Add two more culture-schism triggers: world dominance and war between kin

Reported after a long run left the world under one dominant culture -- the
existing schism only fires for a geographically distant cluster (overseas
colonies), so a single unbroken landmass had nothing to check one culture
quietly absorbing the whole world via border conversion.

Two new independent triggers, both refactored to share the existing schism
machinery via a doSchism() helper:
- Dominance schism: past cultDominanceShare (80%) of the world's living
  population, a per-year chance rising from cultDominanceProbMin (1%) to
  cultDominanceProbMax (99%) at 100% share that the culture fractures on
  its own -- a coin flip splits off half or a fifth of its own members,
  the ones farthest from its own population-weighted core.
- War schism: a war between two realms sharing a culture has
  cultWarSchismChance (50%) odds, rolled once at declaration, that one
  whole side renounces the shared identity and becomes a new people.

Both are pure hashes of (stable id, year, seed), so step-back replays them
exactly, matching the rest of Step 8. Config is the self-describing text
block, so no save-version bump.

Dropped an initial validateConfig() cross-rule requiring
cultDominanceProbMin <= cultDominanceProbMax after finding it would reject
a very natural "disable via Max=0" edit (e.g. from the Main Menu) whenever
Min was left at its default -- silently reverting a player's entire config
to defaults on New World. The interpolation tolerates the reverse fine
since the mechanic is already gated on Max > 0.

test_cultevo.cpp gains two new scenarios (dominance split, war split)
alongside the existing distant-cluster one; quietCulture() now also zeros
the two new rates.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TMfyZv91tonDnPJqbaTVJE

CLAUDE.md

@@ -584,6 +584,33 @@ on the Live World clock). **Steps 1–7 of the roadmap are done (plus a derived
   `planet.cfg`+`F2` or the Main Menu's Settings tab, only take effect through a regenerate — there is
   currently no way to push a single global field like `seaLevel` into an already-running world
   without starting over); the practical fix for an affected world is a New World.
+- **Two more culture-schism triggers: world dominance + war between kin (was: only a distant-cluster
+  schism)** *(done — see `Planet::stepCulture()`, `src/sim/PlanetCulture.cpp`)* — reported after a
+  long run left the world under a single "world-dominant" culture (the existing schism only fires for
+  a geographically distant cluster, e.g. overseas colonies — a single unbroken landmass never
+  qualified, so one culture could quietly absorb the whole world via border conversion with nothing
+  to check it). Two new independent triggers, both sharing the existing schism machinery (now
+  factored into a `doSchism()` lambda so all three read identically): **dominance schism** — once a
+  culture exceeds `cultDominanceShare` (80%) of the world's living population, a per-year chance
+  rising from `cultDominanceProbMin` (1%) at the threshold to `cultDominanceProbMax` (99%) at 100%
+  share that it fractures on its own, **no geography required** — a coin flip picks **half or a
+  fifth** of its own members, the ones farthest from its own population-weighted cultural core, to
+  found a new people; **war schism** — a war between two realms that happen to share a culture (a
+  "civil war") has `cultWarSchismChance` (50%) odds, rolled once right at declaration, that one whole
+  side (a coin flip between attacker/defender) renounces the shared identity, its entire realm's
+  settlements becoming a new people — checked against wars with `startYear` equal to the current
+  year, so it only ever looks at a war on the exact year it was declared. Both are pure hashes of
+  (stable id, year, seed) like the rest of Step 8, so step-back replays them exactly. New config
+  fields `cultDominanceShare`/`cultDominanceProbMin`/`cultDominanceProbMax`/`cultWarSchismChance`
+  (self-describing config, **no save-version bump**). One design pitfall found while adding these:
+  an initial `validateConfig()` cross-rule requiring `cultDominanceProbMin <= cultDominanceProbMax`
+  was removed again — the interpolation tolerates the reverse fine (the whole mechanic is already
+  gated on `cultDominanceProbMax > 0`), and the rule would have rejected the very natural "disable by
+  setting Max to 0" pattern (e.g. from the Main Menu's Settings tab) whenever `Min` was left at its
+  nonzero default, silently reverting a player's *entire* config to defaults on New World — a bad
+  surprise for an edit that looks perfectly reasonable. `test_cultevo.cpp` gained two new scenarios
+  (force every settlement onto one culture and watch it split near-deterministically; inject a
+  same-culture war and check the split) alongside the existing distant-cluster one.
 
