Ehud Shapiro

AAMAS Conference 2026 Conference Paper

Grassroots Federation: Fair Democratic Governance at Scale

• Nimrod Talmon
• Ehud Shapiro

We propose a framework for the fair democratic governance of federateddigitalcommunitiesthatformandevolvedynamically, where small groups self-govern and larger groups are represented by assemblies selected via sortition. Prior work addressed static fairness conditions; here, we formalize a dynamic setting where federations evolve over time through communities forming, joining, and splitting, in all directions—bottom-up, top-down, and middle-out—and adapt the fairness guarantees. The main technical challenge is reconciling integral seat allocations with dynamic, overlapping federations, so that child communities always meet their persistent floors while long-run averages converge to proportional fairness. Overcoming these challenges, we introduce a protocol that ensures fair participation and representation both persistently (at all times) and eventually (in the limit after stabilization), extending the static fairness properties to handle structural changes. Prior work shows that grassroots federations can be specified via atomic transactions among assembly members, and that Constitutional Consensus can realize both these transactions and the democratic processes leading to them. Together, the four works form a complete design for an egalitarian, fairly governed, largescale decentralized sovereign digital community platform.

TARK Conference 2025 Conference Paper

AI-Generated Compromises for Coalition Formation: Modeling, Simulation, and a Textual Case Study

• Eyal Briman
• Ehud Shapiro
• Nimrod Talmon

The challenge of finding compromises between agent proposals is fundamental to AI sub-fields such as argumentation, mediation, and negotiation. Building on this tradition, Elkind et al. (2021) introduced a process for coalition formation that seeks majority-supported proposals preferable to the status quo, using a metric space where each agent has an ideal point. The crucial step in this iterative process involves identifying compromise proposals around which agent coalitions can unite. How to effectively find such compromise proposals, however, remains an open question. We address this gap by formalizing a holistic model that encompasses agent bounded rationality and uncertainty and developing AI models to generate such compromise proposals. We focus on the domain of collaboratively writing text documents -- e. g. , to enable the democratic creation of a community constitution. We apply NLP (Natural Language Processing) techniques and utilize LLMs (Large Language Models) to create a semantic metric space for text and develop algorithms to suggest suitable compromise points. To evaluate the effectiveness of our algorithms, we simulate various coalition formation processes and demonstrate the potential of AI to facilitate large-scale democratic text editing, such as collaboratively drafting a constitution, an area where traditional tools are limited.

AAAI Conference 2025 Conference Paper

Federated Assemblies

• Daniel Halpern
• Ariel D. Procaccia
• Ehud Shapiro
• Nimrod Talmon

A *citizens' assembly* is a group of people who are randomly selected to represent a larger population in a deliberation. While this approach has successfully strengthened democracy, it has certain limitations that suggest the need for assemblies to form and associate more organically. In response, we propose *federated assemblies*, where assemblies are interconnected, and each parent assembly is selected from members of its child assemblies. The main technical challenge is to develop random selection algorithms that meet new representation constraints inherent in this hierarchical structure. We design and analyze several algorithms that provide different representation guarantees under various assumptions on the structure of the underlying graph.

AAMAS Conference 2022 Conference Paper

Foundations for Grassroots Democratic Metaverse

• Ehud Shapiro
• Nimrod Talmon

While the physical lives of many of us are in democracies (one person, one vote — e. g. , the EU and the US), our digital lives are mostly in autocracies (one person, all votes — e. g. , Facebook). Cryptocurrencies promise liberation but stop short, at plutocracy (one coin, one vote). What would it take for us to live in a digital democracy? This paper offers a vision, a theoretical framework, and an architecture for a grassroots network of autonomous, people-owned, people-operated, and people-governed digital communities, namely a grassroots democratic metaverse. It also charts a roadmap towards realizing it, and identifies unexplored territory for MAS research.

EUMAS Conference 2022 Conference Paper

Sybil-Resilient Social Choice with Low Voter Turnout

• Reshef Meir
• Nimrod Talmon
• Gal Shahaf
• Ehud Shapiro

Abstract We address social choice in the presence of sybils (fake or duplicate votes) and low turnout, two behaviors that may each distort the will of the society. To do so we assume the status quo as an ever-present distinguished alternative. We propose a general Reality Enforcing mechanism, which can be combined with arbitrary voting rules and operates by adding virtual votes that support the status quo. We measure the tradeoff between safety and liveness (the ability of non-abstaining non-sybil voters to maintain or to change the status quo, respectively) in a variety of voting domains and show a tight inherent limit to the amount of sybils and abstentions that can be tolerated.

JAIR Journal 2021 Journal Article

Aggregation over Metric Spaces: Proposing and Voting in Elections, Budgeting, and Legislation

• Laurent Bulteau
• Gal Shahaf
• Ehud Shapiro
• Nimrod Talmon

We present a unifying framework encompassing a plethora of social choice settings. Viewing each social choice setting as voting in a suitable metric space, we offer a general model of social choice over metric spaces, in which—similarly to the spatial model of elections—each voter specifies an ideal element of the metric space. The ideal element acts as a vote, where each voter prefers elements that are closer to her ideal element. But it also acts as a proposal, thus making all participants equal not only as voters but also as proposers. We consider Condorcet aggregation and a continuum of solution concepts, ranging from minimizing the sum of distances to minimizing the maximum distance. We study applications of our abstract model to various social choice settings, including single-winner elections, committee elections, participatory budgeting, and participatory legislation. For each setting, we compare each solution concept to known voting rules and study various properties of the resulting voting rules. Our framework provides expressive aggregation for a broad range of social choice settings while remaining simple for voters; and may enable a unified and integrated implementation for all these settings, as well as unified extensions such as sybil-resiliency, proxy voting, and deliberative decision making.

