You trust strangers because a medieval Church banned cousin marriage

You trust strangers because a medieval Church spent centuries telling you who you were forbidden to marry. That is not a metaphor. It is a chain, traced link by link.

Begin with the human default, because trust in strangers is not it. Across most of history and most of the world, the load-bearing unit of loyalty was not the individual and not the nation but the extended kin group - the clan, the lineage, the cousinage. You married in: close, and often repeatedly, because marriage was how a family kept property, alliance and obligation inside the blood. Cousin marriage was not a scandal. It was the wall the whole social order leaned on. Trust ran along bloodlines and stopped where they stopped.

Rome drew a modest line. Roman civil law prohibited marriage within four degrees of consanguinity - roughly, out to first cousins. Narrow, sensible, and no threat to the clan.

Then the Church began to move the line, and kept moving it. Canon law followed Rome until the early ninth century, when the Western Church increased the number of prohibited degrees from four to seven. Count it out and the prohibition reached marriage to anyone up to and including a sixth cousin. In a small medieval village that is very nearly everyone you have ever met. This was not discouragement. It made marrying inside your own kin group structurally impossible.

Now the consequence, which is the whole argument. If you cannot marry within the clan, the clan cannot renew itself as a sealed unit. You have to go outside - first for a spouse, then for everything the clan used to supply: capital, protection, a business partner, a patron, a court to settle your quarrels. The anthropologist Joseph Henrich, of Harvard, calls this bundle of rules the Marriage and Family Program, and his conclusion is blunt: a series of Church edicts on marriage that began in the fourth century undermined the foundations of kin-based society. Break the clan and people are thrown onto strangers and institutions in its place - guilds, chartered towns, monasteries, universities, markets, and in the end the impersonal rule of law. The instinct to trust a stranger is not in the blood. It is the scar tissue left when the Church amputated the alternative.

And it set hard. Henrich's name for the mind this produced is WEIRD - Western, Educated, Industrialized, Rich and Democratic - and its signature can be measured: WEIRD people are more trusting of strangers and more likely to give blood than most humans alive. The Briton who donates his own blood to an anonymous recipient he will never meet is doing a thing most of our species would find faintly insane. He is the far-off echo of a marriage rule from the ninth century.

Even the Church did not hold its furthest line. The Fourth Lateran Council of 1215 decreed a change from seven prohibited degrees back to four. The machinery could be relaxed once the clan was already broken. The wall did its work, and was then lowered. The individualism walked out the other side.

Here is the strongest case against reading it this way - the story the West tells about itself. On this telling, our freedoms and our individualism are simply the fruit of reason: the Enlightenment swept away superstition, and any society converges on the same liberal settlement once it has enough schooling and enough GDP. They think progress is one ladder, and everyone climbs the same rungs.

The difficulty is that the scholar who has done most to trace where the Western mind actually came from reaches the opposite verdict, and the dates back him. The individualism did not arrive by reason defeating dogma. It arrived as the accidental by-product of dogma - a strange, centuries-long ecclesiastical war on marrying your relatives, waged long before anyone could have foreseen parliaments and joint-stock companies at the end of it. No one reasoned their way into trusting strangers. They were forced onto strangers by a rule about weddings, and the psychology followed the plumbing. Which means the Western mind is not the universal terminus of reason. It is a local accident that fired once, here, under peculiar conditions - and there is no ladder underwriting that anyone else, or we ourselves, stay standing on it.

That is the uncomfortable part, and it is the point. If high trust, the nuclear family, the individual conscience and the impersonal rule of law were the natural resting state of any decent society, we could take them as given and spend them without care. They are not the resting state. They are the residue of a long programme that dissolved the clan - a construct, assembled half by accident, and a construct can be dismantled. Nothing in nature holds it up. Corrode the institutions that replaced the clan, drain the trust that lets strangers cooperate, import at scale the kin-first loyalties the West spent centuries taking apart and then decline, out of politeness, to ask anyone to set them down, and the wall will not keep standing from respect for our intentions. Blood-loyalty is the species default. Ours is the exception. Exceptions revert.

The West did not think its way into trusting strangers, and it will not think its way out of losing them. It was built by a rule almost none of us knew was there - and a thing you never knew you had is a thing you will not notice leaving.

An opinion of the house.