Like JRR Tolkien I have a kind of prejudice against the Normans for their near complete obliteration of high Anglo Saxon culture, in particular the Old English mythology and folklore.
However, I am forced to acknowledge that, over a period of roughly 900-1300; the Normans were a very small but stunningly successful people from Normandy in France; astonishingly effective in military and political terms.
The Normans probably comprised some mixture of ex-Scandinavian 'Vikings' with French natives - and despite tiny numbers they conquered (and nearly conquered) lands as wide spread as the British Isles, Southern Italy, Cyprus, and challenged the great capital of Constantinople.
For example, maybe ten thousand or so Normans utterly subdued an English population of maybe one or more than two million! And imposed themselves on the natives as a ruling elite which was almost-wholly unassimilated for about ten or dozen generation.
How did they do this? My impression is that the method seems to have been by superior organization supported by superior technology - especially technology in relation to what might be termed engineering or architecture.
It seems, as a rule, a small minority can only successfully dominate a majority by superior technology; and superior technology implies superior intelligence.
My understanding is that once the Normans had defeated the Anglo-Saxons natives in one locality, they would force the natives to build a castle; from which the minority of Normans could then dominate the surrounding population from a position of safety; then the military force would move on to subdue the next region, and force these new natives to build the castles which would then become the instruments of their own suppression.
Thereby, local and piecemeal superiority was swiftly converted to national and overall superiority. Once a network of castles were dotted over the whole of England, then the Normans could not be defeated.
In a nutshell, it looks to me as if the Normans were simply more intelligent than the Anglo Saxons - and not a little, but a lot more intelligent.
Presumably at least one standard deviation more intelligent than the Native English (i.e. if the native IQ was called 100, the Normans must have had an IQ of 115 or more).
(One SD seems to be the usual (minimum) difference which allows one group to rule another - whether the difference is between classes, castes or races.)
And I am not sure, but I think that Norman cultural achievements are also at least consistent with higher intelligence than the native Anglo-Saxons.
A comparison of literacy rates might be enlightening - however, the extreme differential in social conditions (and the treatment of Anglo Saxons as an inferior and excluded caste) make such measures difficult to interpret.
Probably a 'matched' comparison of the pre-Norman English ruling class (in terms of literacy, technology etc) with the post Norman ruling class, might be the most enlightening line of enquiry.
Why would Normans have had a higher intelligence (assuming that they did)? Perhaps because of the Scandinavian roots - since the harsh conditions and long winters of Scandinavia seem to select for higher intelligence, presumably because low intelligence people cannot survive.
This higher Norse intelligence was not revealed under the harsh Scandinavian conditions (except in terms of sheer survival); but was immediately evident when new and more temperate colonies are founded (as happened in the Northern parts of England - the 'Danelaw', as well as in Normandy); and under more favourable conditions forms of higher cultural expressions begin to develop very rapidly.
Once these intelligent Vikings had conquered a portion of France, they apparently married local women and recruited local men. If we assume that the marriages were 'assortative matings', in which IQs were (somewhat) matched; and if recruitment was of the smarter men (and/ or excluded the least intelligent) - then the high original and Viking-derived intelligence might have been sustained or even amplified as The Normans were formed by a fusion of conquerors and locals.
Maybe, as genetic technologies advance and g may become measurable in terms of comparing large numbers of g-related genes, and if there is availability of suitable DNA from both Norman and Anglo Saxon skeletons; this theory of Normans having very high intelligence relative to Anglo Saxons may become directly testable.
(Note: Some of these ideas were developed in discussion over a prolonged lunch with my friend Peter Andras, now of Keele University - http://www.keele.ac.uk/scm/staff/professors/peterandras/.)