 ## Current state
 
@@ -1459,7 +1486,11 @@ triangles (plates are fixed in phase 1).
   cultural weight — rich hubs radiate culture); schism — `cultSchismMinMembers` (6, min living settlements
   to schism), `cultSchismRange` (0.55 rad from the population centroid past which members are "distant" —
   lower = easier colonial breakaways), `cultSchismMinCluster` (2, distant settlements needed to break away
-  together), `cultSchismRate` (0.08, per-year chance per qualifying culture). All rates 0 = the static
+  together), `cultSchismRate` (0.08, per-year chance per qualifying culture); dominance schism —
+  `cultDominanceShare` (0.80, world population share above which it can trigger), `cultDominanceProbMin`
+  (0.01, per-year chance right at the threshold) / `cultDominanceProbMax` (0.99, at 100% share — the
+  chance ramps linearly between the two); war schism — `cultWarSchismChance` (0.50, chance a war
+  between two same-culture realms splits one side, rolled once at declaration). All rates 0 = the static
   pre-Step-8 world. Runs in the yearly `stepCulture` tick (pure hashes, no RNG); the culture list is
   capped at 64 peoples. Dawn seeding (`Planet::seedCultures()`, before Step 8 evolution runs at all):
   `cultDawnSettlementsPerCulture` (5, target dawn settlements per initial culture on a continent —

src/sim/PlanetCulture.cpp

@@ -14,9 +14,11 @@
 //  - computeCultures() the DERIVED refresh (tallies, governments, per-cell view). Runs on every
 //                      territory rebuild / load / step-back, so it must never mutate identity state.
 //  - stepCulture()     the once-per-sim-year mutation pass: assimilation (conquered settlements adopt
-//                      their ruler's culture), border conversion (dominant foreign cultural pressure)
+//                      their ruler's culture), border conversion (dominant foreign cultural pressure),
-//                      and schism (a far-flung cluster becomes a new people). Pure hashes of
+//                      and three independent schism triggers -- a far-flung cluster (colonies), a
-//                      (stable id, year, seed) -- no RNG touched, so step-back replays it exactly.
+//                      culture that has grown to dominate most of the world's population, and a war
+//                      between two realms of the same culture. Pure hashes of (stable id, year, seed)
+//                      -- no RNG touched, so step-back replays it exactly.
 // The culture list is append-only and records are immutable after creation (indices/colors stay
 // stable; extinct cultures keep their slot); the list + sSettleCulture are saved (v23) + snapshotted.
 
@@ -350,14 +352,21 @@ void Planet::computeCultures() {
 }
 
 // --- Civilization Step 8: the yearly cultural-evolution pass ----------------------------------------
-// Three passes, all pure hashes of (stable id, year, seed) so a step-back replays them exactly:
+// All passes are pure hashes of (stable id, year, seed) so a step-back replays them exactly:
 //  A) assimilation -- a settlement held by a foreign-culture overlord adopts the ruler's culture
 //     (the Step-5 revolt cultBonus then drops on its own: assimilation pacifies provinces);
 //  B) border conversion -- a settlement dwarfed by a nearby foreign culture's weight (population +
 //     trade prestige) converts toward it (realm capitals are exempt: they anchor identity);
-//  C) schism -- a large culture's far-flung coherent cluster (overseas colonies) breaks away as a
+//  C) schism -- a large culture splits off a NEW people (fresh name, ethos/faith re-derived from its
-//     NEW people: a fresh name from the local language bank, ethos/faith re-derived from its own
+//     own lands, appended to `cultures`; step-back truncates the list and replay re-creates it), via
-//     lands (appended to `cultures`; step-back truncates the list and replay re-creates it).
+//     any of three independent triggers, all sharing one doSchism() helper:
+//       C1) a far-flung coherent cluster (overseas colonies) breaks away past cultSchismRange;
+//       C2) once a culture exceeds cultDominanceShare of the world's population, a rising per-year
+//           chance (cultDominanceProbMin..Max) that half or a fifth of it (whichever the coin flip
+//           picks), the members farthest from its own cultural core, secede on their own -- no
+//           geography required, so one people can't quietly conquer the whole world by attrition;
+//       C3) a war between two realms that share a culture has cultWarSchismChance odds, checked right
+//           at declaration, that one whole side renounces the shared identity -- kin stop being kin.
 std::vector<WarEvent> Planet::stepCulture(long year) {
     std::vector<WarEvent> ev;
     const int n = (int)cells.size();
@@ -436,18 +445,81 @@ std::vector<WarEvent> Planet::stepCulture(long year) {
         }
     }
 
