---
title: "Closing the Loop: Bringing Humans into Empirical Computational Social Choice and Preference Reasoning"
authors:
  - "Nicholas Mattei"
doi: "10.24963/ijcai.2020/729"
url: "https://ijcai.org/proceedings/2020/729"
pdf: "https://ijcai.org/proceedings/2020/0729.pdf"
venue: "Proceedings of the Twenty-Ninth International Joint Conference on Artificial Intelligence"
year: 2020
pages: "5169-5173"
track: "Early Career"
keywords:
  - "Agent-based and Multi-agent Systems: Voting"
  - "AI Ethics: Moral Decision Making"
  - "Agent-based and Multi-agent Systems: Algorithmic Game Theory"
  - "Agent-based and Multi-agent Systems: Computational Social Choice"
---

# Closing the Loop: Bringing Humans into Empirical Computational Social Choice and Preference Reasoning

Nicholas Mattei

*Proceedings of the Twenty-Ninth International Joint Conference on Artificial Intelligence*, pages 5169-5173. DOI: [10.24963/ijcai.2020/729](https://doi.org/10.24963/ijcai.2020/729)

## Abstract

Research in both computational social choice and preference reasoning uses tools and techniques from computer science, generally algorithms and complexity analysis, to examine topics in group decision making.  This has brought tremendous progress in the last decades, creating new avenues for research and results in areas including voting and resource allocation.  I argue that of equal importance to the theoretical results are impacts in research and development from the empirical part of the computer scientists toolkit: data, system building, and human interaction. I highlight work by myself and others to establish data driven, application driven research in the computational social choice and preference reasoning areas. Along the way, I highlight interesting application domains and important results from the community in driving this area to make concrete, real-world impact.

## Full text

## 1 Introduction

The Internet enables computers and, by proxy, humans to both communicate and form groups at distances, speeds, and scales that recently lived only in the imagination. These groups of agents, be they human, computer, or a mix of the two, must make collective decisions subject to external and internal constraints and preferences in many important realworld settings. Group decisions have classically been the purview of research in social choice, which concerns itself with the study of how groups, where each member has their own preferences, make decisions that must be then shared by that group [Sen, 2018]. The field of computational social choice (COMSOC) adds to the standard model computational questions and modes of analysis including algorithms and complexity [Brandt et al., 2016a] as well as incorporating work more broadly in artificial intelligence including preference reasoning and machine learning. [Domshlak et al., 2011]. Taken together, this work falls mostly in multiagent systems research [Shoham and Leyton-Brown, 2008; Wooldridge, 2009], where we study not only systems, but the interactions between many systems. This intersection of tools and techniques has led to broad and important results in areas including resource allocation problems such as kidney exchange [Roth et al., 2005; Dickerson et al., 2014], school choice [Abdulkadiro˘glu et al., 2005], routing traffic [Kleinberg et al., 1999], fair division of continuous resources [Aziz and Mackenzie, 2016], allocating radio spectra [Leyton-Brown et al., 2017], fair assignment [Aziz, 2019; Lian et al., 2018], and allocating security resources for patrol [Tambe, 2012]; as well as distributed decision tasks including (multi-winner) voting [Conitzer et al., 2007; Faliszewski et al., 2017], distributed democracy [Brill, 2018], and peer selection [Aziz et al., 2019; Xu et al., 2018].

In a traditional social choice setting we have a finite set of agents N = {1, . . . , n} and a finite set of alternatives A = {1, . . . ,m}. Each agent i ∈ N has an order-able preference ≿i over the alternatives. In social choice we will focus on two important goals: voting and allocation. In voting, the preferences of all agents are combined to select either an element of or an ordering over the set of alternatives A to share amongst the group. In allocation, the items within A are to be distributed or allocated to the set of agents in N. The common thread among these settings is that we have a group of self-interested agents submit their preferences to a centralized or de-centralized authority and outcomes are decided by some mechanism. Each mechanism for group decision making may or may not satisfy various design goals such as fairness and/or efficiency as analyzed with formal models of agent behavior [Brandt et al., 2016a; Shoham and Leyton-Brown, 2008]. In some instances these models of behavior provide a limited or (occasionally) unrealistic view into many real-world settings. Researchers in economics have shown where the predictions of game theory are contradicted by data or experiment; giving rise to behavioral and experimental economics [Kagel and Roth, 1995] Consideration of the human element has lead to significant impacts in the areas of markets and matching including school choice [Abdulkadiro˘glu et al., 2005] and kidney exchanges [Roth et al., 2005]. However, until recently there has not been a data driven, systems oriented research program that directly questions these strong assumptions in COMSOC research.

