not very much, genetic relatedness-wise. i think it might be pretty much like marrying any random person from your ethnic group (i’m not sure — maybe someone out there knows?).
as we’ve seen, once christianity hit europe, certain bans on whom you could marry were instituted. by the sixth century, you could no longer marry your second cousin (if you were going to be a good christian, that is). later that was pushed back to the fourth cousin; and in the eleventh century, it was your sixth cousin. the regulations dropped back to fourth cousin again in the thirteenth century. those are big changes compared to pre-christian days when marrying your first cousin was prolly not uncommon.
so, how do these mating patterns affect the degree of genetic relatedness in a society?
well, looking at the standard coefficients of inbreeding for cousins, i worked out that the coefficient of inbreeding for fourth cousins is 0.000977 (that’s rounded to the sixth decimal place), and sixth cousins is 0.000015. first cousins, btw, is 0.0625.
using falconer’s coancestries recurrence equation, we can calculate, for instance, how inbred an individual is at the end of three generations of the same sort of inbreeding (e.g. first cousin marriage each generation).
if you have three successive generations of first cousins inbreeding (barring any other sort of inbreeding), the math looks like this:
Ft = 0.0625 (1 + 3Ft-1 + Ft-2)
Ft = 0.0625 (1 + 0.1875 + 0.0625) = 0.078125
so, at the end of the third generation, we get individuals with a quite higher coefficient of inbreeding than their cousin-grandparents.
fourth cousins, not so inbred:
Ft = 0.000977 (1 + 3Ft-1 + Ft-2)
Ft = 0.000977 (1 + 0.002931 + 0.000977) = 0.000981 [rounded]
seventh cousins … well, like i said, for all i know this level of inbreeding might be like that between two strangers from the same population:
Ft = 0.000015 (1 + 3Ft-1 + Ft-2)
Ft = 0.000015 (1 + 0.000045 + 0.000015) = 0.000015 [rounded]
imagine having several hundred years of such strong outbreeding. that’s some really loose genetic ties, afaics. imagine several hundred years of strong inbreeding!
here, for comparison’s sake, is what three generations of mbd marriage, the most common form of cousin marriage, looks like (male p.o.v.):
Ft = 0.0635 (1 + 3Ft-1 + Ft-2)
Ft = 0.0635 (1 + 0.1905 + 0.0635) = 0.079629
yup. more inbred than regular first-cousin cousin marriage!
(note: comments do not require an email. or an abacus.)
hbd chick 
From a society where cousin marriage was preferred, was regarded as the ideal; to a society where cousing marriage was evil, was prohibited.
From a society built on nepotism (preference of families); to a society built on an ideal of impartiality (regarding everyone as deserving equal consideration).
From a society based on spontaneous instinct based on genetic relatedness; to a society based on ideology.
But I think the timescale involved, and the only-partial degree of success in implementing this change, means that the ‘selective sweep’ (if there was one) did not go through to completion; leaving a mixed picture.
The question is – what happened to the reproductive success of people who ‘defied’ the ban on cousin/ consanguinous marriage? Did their actual reproductive success suffer? (did they have lower fertility, did more of their babies die?) or was it merely a matter of suffering ‘tut-tut you naughty person’ as nowadays…
@bruce – “The question is – what happened to the reproductive success of people who ‘defied’ the ban on cousin/ consanguinous marriage?”
as for individuals within any particular european society — good question. i dunno. actually, i haven’t given it much thought yet (and even if i do, i might not come up with much! (^_^) ).
as for the fringes of europe — as opposed to the french-german center where most of this really happened — i’m gonna start posting about that in the near future. need to read up on it more first, but fringe places like spain and ireland and southern italy certainly went along a different trajectory altogether re. inbreeding. the irish, for one, pretty much ignored the cousin marriage ban totally until after the normans took over the place, and even then they were none too happy about it.
what was the reproductive success of these different groups? well, they’re still around — and there’s lots o’ italians and irish. (~_^)
interesting that these fringes where inbreeding and clans and the idea of la familia lasted the longest are also the piigs of today. coincidence? hmmmmmmm….