The Folded Tree

They called it the Habsburg jaw. A mandible so deformed by generations of intermarriage that Charles II of Spain, the last of his line, could barely chew his food. Three marriages between uncle and niece figured among his parents, grandparents and great-grandparents. His family tree, when drawn in full, did not fan outward like the branches it was supposed to resemble. It folded back on itself, the same ancestors appearing again and again, until the tree became a loop and the loop became a cage. He died childless in 1700, and the dynasty died with him.

Genealogists have a term for this. They call it pedigree collapse: the phenomenon in which the number of distinct ancestors in a family tree is smaller than the theoretical maximum because the same individuals occupy multiple positions. Go back ten generations and you should, in principle, have 1,024 distinct forebears. In populations where cousin marriage or tight village endogamy was common, the number can be a fraction of that. The tree does not branch. It folds.

For most of the twentieth century, pedigree collapse was treated as a curiosity of royal genealogy or a footnote in population genetics. It took a team led by Jonathan Schulz and Joseph Henrich, publishing in Science in 2019, to argue that it was something far larger: a hidden variable in the architecture of civilisation itself.

Their argument was structural. In most agrarian societies, kinship networks were dense. Cousin marriage was not merely permitted but encouraged, because it kept land, wealth and loyalty within the clan. These were the societies that Geert Hofstede, in his landmark studies of national culture from the 1970s onward, would classify as collectivist: high in conformity, high in deference to authority, oriented toward the group rather than the individual. They were also, as Woodley and Bell demonstrated across seventy nations, the societies least likely to develop democratic institutions.

What broke the pattern, Schulz and Henrich argued, was the medieval Western Church. Beginning with the Council of Agde in 506, Rome extended its marriage prohibitions outward, banning unions between cousins, step relatives, in laws and eventually spiritual kin such as godparents. Over the centuries the bans dissolved Europe’s clan structures, encouraged nuclear families, promoted residential mobility and selected for the psychological profile that Henrich would later call WEIRD: Western, Educated, Industrialised, Rich and Democratic.

The data support the thesis. Across regions of Turkey, Spain, Italy and France, Schulz and his colleagues found that as cousin marriage rates decline, individualism rises, conformity falls and impersonal trust increases. The correlations track closely with Hofstede’s individualism axis. The family tree that does not fold produces the citizen who does not defer.

The thesis is gaining empirical weight. Last month Roberto Ezcurra, publishing in Defence and Peace Economics, found that countries with historically strong kinship systems remain markedly more politically unstable today, extending the argument from cultural psychology into security studies. The mechanism, it seems, does not stop at the ballot box.

This matters for more than intellectual history. The Middle East and North Africa remain home to the highest consanguinity rates in the world.  In Iraq the figure reaches 46 per cent. In Pakistan, over 60 per cent. These are not merely demographic statistics. If the Schulz-Henrich thesis holds, they are structural indicators of institutional capacity, social trust and the prospects for participatory governance. The family tree that folds back on itself does not merely carry genetic risk. It produces a political grammar in which loyalty to kin supersedes loyalty to the commons.

And then there is Israel.

Israel scores 54 on Hofstede’s individualism index, placing it in the individualist half of the global distribution and separating it decisively from every neighbour. The Arab world, by contrast, scores 38. Israel is a democracy, however fractious, embedded in a geography where kinship networks remain the dominant organising principle. How did it get there?

Part of the answer lies in the diaspora itself. Judaism has always valued yichus, genealogical prestige, the careful tracing of lineage through generations. But exile did something that yichus alone could not: it scattered the tree. Communities in Vilna married into communities from Minsk, who had married into communities from Frankfurt. The Sephardi dispersal from Iberia after 1492 sent branches from Salonika to Aleppo to Amsterdam. The family tree branched precisely because it had no single village, no single court, no single plot of land to fold back upon. Pedigree collapse was curtailed not by doctrine but by distance.

When those scattered branches reassembled in Mandate Palestine and then in the state, they brought with them the institutional instincts that the model would predict: low conformity, high tolerance for disagreement, the argumentative tradition that the Talmud had been rehearsing for two millennia. The kibbutzim reinforced the pattern. Research by the anthropologist Joseph Shepher found that children raised together in the same communal peer group almost never married one another, a natural expression of exogamy enforced not by prohibition but by proximity. Unit 8200 and Talpiot, the military programmes that became incubators for Israel’s technology sector, selected for individual initiative within collective purpose, a combination that societies organised around intensive kinship struggle to produce.

None of this makes Israel’s democracy secure, or its society harmonious, or its politics admirable. The tension is visible within Israel itself: ultra-Orthodox communities maintain high rates of endogamy, and the friction between their communal kinship structures and the state’s liberal institutions is one of the country’s most persistent internal fault lines. The tree did not unfold uniformly. But what the broader pattern suggests is that the roots of Israel’s institutional distinctiveness in the region may run deeper than ideology or immigration policy or security doctrine. They may run all the way down to the structure of the family tree.

Charles II died in a room in the Royal Alcázar of Madrid. Thirty-four years later the building burned to the ground. The dynasty was gone and then the room was gone. The tree had folded so tightly it left nothing behind. At the Weizmann Institute in Rehovot, researchers now map the branching patterns that two thousand years of exile preserved. The room is quiet. The tree, for once, is open.

Appendix: The Evidence Chain

Table 1. From Kinship to Institutions

Table 2. Country Comparisons