After a re-read of the most delightful The Grand Sophy (Georgette Heyer) and its habit of making my mind drift towards the icky thought of cousins marrying, I decided to do a little research on consanguinity to see what were the home truths about it in the Regency era.
Here is a sciency definition of first cousin marriage: Globally, the most common form of consanguineous union contracted is between first cousins, in which the spouses share 1/8 of their genes inherited from a common ancestor, and so their progeny are homozygous.
And another wikipedia gem on cousin marriage today: Such marriages are often highly stigmatized today in the West, but marriages between first and second cousins nevertheless account for over 10% of marriages worldwide. They are particularly common in the Middle East, where in some nations they account for over half of all marriages.
In Regency England, the laws of marriage concerning consanguinity were ruled by the church. Here is a helpful list of who a bride and groom could not marry: http://www.eskimo.com/~lhowell/bcp1662/misc/kindred.html
I still can’t get over the thought of marrying one of my cousins, but I guess thinking about it in terms of 1/8 relation makes it not quite as knee jerkingly taboo. In Regency culture, when families were separated by distance and upbringing (as in the cases of The Grand Sophy) cousins met often in adulthood, rather than being raised together (as in the case of Mansfield Park, which the brother and sister like relationship is truly oo-cky).
What are your thoughts about this delicate matter? When looking at characters (whether reading or writing) how do you approach this subject?
For further reading on cousin marriage, check out this bibliography: https://docs.google.com/viewer?a=v&q=cache:ntPMv-q1aecJ:www.consang.net/images/f/f4/Consanguinity_Historical.pdf+history+of+Consanguinity+in+england&hl=en&gl=us&pid=bl&srcid=ADGEESgTlOg9GE4R09bDvZei_X6Ytp8aDSV0UTW1K57iKDWUf6V9tHYbTmzyA0InCeDzW3zGTWnJovpxd-07n2nwciZ6VH5hw9a0HQ6B5LQZi1nhbEiVk-AcTTib67JAihUS0nZ0VXib&sig=AHIEtbRnl4TAcKKbqdLdkUvv4c9c0ivsUQ
Regency Reader 
Eeeewwwww. At least your idea explains how such marriages occur. My cousin! Blah! I remember too much of what a snot he was as a child.
When I read Heyer’s books that involve cousin marriage I (“Grand Sophy”, “Unknown Ajax” and the more modern “Behold, Here’s Poison” to name a few) I try not to dwell on the “ick factor” and sort of pretend the main characters are not really blood related so I can enjoy the wonderful stories. It does make me wonder about Heyer though. Cousin marriage wasn’t THAT common even in the Regency period so why does she use that plot device so why did she use that plot devise so relatively often?
I similarly try to avoid placing too much thought into the cousin relationship while reading Grandy Sophy (of course over and over). I can only think it was in some way an homage to Mansfield Park.
I saw an episode of some cable station show Forbidden Sex (I think the science channel) where when siblings or other close relations are raised separately and unknown to each other and meet in adulthood they often find themselves attracted to each other and engage in a relationship. Naturally, a kinship biologically would operate as a sort of magnet to our relations, as we are often told not only do we tend to fall in love with people who are our “equal” in looks and health, but we also feel a natural affinity for those prepossessed to share our interests, etc. The argument is, that only being raised together we “learn” the appropriate boundaries.
I think in the case of Grand Sophy, this is very much the case, and that Sophy feels no more related to her stiff cousin than to any other bloke. However, biology helps along their feelings of understanding each other, even though they fight against it with over the top bickering.
Does make you wonder, though, as you say.