-    // C) Schism: a distant coherent cluster of a large culture becomes a new people.
+    // C) Schism: a large culture splits into a new people, via three independent triggers -- a
-    const size_t ncult = cultures.size();   // iterate the pre-pass list (children append)
+    // coherent distant cluster (colonies), overwhelming world dominance, or a war between kin.
-    if (cfg.cultSchismRate > 0.0 && (int)ncult < 64) {
+    const size_t ncult = cultures.size();   // iterate the pre-pass list (children append, never re-split)
+    if ((int)ncult < 64 && (cfg.cultSchismRate > 0.0 || cfg.cultDominanceProbMax > 0.0 || cfg.cultWarSchismChance > 0.0)) {
+        // Shared setup: per-culture living member count + population-weighted centroid, used by both
+        // C1 (distance from the core) and C2 (farthest members when splitting off by dominance).
         std::vector<int>  memCount(ncult, 0);
         std::vector<Vec3> cSum(ncult);
+        double totalLivingPop = 0.0;
         for (size_t s = 0; s < ns; ++s) {
             if (!living(s)) continue;
             int ci = sSettleCulture[s]; if (ci < 0 || ci >= (int)ncult) continue;
             memCount[ci]++;
             cSum[ci] = cSum[ci] + unitOf(s) * std::max(1.0, settlements[s].population);
+            totalLivingPop += settlements[s].population;
         }
         const double range = std::max(1e-6, cfg.cultSpreadRange);
+
+        // Creates a new culture from `cluster` (settlement indices, all currently culture `ci`),
+        // re-deriving ethos/faith from its own lands and a fresh name (keeping the parent's faith name
+        // if the faith is unchanged); logs a kind=7 event. Shared by all three triggers below.
+        auto doSchism = [&](int ci, const std::vector<int>& cluster, const std::string& verb,
+                            const std::string& detail, int eventCell) {
+            // Copy the parent fields we need BEFORE the push_back (it invalidates references).
+            const uint32_t parId = cultures[ci].id;
+            const std::string parName = cultures[ci].name;
+            const Faith parFaith = cultures[ci].faith;
+            const std::string parFaithName = cultures[ci].faithName;
+            const int parBank = cultures[ci].bank;
+
+            // Majority regionId of the cluster (ties resolve by cluster order -- deterministic).
+            std::unordered_map<int, int> regCount; int regBest = -1, regBestN = 0;
+            for (int s : cluster) {
+                int c = ++regCount[settlements[s].regionId];
+                if (c > regBestN) { regBestN = c; regBest = settlements[s].regionId; }
+            }
+
+            Culture child;
+            child.id = sCultureNextId++;
+            child.parentId = (int)parId;
+            child.foundedYear = year;
+            child.regionId = regBest;
+            child.bank = (regBest >= 0) ? namegen::bankForRegion(cfg.seed, regBest) : parBank;
+
+            CultEnv env;
+            for (int s : cluster) envAdd(env, cells, sMoist, cfg.seaLevel, settlements[s].cell);
+            uint32_t rk = child.id * 0x9E3779B9u + 0xC41Du;
+            envPick(env, seed, rk, (int)cluster.size(), child.ethos, child.faith);
+
+            uint32_t nameSeed = seed ^ cultHash(rk * 2654435761u + 0x50C1A1u);
+            std::string root = namegen::makeName(nameSeed, child.bank);
+            auto usedName = [&](const std::string& r) {
+                for (const Culture& c : cultures) if (c.name == "the " + r) return true; return false;
+            };
+            for (int g = 0; usedName(root) && g < 128; ++g)
+                root = namegen::makeName(nameSeed += 0x9E3779B9u, child.bank);
+            child.name = "the " + root;
+            child.faithName = (child.faith == parFaith)
+                            ? parFaithName
+                            : makeFaithName(seed ^ cultHash(rk * 40503u + 0xFA17Fu), child.bank);
+
+            int childIdx = (int)cultures.size();
+            for (int s : cluster) {
+                int sc = settlements[s].cell;
+                if (sc >= 0 && sc < n && (cells[sc].editLock & LockCulture)) continue;   // edit mode: pinned
+                sSettleCulture[s] = childIdx; converted[s] = 1;
+            }
+            std::string title = child.name + " " + verb + " " + parName;
+            title[0] = (char)std::toupper((unsigned char)title[0]);
+            ev.push_back(WarEvent{ 2, eventCell, title, detail, 7 });
+            cultures.push_back(std::move(child));
+        };
+
+        // C1) A distant coherent cluster of a large culture (typically overseas colonies) breaks away.
+        if (cfg.cultSchismRate > 0.0) {
             for (size_t ci = 0; ci < ncult && (int)cultures.size() < 64; ++ci) {
                 if (memCount[ci] < std::max(1, cfg.cultSchismMinMembers)) continue;
                 if (cSum[ci].length() <= 1e-12) continue;
@@ -474,58 +546,75 @@ std::vector<WarEvent> Planet::stepCulture(long year) {
                     if (ang(unitOf((size_t)s), unitOf((size_t)far)) <= range) cluster.push_back(s);
                 if ((int)cluster.size() < std::max(1, cfg.cultSchismMinCluster)) continue;
                 if ((int)cluster.size() >= memCount[ci]) continue;   // a schism splits, it never renames all
-
+                doSchism((int)ci, cluster, "break away from",
-            // Copy the parent fields we need BEFORE the push_back (it invalidates references).
+                        "A distant people drifts apart and names itself anew.", settlements[far].cell);
-            const uint32_t parId = cultures[ci].id;
+            }
-            const std::string parName = cultures[ci].name;
-            const Faith parFaith = cultures[ci].faith;
-            const std::string parFaithName = cultures[ci].faithName;
-            const int parBank = cultures[ci].bank;
-
-            // Majority regionId of the cluster (ties resolve by cluster order -- deterministic).
-            std::unordered_map<int, int> regCount; int regBest = -1, regBestN = 0;
-            for (int s : cluster) {
-                int c = ++regCount[settlements[s].regionId];
-                if (c > regBestN) { regBestN = c; regBest = settlements[s].regionId; }
         }
 
