively buffered against dysgenics.
Therefore, when deleterious genetic mutations are accumulating in a population, we would not expect national personality to change as rapidly as national intelligence will change.
And indeed that seems to be what we observe! English national characteristics seem to have remained similar for several generations - indeed until about the middle of the twentieth century; despite the reduction in English intelligence and the collapse and near-disappearance of English genius.
But nowadays, at last - after several generations of buffering, we are seeing the consequences of mutation accumulation on personality, and on national character; with the English population - on average and en masse - showing many signs of change, and indeed decline into dysfunctionality and overt pathology.