Over the next several weeks, I will share the results of my first study, into the rates of consanguineous marriage among known members of the Mafia in Corleone, and a control group matched by year of marriage.
I’ve noticed that Mafia members in Corleone appear to marry into other known Mafia families, engage in double in law marriages, and marry close blood relations, all at higher rates than is typical for the town as a whole. A few months ago, I decided to quantify my observations beginning with a controlled study of the blood relations between members of the Fratuzzi and their wives.
Cavalli-Sforza and his co-authors have written the definitive text on consanguineous marriage in Italy, and have paid special attention to Sicily which, owing to its geography and political history, is more isolated than the Italian peninsula. I planned to use their results as a baseline for my own research.
The genealogical documentation that accompanies every marriage is created by a priest, using the baptismal and marriage records. The results were used by the Church to determine degree of relation, which it labeled in degrees I through IV. First degree is the closest relations that are ever allowed to marry, and fourth is the most distant relations still requiring dispensation to marry.
Cavalli-Sforza et al’s research is based on these marriage records created by the Church. The authors are geneticists, with different research aims, and so their categories are slightly different, but map neatly onto the Church’s:
|Church degrees of relation||Cavalli-Sforza|
|II. first cousins||22|
|III. first cousins once removed||23|
|IV. second cousins||33|
In this table from the Cavalli-Sforza text, you can see the types of consanguineous marriages are coded by number. The last two categories, “34” and “44,” are second cousins once removed and third cousins, respectively.
At first, I interpreted this table as saying that half of all marriages required dispensation, due to the bride and groom being first cousins (“22”). In fact, that column shows not the percentage of all marriages, but of all marriages requiring dispensation. The total percentage of marriages that were consanguineous is in the second to last column of this table.
Overall, the incidence of marriage in Sicily between third cousins or closer relations has historically been around five percent of all marriages, according to Cavalli-Sforza’s figures, peaking at around ten percent, and returning to those levels later in the century.
Typically, 25% of your genes will also be found in any one of your aunts’ or uncles’ DNA. You share half as much of your DNA with one of your parents’ siblings as you do with one of your own brothers or sisters. If you have a half-sibling, your coefficient of relationship is also 25%. In other words, you have as much in common with a half-sibling, genetically, as you do with one of your parents’ full siblings. Half-siblings were not allowed to marry, but uncle-niece or aunt-nephew pairings were, with a dispensation from the Church.
Double first cousins are the products of double in-law marriages. When two siblings from one set of parents marry two siblings from another set of parents, the offspring of these marriages are not just first cousins, but double first cousins. They share as much in common, genetically, as half-siblings, or a man and his niece, or a woman and her nephew. While you and your first cousins share just one set of grandparents in common, double first cousins have both sets of grandparents in common. The Church does not restrict double in-law marriages.
|0||identical twins; clones||100%|
|2||full siblings||50% (2−2+2−2)|
|2||3/4 siblings or sibling-cousins||37.5% (2−2+2−3)|
|2||half siblings||25% (2−2)|
|4||double first cousins||25% (4⋅2−4)|
|3||great grandparent-great grandchild||12.5% (2−3)|
|4||first cousins||12.5% (2⋅2−4)|
|6||quadruple second cousins||12.5% (8⋅2−6)|
|6||triple second cousins||9.38% (6⋅2−6)|
|4||half-first cousins||6.25% (2−4)|
|5||first cousins once removed||6.25% (2⋅2−5)|
|6||double second cousins||6.25% (4⋅2−6)|
|6||second cousins||3.13% (2⋅2−6)|
|8||third cousins||0.78% (2⋅2−8)|
|10||fourth cousins||0.20% (2⋅2−10)|
Table source: Wikipedia
Cavalli-Sforza categorize marriages between double first cousins as “Multiple,” indicating that the bride and groom are related through multiple common ancestors.
Since the priests would go back at most three or four generations to complete the documentation supporting a marriage, any common ancestors revealed by the supporting documentation are close enough relations to have a significant impact on a potential couple’s shared coefficient of relation. While discussion of the existing research has focused on the rates of marriage in the first two categories (uncle-niece and first cousin marriages, respectively), and Cavalli-Sforza describe the first category as having an unusually high number of marriages, there’s no telling how much consanguinity lies unmeasured under the heading of “Multiple,” especially if, as I have observed, double in-law marriages are common in the populations being studied.
I have hypothesized that my control group in Corleone would turn out to “marry in” at roughly the same frequency as the rest of Sicily, i.e. around 5-10%, and that Mafia marriages would prove more consanguineous, on average, than the control group. Now, the genealogical research is complete. Over the next several weeks, I will share the results.
- Luigi Luca Cavalli-Sforza, Antonio Moroni, and Gianna Zei. Consanguinity, Inbreeding, and Genetic Drift in Italy. Princeton University Press, 2004.
- “Coefficient of relationship.” Wikipedia. https://en.wikipedia.org/wiki/Coefficient_of_relationship Accessed 19 August 2016.
Featured Image: “The Olive Trees,” by Vincent Van Gogh