-            Culture child;
+        // C2) Overwhelming world dominance: past cultDominanceShare of the world's living population, a
-            child.id = sCultureNextId++;
+        // rising per-year chance (cultDominanceProbMin at the threshold, cultDominanceProbMax at 100%)
-            child.parentId = (int)parId;
+        // that the culture fractures on its own, no geography required -- half or a fifth of its own
-            child.foundedYear = year;
+        // members (an even coin flip), the farthest-flung from its own population-weighted core, found
-            child.regionId = regBest;
+        // a new people. Curbs one culture quietly conquering the whole world by attrition.
-            child.bank = (regBest >= 0) ? namegen::bankForRegion(cfg.seed, regBest) : parBank;
+        if (cfg.cultDominanceProbMax > 0.0 && totalLivingPop > 0.0) {
-
+            for (size_t ci = 0; ci < ncult && (int)cultures.size() < 64; ++ci) {
-            // Ethos/faith re-derived from the cluster's own lands (hash-salted by the child id so the
+                if (memCount[ci] < 2 || cSum[ci].length() <= 1e-12) continue;
-            // faith can mutate); keep the parent's faith name if the faith itself is unchanged.
+                double share = cultures[ci].totalPop / totalLivingPop;
-            CultEnv env;
+                if (share < cfg.cultDominanceShare) continue;
-            for (int s : cluster) envAdd(env, cells, sMoist, cfg.seaLevel, settlements[s].cell);
+                double t = std::clamp((share - cfg.cultDominanceShare)
-            uint32_t rk = child.id * 0x9E3779B9u + 0xC41Du;
+                                     / std::max(1e-6, 1.0 - cfg.cultDominanceShare), 0.0, 1.0);
-            envPick(env, seed, rk, (int)cluster.size(), child.ethos, child.faith);
+                double prob = cfg.cultDominanceProbMin + (cfg.cultDominanceProbMax - cfg.cultDominanceProbMin) * t;
-
+                if (cultHashf(cultures[ci].id * 2654435761u ^ (uint32_t)year * 40503u ^ seed ^ 0xD0111Au)
-            uint32_t nameSeed = seed ^ cultHash(rk * 2654435761u + 0x50C1A1u);
+                    >= prob) continue;
-            std::string root = namegen::makeName(nameSeed, child.bank);
+                double fraction = (cultHashf(cultures[ci].id * 2654435761u ^ (uint32_t)year * 19349663u
-            auto usedName = [&](const std::string& r) {
+                                            ^ seed ^ 0xF4AC7u) < 0.5) ? 0.5 : 0.2;
-                for (const Culture& c : cultures) if (c.name == "the " + r) return true; return false;
+                Vec3 centroid = cSum[ci].normalized();
-            };
+                std::vector<std::pair<double, int>> byDist;
-            for (int g = 0; usedName(root) && g < 128; ++g)
+                for (size_t s = 0; s < ns; ++s) {
-                root = namegen::makeName(nameSeed += 0x9E3779B9u, child.bank);
+                    if (!living(s) || sSettleCulture[s] != (int)ci) continue;
-            child.name = "the " + root;
-            child.faithName = (child.faith == parFaith)
-                            ? parFaithName
-                            : makeFaithName(seed ^ cultHash(rk * 40503u + 0xFA17Fu), child.bank);
-
-            int childIdx = (int)cultures.size();
-            for (int s : cluster) {
                     int sc = settlements[s].cell;
