Dancing parameters

Biology is sloppy. I always say "all parameters held equal" or "all variables controlled" because there are so many factors to consider. I am now reading a classic, The Genetics of Human Poulations, by L. L. Cavalli-Sforza and W. F. Bodmer, and here is an interesting bit from the chapter on population structure: consanguineous rates of marriage were extremely low throughout much of Europe up until the 19th century, at which point their frequency rose sharply, before dropping again during the 20th century. What was happening here? The authors note that the 19th century abolition of primogeniture and the relaxation of the granting of dispensations by the Roman Catholic Church for incestous marriages resulted in positive incentives toward cousin marriage (partible inheritance tends to break up wealth) concurrent with the removal of traditional constraints (the Catholic Church derived a great deal of revenue and acquired leverage from granting dispensations to ...