My work focuses on:

1. closing the gap between these idealized models of agent behavior and reality by using empirical, data driven studies focusing on how humans really make decisions;
2. building concrete, real-world systems based on these

- findings to encourage broad dissemination and availability of the research results; and
3. promoting empirical and data driven work in the community by building websites like www.preflib.org and running data focused workshops like the Exploring Beyond the Worst Case in Computational Social Choice (EXPLORE) and the Games, Agents, and Incentives Workshop (GAIW).

One of the most exciting things about computer science research is that we can both collaborate broadly with researchers in economics, psychology, biology and beyond and use our engineering skills to create real systems that can be completely automated or interact with humans at many different scales. I argue that a focus on data and real-world system building leads to the largest research impacts. This is an engineering mindset: to discover new theory and novel techniques to build systems that make the world a better place by advancing the state of the art in intelligent systems.

## 2 Empirical Computational Social Choice and Preference Reasoning

A key difference between social choice and computational social choice (COMSOC) is the focus on the concerns of a computer scientist: approximations, algorithms, and, I would argue, systems. The focus of my work has been to build systems and build theoretical models that help people and agents make decisions. In order to do this effectively we need to engage with the messy and sometimes downright irrational ways that choices and systems can be used in the real world - and integrate these aspects into preference reasoning and social choice research [Popova et al., 2013; Allen et al., 2015]. I argue that the most successful research projects are a blend of theoretical innovations, experimental analysis, and practical tool building. We should walk the same road that Kagel and Roth [1995] describe for experimental economics: evolving from theory, to simulated or re-purposed data, to full fledged laboratory and field experiments. This progression has enabled a "conversation" between experimental and theoretical researchers that, in turn, significantly advanced the field. In this section I detail some efforts to advance down this road.

Within the broader social choice field looking at voting and allocations, there has been work that analyzed large datasets to understand human behavior. Regenwetter et al. [2007] collected data from elections of professional societies, analyzing these elections for paradoxes and evidence of various proposed voter utility models. Likewise, the work of Plassmann and Tideman [2014] collected a number of historic elections in an effort to quantify the likelihood of various election paradoxes. In areas such as recommender systems and machine learning, using data to understand user decisions was already well established [Bache and Lichman, 2013; Kamishima, 2003]. Finally, in the broader matching community there is significant work on human behavior for market systems [Abdulkadiro˘glu et al., 2005; Roth et al., 2005]. However, within COMSOC there had not been a broad, data driven program of research.

When I started my research, I wanted to see empirically how these theories and models of agent behavior for, e.g., manipulation and voting paradoxes, manifested in the real world. I was struck by the small number of empirical research papers in the COMSOC community and wanted to expand the use of data [Walsh, 2011]. I performed empirical studies of voting systems, mining large sets of real-world preference data to create elections from user preferences, and found that many common assumptions about worst-case agent behavior, preference domain restrictions, and election paradoxes, did not occur in practice [Mattei, 2011; Mattei et al., 2012]. To encourage more researchers to include experimental and empirical verification in their work, Toby Walsh and I established PREFLIB.ORG and PREFLIB TOOLS for working with real-world preference data [Mattei and Walsh, 2017; Mattei and Walsh, 2013]. To date we have gathered thousands of instances of data from real-world domains where agents express preferences in domains including voting, resource allocation, ranking, and rating. We unified this data into a common format, making it easy to access and reuse this data.

The use of PREFLIB and the broader mindset of empirical verification has expanded across the COMSOC community and beyond, feeding back into social choice and other domains. Examples include the area of multiwinner voting, which is typically a computationally hard problem, where Skowron et al. [2015] and others have used data and experiments to understand when certain multiwinner voting rules admit good approximations in practice or are computationally tractable. Bredereck et al. [2019] use experiments on PREFLIB and simulated data to see when committees, selected using multiwinner voting rules, satisfy important properties such as justified representation. Finally, Szufa et al. [2020] use models generated from agent preferences to define notions of distances between elections, and then experimentally use optimization techniques to create novel multiwinnner voting rules that satisfy good properties across a range of voting behavior. More than just enabling experiments, PREFLIB has provided a large library of data to use for looking at election paradoxes and inconsistencies. The data found in PREFLIB in conjunction with advanced automated proof finding techniques have led to a deeper understanding of the causes of voting paradoxes [Brandt et al., 2016b].