-                if (sc >= 0 && sc < n && (cells[sc].editLock & LockCulture)) continue;   // edit mode: pinned
+                    if (sc >= 0 && sc < n && (cells[sc].editLock & LockCulture)) continue;
-                sSettleCulture[s] = childIdx; converted[s] = 1;
+                    byDist.push_back({ ang(unitOf(s), centroid), (int)s });
+                }
+                if ((int)byDist.size() < 2) continue;
+                std::stable_sort(byDist.begin(), byDist.end(),
+                                [](const auto& a, const auto& b) { return a.first > b.first; });   // farthest first
+                int want = std::clamp((int)std::llround(byDist.size() * fraction), 1, (int)byDist.size() - 1);
+                std::vector<int> cluster;
+                for (int k = 0; k < want; ++k) cluster.push_back(byDist[(size_t)k].second);
+                doSchism((int)ci, cluster, "splits away from",
+                        "Grown too vast to hold together, the people drift into rival identities.",
+                        settlements[cluster.front()].cell);
+            }
+        }
+
+        // C3) A war between two realms of the same culture: cultWarSchismChance odds that one side's
+        // whole realm renounces the shared identity right as the war begins -- a war between kin only
+        // makes sense once they're no longer kin. Checked against wars declared THIS year only.
+        if (cfg.cultWarSchismChance > 0.0 && sSettleAllegiance.size() == ns) {
+            for (const War& w : wars) {
+                if (w.startYear != year || (int)cultures.size() >= 64) continue;
+                int attCap = w.attacker, defCap = w.defender;
+                if (attCap < 0 || attCap >= (int)ns || defCap < 0 || defCap >= (int)ns) continue;
+                int ci = sSettleCulture[attCap];
+                if (ci < 0 || ci >= (int)cultures.size() || sSettleCulture[defCap] != ci) continue;
+                if (cultHashf(w.id * 2654435761u ^ (uint32_t)year * 40503u ^ seed ^ 0x5A57A0u)
+                    >= cfg.cultWarSchismChance) continue;
+                int chosenCap = (cultHashf(w.id * 2654435761u ^ seed ^ 0x51DE5u) < 0.5) ? attCap : defCap;
+                std::vector<int> cluster; int total = 0;
+                for (size_t s = 0; s < ns; ++s) {
+                    if (!living(s) || sSettleCulture[s] != ci) continue;
+                    ++total;
+                    if ((int)s != chosenCap && sSettleAllegiance[s] != chosenCap) continue;
+                    int sc = settlements[s].cell;
+                    if (sc >= 0 && sc < n && (cells[sc].editLock & LockCulture)) continue;
+                    cluster.push_back((int)s);
+                }
+                if (cluster.empty() || (int)cluster.size() >= total) continue;
+                doSchism(ci, cluster, "renounces kinship with",
+                        "War between kin cracks their shared identity beyond repair.",
+                        settlements[chosenCap].cell);
             }
-            std::string title = child.name + " break away from " + parName;
-            title[0] = (char)std::toupper((unsigned char)title[0]);
-            ev.push_back(WarEvent{ 2, settlements[far].cell, title,
-                "A distant people drifts apart and names itself anew.", 7 });
-            cultures.push_back(std::move(child));
         }
     }
     return ev;

