Some years ago in my search for causes of the West’s prosperity I came across Deepak Lal’s 1999 book Unintended Consequences. The book planted the seed of an idea that has recently borne some exotic fruit.
According to Lal, in 597 AD Catholic missionaries were trying to convert the pagan Anglo-Saxons in England. Augustine, a monk who later became Archbishop of Canterbury, wrote to Pope Gregory I asking him whether several of the converts’ marriage habits were allowed.
In his reply, the pope was strict. He did the following: (1) he rejected marriage to close relatives or to close in-laws (called affines by anthropologists), (2) he banned the adoption of children, and (3) he prohibited concubines. (Divorce was already prohibited, based on scripture). Why?
According to Lal, who relied on anthropologist Jack Goody, the goal was to gain property for the Church. In most early societies, the rules of kinship, such as those allowing close relatives to marry, helped ensure that families had heirs into the future. But the Church opposed those rules. Lal quotes Goody, “[P]rohibit close marriage, discourage adoption, condemn polygyny, concubinage, divorce and remarriage, and 40 percent of families will be left with no immediate heirs.”
With no immediate heirs, it was likely that the Church would obtain more property, especially because making bequests to the Church helped a person to be sure of salvation after death. Over the long run, the pope’s decisions led to the nuclear family and to an individualistic spirit that contributed to the West’s economic growth.
I found this claim a bit cynical but plausible. I suspected that Lal’s Indian origins enabled him to be more objective about the West and the Catholic Church than other economic historians, although that doesn’t explain anthropologist Jack Goody’s observations. (Lal also commented in a footnote that the Roman Catholic Church was “probably the longest-lasting and most global and financially successful business corporation the world has ever known.”)
The impact of Pope Gregory’s decision has just been affirmed and expanded by a new book, The WEIRDest People in the World, by evolutionary psychologist Joseph Henrich. As Henrich describes it, Pope Gregory I’s answer was merely the beginning of the Church’s mounting restrictions on whom one could marry. These restrictions were mostly “incest taboos,” with incest defined ever more broadly. For example, early on it became illegal to marry the widow of one’s brother who had died (even though in the Hebrew Bible that is called levirate marriage and was acceptable, even expected).
By 1063, the Synod of Rome (that is, the assembly of bishops for Italy) prohibited marriages between sixth cousins. Who is your sixth cousin? You and your sixth cousin have the same great-great-great-great-great-grandparent. And if you found a seventh cousin to marry, you had to be sure that he or she was not a godparent or in-law.
Henrich sees this progression as the foundation of the West. He sees the Christian Church as the prime (and perhaps only) example in the world of the complete obliteration of traditional kinship ties. As a result, he says, people descended from those western Europeans are WEIRD. Their culture has primed them to be: Western, Educated, Industrialized, Rich, and Democratic—utterly different from all other cultures.
It is well known that northwestern Europe developed a distinctive monogamous nuclear family by the 1700s, if not earlier. Before they married, young sons and daughters would go to another family as hands or domestic help. When they married, they would start their own family with the savings they had built up. In contrast, in China, for example, married women remained with the father’s family and were rarely allowed to work outside the family.
But Henrich’s 680-page book goes much further in describing the results of what he calls the West’s (that is, the Church’s) MFP, or Marriage and Family Program. Henrich gathers up hundreds of studies, including many personality tests, to argue that the destruction of traditional kinship led people to develop a wide range of unusual characteristics. They became “increasingly individualistic, independent, self-focused, nonconformist, and relationally mobile.” And that’s just for starters. 
Without the support of clans, Europeans in the Middle Ages created voluntary associations such as guilds, monasteries, and universities, and many moved from agriculture to towns and cities. Because they learned to deal with people outside their kin, they were able to develop markets, vastly increasing their access to new products—and to new ideas. No longer did they have to deal only with family members because only family members could be trusted. WEIRD people learned that outsiders could be trusted—and, to make sure of it, they developed institutions like the “law merchant” and contracts. They developed other useful habits like attention to punctuality. (Henrich devotes several pages to the history of clocks and awareness of time.)
So, such characteristics led to markets, economic growth, and innovation. Many of us have credited the “Protestant ethic” for economic growth, and to some extent we should. However, Henrich cautions, Protestantism itself was the result of the mental changes fostered by the Church. Protestantism was “like a booster shot for many of the WEIRD psychological patterns.”  For example, he says that even today Protestants work longer hours than Catholics, a trait that he traces to the boost, or greater intensity, that Protestantism gave to MFP characteristics.
I’ve only touched on the highlights of this vast book, which even addresses the problems that the West faces as the world becomes more globally integrated, but still culturally different. In any case, I will add it to my shelves along with such books as How the West Grew Rich and The Great Divergence. And, of course, Unintended Consequences.
 Deepak Lal, Unintended Consequences: The Impact of Factor Endowments, Culture, and Politics on Long-Run Economic Performance (Cambridge, MA: MIT Press, 1999).
 Lal, 84.
 Lal, 223, n65.
 John Henrich, The WEIRDEST People in the World: How the West Became Psychologically Peculiar and Particularly Prosperous (New York: Farrar, Straus and Giroux, 2020), 55.
 Henrich, 350.
 Henrich, 418.