How can structured inequalities arise from initially equal societies?
This project explores that question through agent-based simulations inspired by:
- 🐝 The Bonabeau model (1995) — hierarchy from local competition and memory decay
- ⚖️ The Sadurní model (2024) — two-group rivalry with normalized prestige
We replicate, analyze, and extend these models with two original contributions:
- Unequal importance of winning depending on the group
- Internal (intra-group) competition dynamics
- Bonabeau: agents gain/lose fitness in fights; hierarchy emerges when density is high enough
- Sadurní: fitness is exchanged between two groups only; total fitness is conserved
- Our extensions: add asymmetric exchange rules and intra-group competition
Hierarchy emerges when density crosses a critical threshold

Low η → no structure; High η → clear dominance
| η = 1 (Low determinism) ➡️ No structure, random dynamics |
η = 5 (Moderate determinism) ➡️ One dominant leader emerges |
η = 10 (High determinism) ➡️ Temporary multi-leadership, then stability |
|---|---|---|
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Minority groups tend to develop stronger inequality (higher Gini coefficients)

When the fitness exchanged depends on the winner’s group:
- If one group loses significantly more in case of defeat, its internal dynamics become unstable.
- In this group, leaders emerge quickly but are replaced just as fast, producing short-lived hierarchies.
This models fragile societies, where power is volatile and leadership is constantly shifting.
Adding intra-group interactions introduces new dynamics:
- When inter-group competition dominates, hierarchy forms through repeated external victories.
- When intra-group rivalry dominates, strong internal stratification appears early and persists.
This mechanism alters how hierarchies form, even if the overall inequality remains similar.