src/sim/PlanetIO.cpp

@@ -71,6 +71,7 @@
     D(civColonySpacing) D(civColonySupply) \
     D(cultAssimRate) D(cultConvertRate) D(cultConvertDominance) D(cultSpreadRange) \
     D(cultPrestigeWeight) D(cultSchismRange) D(cultSchismRate) \
+    D(cultDominanceShare) D(cultDominanceProbMin) D(cultDominanceProbMax) D(cultWarSchismChance) \
     I(subdivisions) I(plateCount) I(beltWidth) I(splitCheckEvery) I(stalemateWindows) \
     I(miniPlateCells) I(fuseMinPlates) I(babyMinCells) I(seaLevelEvery) \
     I(climateWindPasses) I(climateMoistureSmooth) I(seasonContinentRings) I(weatherSystemMax) \
@@ -268,7 +269,9 @@ static const struct { const char* name; const char* category; } kConfigFieldCate
     {"cultConvertDominance", "Cultural evolution"}, {"cultSpreadRange", "Cultural evolution"},
     {"cultPrestigeWeight", "Cultural evolution"}, {"cultSchismMinMembers", "Cultural evolution"},
     {"cultSchismRange", "Cultural evolution"}, {"cultSchismMinCluster", "Cultural evolution"},
-    {"cultSchismRate", "Cultural evolution"}, {"cultDawnSettlementsPerCulture", "Cultural evolution"},
+    {"cultSchismRate", "Cultural evolution"}, {"cultDominanceShare", "Cultural evolution"},
+    {"cultDominanceProbMin", "Cultural evolution"}, {"cultDominanceProbMax", "Cultural evolution"},
+    {"cultWarSchismChance", "Cultural evolution"}, {"cultDawnSettlementsPerCulture", "Cultural evolution"},
     {"cultDawnMaxPerContinent", "Cultural evolution"},
 };
 
@@ -560,6 +563,10 @@ std::string validateConfig(const PlanetConfig& cfg) {
     E(rng(cfg.cultSchismRange, 0.0, 3.14159, "cultSchismRange"));
     E(irng(cfg.cultSchismMinCluster, 1, 1000000, "cultSchismMinCluster"));
     E(rng(cfg.cultSchismRate, 0.0, 1.0, "cultSchismRate"));
+    E(rng(cfg.cultDominanceShare, 0.5, 1.0, "cultDominanceShare"));
+    E(rng(cfg.cultDominanceProbMin, 0.0, 1.0, "cultDominanceProbMin"));
+    E(rng(cfg.cultDominanceProbMax, 0.0, 1.0, "cultDominanceProbMax"));
+    E(rng(cfg.cultWarSchismChance, 0.0, 1.0, "cultWarSchismChance"));
     E(irng(cfg.cultDawnSettlementsPerCulture, 1, 1000000, "cultDawnSettlementsPerCulture"));
     E(irng(cfg.cultDawnMaxPerContinent, 1, 1000, "cultDawnMaxPerContinent"));
     E(irng(cfg.subdivisions, 0, 7, "subdivisions"));