AAMAS Conference 2021 Conference Paper

Egalitarian and Just Digital Currency Networks

• Gal Shahaf
• Ehud Shapiro
• Nimrod Talmon

We explore the possibility of an alternative cryptocurrency that is egalitarian in control, just in the distribution of its created wealth, and forms and grows in a grassroots way; we analyze it via a mathematical entity called a currency network. Our main result states sufficient conditions for the establishment of 1: 1 exchange rates and distributive justice in a currency network.

AAMAS Conference 2021 Conference Paper

How to Amend a Constitution? Model, Axioms, and Supermajority Rules

• Ben Abramowitz
• Ehud Shapiro
• Nimrod Talmon

A self-governed society must have decision rules by which group decisions are made, and these rules are often codified in a written constitution. One of the defining features of a constitution is its degree of entrenchment, or how hard it is to change by amendment. If it is too easy to make amendments, then the constitution can change too frequently, leading to chaos. On the other hand, if it is too hard to make amendments, then this can also be destabilizing, as voters may begin to see the rules as less legitimate, or even seek to overturn the status quo in a revolt. As norms, priorities, and circumstances change over time and over generations a constitution must be able to adapt. Our work considers a stylized model of constitutions that use reality-aware supermajority rules to make decisions. We propose principles for designing amendment procedures for changing decision rules in these constitutions and propose a novel procedure based on these principles.

AAAI Conference 2021 Conference Paper

United for Change: Deliberative Coalition Formation to Change the Status Quo

• Edith Elkind
• Davide Grossi
• Ehud Shapiro
• Nimrod Talmon

We study a setting in which a community wishes to identify a strongly supported proposal from a large space of alternatives, in order to change the status quo. We describe a deliberation process in which agents dynamically form coalitions around proposals that they prefer over the status quo. We formulate conditions on the space of proposals and on the ways in which coalitions are formed that guarantee deliberation to succeed, that is, to terminate by identifying a proposal with the largest possible support. Our results provide theoretical foundations for the analysis of deliberative processes in systems for democratic deliberation support, such as, e. g. , LiquidFeedback or Polis.

IJCAI Conference 2019 Conference Paper

Sybil-Resilient Reality-Aware Social Choice

• Gal Shahaf
• Ehud Shapiro
• Nimrod Talmon

Sybil attacks, in which fake or duplicate identities (a. k. a. , Sybils) infiltrate an online community, pose a serious threat to such communities, as they might tilt community-wide decisions in their favor. While the extensive research on sybil identification may help keep the fraction of sybils in such communities low, it cannot however ensure their complete eradication. Thus, our goal here is to enhance social choice theory with effective group decision mechanisms for communities with bounded sybil penetration. Inspired by Reality-Aware Social Choice, we use the status quo as the anchor of Sybil Resilience, characterized by Sybil Safety -- the inability of sybils to change the status quo against the will of the genuine agents, and Sybil Liveness -- the ability of the genuine agents to change the status quo against the will of the sybils. We consider the social choice settings of deciding on a single proposal, on multiple proposals, and on updating a parameter. For each, we present social choice rules that are sybil-safe and, under certain conditions, satisfy sybil-liveness.

AAMAS Conference 2018 Conference Paper

Incorporating Reality into Social Choice

• Ehud Shapiro
• Nimrod Talmon

When voting on a proposal one in fact chooses between two alternatives: (i) A new hypothetical social state depicted by the proposal and (ii) the status quo (henceforth: Reality); a Yes vote favors a transition to the proposed hypothetical state, while a No vote favors Reality. Social Choice theory generalizes voting on one proposal to ranking multiple proposals; that Reality was forsaken during this generalization is, in our view, inexplicable. Here we propose to rectify this neglect and incorporate Reality into Social Choice, distinguishing Reality from hypothesis. We show that doing so: (i) Offers a natural resolution to Condorcet’s paradox; (ii) Explains what approval voters approve; (iii) Produces a simple and efficient Condorcet-consistent show-of-hands agenda; (iv) Produces democratic action plans, which start with Reality and proceed in democratically-supported transitions; and (v) Nullifies Independence of Irrelevant Alternatives and hence abdicates Arrow’s Theorem. Arrow’s theorem was taken to show that democracy, conceived as government by the will of the people, is an incoherent illusion. Incorporating Reality into Social Choice may clear this intellectual blemish on democracy and offer a coherent, simple, efficient, easy to communicate, and trustworthy path forward to democracy.

TCS Journal 2004 Journal Article

BioAmbients: an abstraction for biological compartments

• Aviv Regev
• Ekaterina M. Panina
• William Silverman
• Luca Cardelli
• Ehud Shapiro

Biomolecular systems, composed of networks of proteins, underlie the major functions of living cells. Compartments are key to the organization of such systems. We have previously developed an abstraction for biomolecular systems using the π-calculus process algebra, which successfully handled their molecular and biochemical aspects, but provided only a limited solution for representing compartments. In this work, we extend this abstraction to handle compartments. We are motivated by the ambient calculus, a process algebra for the specification of process location and movement through computational domains. We present the BioAmbients calculus, which is suitable for representing various aspects of molecular localization and compartmentalization, including the movement of molecules between compartments, the dynamic rearrangement of cellular compartments, and the interaction between molecules in a compartmentalized setting. Guided by the calculus, we adapt the BioSpi simulation system, to provide an extended modular framework for molecular and cellular compartmentalization, and we use it to model and study a complex multi-cellular system.

STOC Conference 1991 Conference Paper

Separating Concurrent Languages with Categories of Language Embeddings (Extended Abstract)

• Ehud Shapiro