Now we have another big, monocausal explanation for Western Europe’s rise that is utterly different from the rest, although it has faint echoes of Max Weber. The WEIRDest People in the World, by cultural psychologist Joseph Henrich, gives the credit for European prosperity to the Catholic Church. Why? Because the Church completely obliterated traditional kinship ties through increasingly rigid restrictions on whom one could marry.

The process started in 597 AD when a monk, Augustine of Canterbury, was trying to convert the Anglo‐​Saxons of England. He wrote to Pope Gregory I (now St. Gregory the Great), asking if some prevalent marriage customs would be allowed once the natives became Christian. The pope’s reply was strict. He rejected marriage to close relatives or to close in‐​laws (in‐​laws are called “affines” by anthropologists). He banned the adoption of children. He prohibited concubines. (Divorce was already prohibited, based on Jesus’s words in Scripture.)

Exactly why the pope gave these answers can be debated, but one effect of the practice was to reduce the number of heirs. And one result of fewer heirs was that more people bequeathed their wealth to the Church, motivated in part by the hope that such gifts could ease their way into heaven.

Individual over clan/ To Henrich, the pope’s instructions launched the Church’s “Marriage and Family Program” (MFP), which ultimately destroyed the extended family. He does not discuss the reasons for this policy, but he presents plenty of evidence for it. He includes a timeline of the steps, both local and centralized, that the Church took to eliminate marriages within one’s “clan.”

By 1063, the Synod of Rome (the assembly of bishops for Italy) prohibited marriages between all cousins up to sixth cousins. Certainly, that was extreme. Who is your sixth cousin? Someone who has the same great‐​great‐​great‐​great‐​great‐​grandparent. It is unlikely that you would know who such a relative is. And if you found a seventh cousin to marry, you had to be sure that he or she was not also an affine or a godparent. (The sixth‐​cousin “incest” rule was later relaxed to prohibit just third and closer cousins from marrying one another. While the rule was in force, the convenient “discovery” that one was married to a distant cousin could be reason for an annulment.)

The effect of the MFP, according to Henrich, was enormous. Without dependence on a broad collection of relatives and without obligations to them, people were forced to act on their own or with just a nuclear family (if they managed to form one). Thus, the MFP led people to create associations with non‐​relatives, launching medieval guilds, monasteries, and universities. They formed towns and cities. They entered into commercial and political relationships with strangers that would otherwise not even be considered. Towns and commerce grew and practices to ensure trustworthiness grew with them.

These people became individualistic, less willing to bend to authority, and more eager to develop distinctive personal characteristics and achievements because they were competing with others for wealth, prestige, and spouses. They became more “moral” (that is, they adopted a commitment to “impartial principles”). If they didn’t follow those principles, they felt guilty. They even came up with a new collection of Christian sects, Protestantism, that emphasized individualism: Protestants wanted to have a personal relationship with God through the Bible. That led to an obligation to teach as many people to read as possible, which in turn spurred massive changes (reading even changes the brain, Henrich notes).

They also became more analytical, thinking in terms of abstract rules and separating things into categories. Most other cultures are more holistic; they look at relationships, how the parts of their surroundings fit together, says Henrich. Analytical thinking laid the foundation for law, science, and technology.

The process became unstoppable by 1500, maybe even earlier. As a result, Henrich says, people whose culture was formed by those Catholic Western Europeans are WEIRD. That is, their culture has primed them to be Western, Educated, Industrialized, Rich, and Democratic — utterly different from all other cultures.

Kinship connectedness/ Henrich’s 680‐​page argument is built upon an enormous amount of data, including psychological experiments, surveys, game theory, and historical and anthropological examples, as well as interpretations of this material. One reason the book is so long is that he has to explain and illustrate sometimes complicated experiments (and even with 680 pages, a magnifying glass will help you read graphs that are barely 2 inches by 2 inches).

One step in his argument involves measuring societies for their kinship connectedness. Henrich and his associates have developed a Kinship Intensity Index (KII) based on information from the Ethnographic Atlas, a database about cultures “prior to industrialization,” built from information gathered around 1900. Intense kinship cultures allow and encourage marriages between cousins, multiple wives, endogamous marriage (i.e., within the clan), and patrilineal or matrilineal inheritance. Low‐​intensity kinship cultures have minimal cousin marriages, monogamy, a wide choice in marriage, and bilateral inheritance. Unlike high‐​intensity kinship cultures, they have “neolocal” marriages: newly married couples readily leave their home to form their own nuclear marriage.

Examples of high‐​KII countries are Iran, Ghana, and Pakistan. Examples of countries that are low in kinship intensity are Sweden, England, the United States, and Australia.

Then, Henrich and his associates used a cache of psychological tests to identify personal characteristics.