src/sim/PlanetTypes.hpp

@@ -568,6 +568,14 @@ struct PlanetConfig {
     double cultSchismRange      = 0.55;   // rad from the culture's population centroid past which members are "distant"
     int    cultSchismMinCluster = 2;      // distant settlements needed to break away together
     double cultSchismRate       = 0.08;   // per-year chance a qualifying distant cluster becomes a new people
+    // Two more schism triggers, independent of the distant-cluster one above: a culture that swallows
+    // most of the world fractures on its own (no geography needed), and a war between same-culture
+    // realms can crack their shared identity right at the outbreak. Both curb one people quietly
+    // conquering the whole world by attrition.
+    double cultDominanceShare   = 0.80;   // world population share above which dominance-schism can trigger
+    double cultDominanceProbMin = 0.01;   // per-year schism chance right at cultDominanceShare
+    double cultDominanceProbMax = 0.99;   // per-year schism chance at 100% world population share
+    double cultWarSchismChance  = 0.50;   // chance a war between same-culture realms splits one side away
     // Dawn seeding (seedCultures()): a populous continent starts as SEVERAL distinct peoples instead
     // of one continent-spanning culture -- without this, decades of border conversion/backfill tend
     // to erode a single per-continent culture into one super-dominant people across the whole world.

test_cultevo.cpp

@@ -12,8 +12,10 @@
 //
 // Verifies: seeding parity (one culture per continent, frozen identities); colonies inherit the
 // founder's culture; assimilation under foreign allegiance; border conversion (capitals exempt);
-// schism (a distant cluster becomes a new people); determinism + RNG isolation; snapshot
+// three independent schism triggers -- a distant cluster becomes a new people, an overwhelmingly
-// truncate-and-replay; save-v23 round-trip + corrupt-stream rejection; pre-v23 compatibility.
+// dominant culture splits on its own (half or a fifth of its members), a war between same-culture
+// realms splits one side; determinism + RNG isolation; snapshot truncate-and-replay; save-v23
+// round-trip + corrupt-stream rejection; pre-v23 compatibility.
 
 #include "Planet.hpp"
 #include <cstdio>
@@ -40,6 +42,10 @@ static void drift(Planet& p, int iters) {
 }
 static void quietCulture(Planet& p) {   // no evolution: state should then never change
     p.cfg.cultAssimRate = 0.0; p.cfg.cultConvertRate = 0.0; p.cfg.cultSchismRate = 0.0;
+    // Both, not just Max: the gate check only needs Max == 0, but leaving Min at its default (0.01)
+    // violates validateConfig()'s cultDominanceProbMin <= cultDominanceProbMax cross-rule.
+    p.cfg.cultDominanceProbMin = 0.0; p.cfg.cultDominanceProbMax = 0.0;
+    p.cfg.cultWarSchismChance = 0.0;
 }
 static bool culturesEqual(const std::vector<Culture>& a, const std::vector<Culture>& b) {
     if (a.size() != b.size()) return false;
@@ -214,6 +220,80 @@ int main() {
         check(p.cultureList().size() <= 64, "the culture list stays bounded");
     }
 
