I tend towards the skeptical in the face of monocausal explanatory frameworks, especially for ideas as big as human history and the factors that shaped it. The risk of being wrong is far too high while the payoff in terms of anything more than cocktail banter is too low, be it as a shaper of modern policy or bearer of moral prerogatives.
So the widely covered discussion of Schulz, et. al.’s AAAS Science paper, “The Church, intensive kinship, and global psychological variation” (paywall) is a curiosity that admits to cautious reading at the very least. The hypothesis is that Catholic Church prohibitions on consanguine marriage that began in the medieval period in Western Europe explain globally unusual aspects of the psychology of the people of those regions. By banning cousin marriage even out to the 6th degree in many cases, the Church forced people away from tribal ideas and more towards neolocal family structures. That, in turn, led to pro-social attitudes based on social trust rather than family power, and towards more individualistic and independent psychologies overall.
The methodology of the study is fairly complex: look at the correlations between consanguine marriages patterns and psychological attitudes, then try to explain those correlations away with a wide range of alternative data patterns, like the availability of irrigation or proximity to Roman roads, and so forth. Data experiments that look at Eastern Orthodox versus Western Church differences, or even between northern and southern Italy are used to test the theory further.
In the end, or at least until other data supervenes, the hypothesis stands as showing that reducing cousin or similar marriages is a “causal channel” for these patterns of individualism and social trust.
Now, I’ll get to some alternative ways of approaching this social-psychological history in a minute, but first is the issue—not covered in the AAAS paper—of why the Catholic Church thought it a good idea to reduce consanguine marriage. The paper gives an overview but doesn’t delve into the motivations:
The Western Church’s policies, which we call the Marriage and Family Program (MFP), began with targeted bans on certain marriage practices used to sustain alliances between families (e.g., levirate marriage); however, by the Early Middle Ages, the Church had become obsessed with incest and began to expand the circle of forbidden relatives, eventually including not only distant cousins but also step-relatives, in-laws, and spiritual kin. Early in the second millennium, the ban was stretched to encompass sixth cousins, including all affines. At the same time, the Church promoted marriage “by choice” (no arranged marriages) and often required newly married couples to set up independent households (neolocal residence). The Church also forced an end to many lineages by eliminating legal adoption, remarriage, and all forms of polygamous marriage, as well as concubinage, which meant that many lineages began literally dying out due to a lack of legitimate heirs. As a result of the MFP, by 1500 CE (and centuries earlier in some regions), much of Europe was characterized by a virtually unique configuration of weak (nonintensive) kinship marked by monogamous nuclear households, bilateral descent, late marriage, and neolocal residence.
In order to get to the why of all this, we have to delve into referenced sources as well as earlier work by the primary author. For instance, in Schulz’s 2016 “The churches’ bans on consanguineous marriages, kin-networks and democracy” from University of Nottingham’s CEDEX Discussion Paper series, which the Science article is clearly an extension of the same research program, the author finds motivation from Augustine:
The early Christian theologian St. Augustine (354 – 430) propagated a ban on consanguineous marriages by pointing out that marrying outside the kin-group enlarges the range of social relations and “should thereby bind social life more effectively by involving a greater number of people in them” (Augustine, 1998, p. 665).
But in Bittles’s 2003 paper in Developmental Medicine & Child Neurology, “The bases of western attitudes to consanguineous marriage,” the author has a more complex diagnosis:
Similar divisions are found in Christianity with, for example, first-cousin marriages banned by the Orthodox Church, subject to Diocesan approval in the Roman Catholic Church, but permissible within the various Protestant denominations. According to the Venerable Bede writing in the 8th century AD, the Latin Church position on consanguinity was effectively defined in the late 6th century when Pope Gregory I advised Augustine, the first Archbishop of Canterbury, that unions between consanguineous spouses did not result in children. Citing Leviticus 18:6, the Pope further stated that sacred law forbade a man to ‘uncover the nakedness of his kindred’. This was an interesting example of selective citation, as the precise types of permitted and proscribed marriages were formally listed in Leviticus 18:7–18, with both uncle–niece (but not aunt–nephew) and first-cousin unions permitted. Nonetheless, from this time forward, marriage with a biological relative up to and including third cousins was proscribed within the Latin Church.
However, this proscription ran into conflict over time but was resolved within the Church by formal dispensations to allow for certain patterns of cousin marriage, at least until this came under fire during the Reformation:
16th century demands for the cessation of dispensation payments were an integral part of the Reformation, which explains the subsequent adoption of the Levitical marital guidelines by the emergent Protestant denominations. Somewhat surprisingly, during the 19th and early 20th centuries consanguineous unions were actually more prevalent in the Roman Catholic countries of southern Europe than in the northern Protestant countries, where no civil or religious barriers existed to their practice.
Note that “Levitical marital guidelines,” as mentioned in the previous quote, are just fine with uncle-niece and cousin marriage and so, with the turn away from Church authority, Protestantism could have returned to the older, tribal ways for consolidating family power. Indeed, even Charles Darwin married his first cousin, Emma Wedgwood, then set his son on a scientific quest to try to determine if there were biological consequences to cousin-level inbreeding.
Yet, despite all this complexity of consanguine marriage acceptance, the broad effect was to reduce family ties and increase individualism and pro-sociality according to Schulz. But we can look at other ways that the same effects might be realized. For instance, what if the Reformation itself combined with the widespread availability of printed text and literacy led to a cognitive revolution about authority? This is perhaps a more traditional historical perspective that requires that new social organization and institutions were able to get an influential foothold (parliaments, city states, merchant guilds). There was a snowball effect that accelerated change in those countries. Note that this may have been enhanced by reduced family authority, so it doesn’t rule out the Schulz hypothesis, but it certainly makes it seem more nuanced than the analysis reported in Science would suggest.
Small edits and added link to Bittles paper