Consider the Asch Conformity Experiment (devised in the 1950s). College students are shown three vertical lines of different lengths. They are also shown a “target” line that matches the length of one of the other three lines. The subjects are expected to say out loud which of the three lines is the same length as the target line. This is an easy task and 98% of people who take it get it right when they are alone.

But in the experiment, several other “participants,” who really are test confederates, announce their answers before the individual taking the test speaks. They all give the same wrong answer. Will the student “correct” his or her answer to conform to the others’ (wrong) assessments, or will the student stick with what he or she believes to be the right answer?

These tests are taken by students from numerous countries. It turns out that about one‐​quarter of all students “conform” to the answers of the others. However, “WEIRD” students conform less. (In this example, Henrich doesn’t specify the countries that the WEIRD students come from.) They give the wrong answer less often than those from other backgrounds. Students from Brazil, for example, go along with the wrong answer about 50% more often than the WEIRD students, and students from Zimbabwe give the wrong answer about 200% more often.

The Asch study is one of many, many measurements that Henrich uses. Another is the World Values Study. It asks people to describe themselves as more or less traditional and to say whether obedience is an important value to them or not. As you would expect, WEIRD people are less committed to tradition and obedience than others. Preference for tradition and obedience is correlated with heavily kinship‐​based societies.

Kinship and the Church/ Having found positive correlations between WEIRD characteristics and low kinship intensity, Henrich wanted to confirm that cultural contact with the Catholic Church over centuries correlated with the WEIRD personal characteristics.

First, he had to identify which groups of people today are part of a culture that was in close contact with the Church between 500 and 1500 AD. In Europe, he did this by mapping the expansion of the Catholic Church during the period, using as his source the formation of bishoprics (which were often created when a chief or king converted to Christianity). That told him which parts of Europe had been affected by the MFP, and when.

Henrich also went beyond Europe. Using an existing “migration matrix,” he and his associates calculated an MFP level for each modern country in the world. If a large group of people had come to a country from a place that had contact with the Catholic Church in the 500‑1500 AD period (e.g., the United States or Australia), that country would have a high MFP level. He writes:

The data show that the longer a country’s population was exposed to the Church the weaker its kin‐​based institutions.… In fact, of all the agricultural, ecological, climatic, geographic, and historical factors that we explored in trying to understand the global variation in kinship intensity, the biggest factor — though not the only — was historical exposure to the Church.

Henrich was even able to detect a difference between the effects of the Western Catholic Church, with its demanding marriage restrictions, and those of the Eastern Orthodox Church. The Church split in 1054. While the Eastern Church had marriage restrictions, they were less rigid than those of the Western Church (“MFP lite,” Henrich calls them). Those who descended from the Eastern Church’s culture are less WEIRD.

A similar exercise led him to find a cause‐​and‐​effect connection between the growth of Protestantism and greater literacy. Protestantism, he says, gave a “boost” to the MFP, moving the WEIRD characteristics to another level.

In 1517, Luther presented his Ninety‐​five Theses in Wittenberg (according to legend, nailed to the Cathedral door). Henrich was able to determine that Protestant influence spanned out (almost in ripples) from Wittenberg. Over 350 years later, in 1871, the number of Protestants in each Prussian county followed a pattern: the number decreased by about 10% for every 100 kilometers’ distance from Wittenberg. Henrich then correlated the level of Protestantism in each county (in 1871) with the level of literacy (also in 1871), obtaining what he calls “striking” results. “Not only do Prussian counties closer to Wittenberg have higher shares of Protestants, but those additional Protestants are associated with greater literacy and more schools.”

Guilt and shame/ Speaking very broadly, Henrich says that WEIRD societies fear guilt, while kinship‐​intense societies fear shame. This is a point we have often heard before when comparing Western and Eastern societies. But why does it happen?

Henrich says that individualism leads WEIRD people to expect themselves and others to be consistent in all their relationships; those who don’t are “two‐​faced” or dishonest. If WEIRD people act inconsistently, they experience “cognitive dissonance” and often engage in rationalizations to overcome feelings of guilt.

“Other populations,” writes Henrich, “focus more on actions and outcomes over what’s ‘inside.’ ” Actions that might not especially bother a person internally (such as failing a test) may cause shame because of the reaction of kin; and because there are so many kin, and many kin are close to one another, the shame may be widespread and severe.

There are many other arguments, graphs, claims, and descriptions in this book, including aspects of European history. For much of its history, Europe was involved in almost constant wars. But because of the presence of MFP, the wars spurred even more involvement in associations: religious, political, social, and work‐​oriented. Ultimately, the associations led to more “benign forms of intergroup competition,” one of which was the market.

There is much more to be mined in the book — and, probably, debated. In the meanwhile, I consider much of what Henrich says to be persuasive. I’ll put his book on my shelf along with Diamond’s Guns, Germs, and Steel and North and Thomas’s The Rise of the Western World, and wait for more to come.