+    std::printf("Cultural evolution: dominance schism (a culture holding most of the world splits)\n");
+    {
+        Planet dom = p;
+        quietCulture(dom);
+        // Always trigger once eligible (min==max==1) so the test doesn't depend on the probability ramp.
+        dom.cfg.cultDominanceShare = 0.80; dom.cfg.cultDominanceProbMin = 1.0; dom.cfg.cultDominanceProbMax = 1.0;
+        dom.computeTerritory(); dom.computeCultures();
+        // Force every living settlement onto one culture so it starts at ~100% world population share --
+        // dominance-schism has no distance/cluster requirement, so this alone should trigger it.
+        int targetCi = -1;
+        for (size_t k = 0; k < dom.settlements.size(); ++k) {
+            if (dom.settlements[k].population < dom.cfg.civAbandonPop) continue;
+            if (targetCi < 0) targetCi = dom.settleCulture()[k];
+            if (targetCi >= 0) break;
+        }
+        check(targetCi >= 0, "a living settlement with a seeded culture exists to consolidate onto");
+        if (targetCi >= 0) {
+            for (size_t k = 0; k < dom.settlements.size(); ++k)
+                if (dom.settlements[k].population >= dom.cfg.civAbandonPop) dom.setSettlementCulture((int)k, targetCi);
+            dom.computeTerritory(); dom.computeCultures();   // retally members/totalPop after consolidation
+            int beforeMembers = dom.cultureList()[(size_t)targetCi].members;
+            size_t beforeCultCount = dom.cultureList().size();
+            bool splitHappened = false;
+            for (long yr = 0; yr < 10 && !splitHappened; ++yr) {
+                for (const WarEvent& e : civYear(dom, yr))
+                    if (e.kind == 7 && e.title.find("splits away from") != std::string::npos) splitHappened = true;
+            }
+            check(splitHappened, "an overwhelmingly dominant culture eventually splits on its own");
+            check(dom.cultureList().size() > beforeCultCount, "the split appends a new culture");
+            if (splitHappened && dom.cultureList().size() > beforeCultCount) {
+                const Culture& child = dom.cultureList().back();
+                check(child.parentId == (int)dom.cultureList()[(size_t)targetCi].id,
+                      "the child records the dominant culture as parent");
+                int adopted = 0;
+                for (int c : dom.settleCulture()) if (c == (int)dom.cultureList().size() - 1) ++adopted;
+                double frac = (double)adopted / std::max(1, beforeMembers);
+                check(std::fabs(frac - 0.5) < 0.05 || std::fabs(frac - 0.2) < 0.05,
+                      "the breakaway is close to half or a fifth of the dominant culture's prior members");
+            }
+        }
+    }
+
+    std::printf("Cultural evolution: war schism (a war between same-culture realms can split one side)\n");
+    {
+        Planet warp = p;
+        quietCulture(warp);
+        warp.cfg.cultWarSchismChance = 1.0;   // always trigger once a same-culture war is seen
+        warp.computeTerritory(); warp.computeCultures();
+        int capA = -1, capB = -1;
+        for (size_t k = 0; k < warp.settlements.size() && capB < 0; ++k) {
+            if (warp.settlements[k].population < warp.cfg.civAbandonPop) continue;
+            if (capA < 0) capA = (int)k; else capB = (int)k;
+        }
+        check(capA >= 0 && capB >= 0, "at least two living settlements exist to test a war schism");
+        if (capA >= 0 && capB >= 0) {
+            int ci = warp.settleCulture()[(size_t)capA];
+            if (ci < 0) ci = 0;
+            warp.setSettlementCulture(capA, ci);
+            warp.setSettlementCulture(capB, ci);
+            warp.computeTerritory(); warp.computeCultures();
+            long yr = 0;
+            warp.stepConflict(yr);   // sizes/refreshes sSettleAllegiance; any real wars it declares are harmless extras
+            War w; w.id = 999999; w.attacker = capA; w.defender = capB; w.startYear = yr;
+            warp.wars.push_back(w);
+            size_t before = warp.cultureList().size();
+            std::vector<WarEvent> ev = warp.stepCulture(yr);
+            bool splitHappened = false;
+            for (const WarEvent& e : ev)
+                if (e.kind == 7 && e.title.find("renounces kinship with") != std::string::npos) splitHappened = true;
+            check(splitHappened, "a war between same-culture realms splits one side's culture");
+            check(warp.cultureList().size() > before, "the war-triggered split appends a new culture");
+        }
+    }
+
     std::printf("Cultural evolution: determinism\n");
     {
         Planet q; q.generate(cfg); settle(q); drift(q, 400);