Are you rational, or merely rehearsing your tribeโs catechism? Bayesโ theorem insists we should all update our beliefs the same way when presented with the same evidence. Yet in todayโs political divide, identical events harden opposing convictions. The problem isnโt the mathโitโs the priors. When your starting assumptions are inherited, acculturated, or indoctrinated, no amount of โevidenceโ will move you into enemy territory.
A Bayesian Sketch of the Divide
- Let
be a contested claim (pick your poison: โthe election was fair,โ โimmigration helps,โ whatever).
- People in Camp R and Camp B begin with different priors
and
. Thatโs acculturation if youโre being polite, indoctrination if youโve run out of patience.
- They observe evidence
(news, a court ruling, a video clip, a statistic).
- They update:
posterior odds = prior odds ร
Except they donโt, not cleanly, because trust in sources warps the likelihoods.
I love Jonny’s content, which is why I reference it so often. He and I have such different philosophical worldviews. Vive la diffรฉrence (or diffรฉrance).
Why this locks in polarisation
1. Wildly different priors.
If Camp R starts at and Camp B at
, then even moderately pro-
evidence (say likelihood ratio
) yields:
- R: prior odds
- B: prior odds
Same evidence, one camp โsettled,โ the other still unconvinced. Repeat ad infinitum, preferably on primetime.
2. Identity-weighted likelihoods.
People donโt evaluate ; they evaluate
. Disconfirming evidence is down-weighted by a factor
. This is called โbeing rationalโ on your own planet and โmotivated reasoningโ on everyone elseโs.
3. Different hypothesis sets.
Camps donโt just disagree on ; they entertain different
s. If one sideโs model includes โcoordinated elite malfeasanceโ and the otherโs does not, then identical data streams update into different universes.
4. Selective exposure = selection bias.
Evidence isnโt i.i.d.; itโs curated by feeds, friends, and fury. You are sampling from your own posterior predictive distribution and calling it โreality.โ
5. Asymmetric loss functions.
Even if beliefs converged, choices wonโt. If the social cost of dissent is high, the decision threshold moves. People report a โbeliefโ that minimises ostracism rather than error.
6. No common knowledge, no convergence.
Aumann told us honest Bayesians with common priors and shared posteriors must agree. Remove eitherโcommon priors or the โwe both know we both saw the same thingโ bitโand you get the modern news cycle.
โAcculturationโ vs โIndoctrinationโ
Same mechanism, different moral valence. Priors are installed by families, schools, churches, unions, algorithms. Call it culture if you approve of the installers; call it indoctrination if you donโt. The probability calculus doesnโt care. Your tribal totems do.
Two quick toy moves you can use in prose
- Likelihood hacking:
โWhen evidence arrives, the tribe doesnโt deny the datum; it edits the likelihoods. โIf my side did it, itโs an outlier; if your side did it, itโs a pattern.โ This is not hypocrisy; itโs a parameter update where the parameter is loyalty.โ - Posterior divergence despite โfactsโ:
โGive two citizens the same court ruling. One updates towards legitimacy because courts are reliable; the other away from legitimacy because courts are captured. The ruling is constant; the trust vector is not.โ
If one wanted to reduce the split (perish the thought)
- Forecast, donโt opine. Run cross-camp prediction markets or calibration tournaments. Bayes behaves when you pay people for accuracy rather than performance art.
- Adversarial collaboration. Force both sides to pre-register what evidence would move them and how much. If someoneโs
for disconfirming evidence is effectively zero, youโve identified faith, not inference.
- Reference classes, not anecdotes. Pull arguments out of the single-case trap and into base-rate land. Yes, itโs boring. So is surgery, but people still do it.
The punchline
Polarisation isnโt the failure of reason; itโs what happens when reason is strapped to identity. Priors are social. Likelihoods are political. Posteriors are performative. You can call it acculturation if you want to feel civilised, indoctrination if you want to throw a brick, but either way youโre watching Bayesโ theorem run inside a culture war. The maths is sober; the humans are not.


