More than just re-purposing existing data, researchers in social choice have conducted experiments to gather data about how people really make decisions and use this knowledge to feed back into the models used for theoretical research. Meir et al. [2020b] detail the design and implementation of an experimental system where choices of humans faced with group choice settings with access to uncertain poll information about an election are recorded. The goal of these experiments is to quantify how often and what types of strategic behavior people use in practice. Likewise, Mennle et al. [2015] ran a set of experiments in the assignment setting where agents could strategically misreport their preferences in order to gain more utility. The setting involved a computationally hard manipulation problem so most agents relied on simpler heuristics to find better, but not the best, strategic profiles. Finally, Scheuerman et al. [2019] have conducted a series of experiments on Mechanical Turk to understanding what factors of the decision environment including uncertainty and payoff amounts affect when and how agents will act strategically in approval voting settings.

This use of empirical research has begun to feed back into the theoretical research of how agents make decisions when faced with specific group choice environments and mechanisms. Zou et al. [2015] use data from Doodle, arguably the most widely used system to make group decisions, and determined behavioral effects which led to different voting patterns such as the difference in approvals of public versus private polls. Zhao et al. [2018] make use of the OPERA system to devise a querying methodology for group decision making that minimizes the number of queries of individual users in order to come to a group decision quickly. Finally, Fairstein et al. [2019], present an analysis of the experimental data from Meir et al. [2020b] to propose a novel voter decision model which more accurately accounts for how agents make decisions in strategic voting environments.

## 3 Use Inspired Systems: Building Better Choices

In addition to a broader culture of experimentation with data, there has been a growing focus on COMSOC research that targets specific end users and systems. Perhaps some of the greatest successes in this area have been in the field of matching and resource allocation including kidney exchanges [Roth et al., 2005], spectrum auctions [Leyton-Brown et al., 2017], and security games [Tambe, 2012].

Since PREFLIB TOOLS a number of systems have been developed for general use. The Spliddit [Goldman and Procaccia, 2014] website allows any user to use some of the best algorithms in fair division research to split credit, rent, or even the cost of a cab. We have used similar ideas in order to help businesses in Australia divide the cost of on demand pickup and delivery problems, devising new heuristics and tools [Aziz et al., 2016a]. For voting and group decision making the website OPERA [Chen et al., 2020] provides a user friendly, interactive system to deploy a range of single and multiwinner voting rules for use in group decision making. Likewise, users interested in deploying results from randomized social choice can use the Pynx system [Brandt et al., 2015]. All of these systems are highly polished, provide mobile and internet interfaces, and are used around the world.

Another significant application of research in COMSOC has been results in peer selection, conference reviewing, and other work assignment settings. The broad area of artificial intelligence is experiencing unprecedented growth in the number of papers written and submitted for review and new ideas are surfacing on how to handle this load [Shah et al., 2018]. One idea for improving these systems is to create a market for the bids of agents on papers, as many papers are chronically under-demanded, leading to poor review quality [Meir et al., 2020a]. Once papers have bids, there is still the assignment problem left to be solved. There have been a number of recent works that use real-world data to test novel objectives for fair and efficient assignments [Lian et al., 2018; Aziz, 2019] as well as handle the discrepancy between scores assigned by the reviewers [Wang and Shah, 2019].

One idea to handle the load has been to enforce that submitters must also participate in the review process itself. This creates a peer selection problem, where agents evaluate one another and, based on these evaluations, a subset of agents are selected as winners. An obvious incentive problem arises in this setting: an agent may lie about their valuation for other agents in order to increase their chances of being selected. Peer selection has become an important topic in recent years for numerous applications: academic peer review including NSF grant reviewing; crowd-sourcing corporate or internal brainstorming sessions; and performing peer review for MOOCs. There are a number of novel strategyproof mechanisms in this setting including partitioning methods [Kurokawa et al., 2015], methods to handle complex interaction graphs between agents [Xu et al., 2018], partitioning methods that re-balance themselves [Aziz et al., 2016b; Aziz et al., 2019], and methods that relax exactness to achieve better overall efficiency [Mattei et al., 2020]. These results on assignment and selection will have broad impact not only for conference reviewing but hopefully the broader scientific enterprise and anywhere where work needs to be distributed and reviewed.

## 4 Human Centered Choices and Preference Reasoning

An important topic that emerges when we focus on use inspired systems is understanding how, when, and where these systems are used. One aspect of this topic where I have been working is in helping our students think creatively about technological systems and their impacts [Burton et al., 2018]. On the development side, economics and social choice have long had important things to say about the trade-offs sometimes necessary between efficiency and fairness including work focusing on the division of continuous resources such as land or water [Moulin, 2004] and on more discrete, indivisible settings such as goods and services [Thomson, 2011]. In political science and political economy we see extensive research into the fairness and equality of voting and other representation schemes [Brams, 2008] and more fundamentally in the areas of political economy having to do with justice and fair distribution of resources to individuals [Rawls, 1971].

As algorithms make more decisions in our daily lives, and the consequences of these decisions become more fraught [Corbett-Davies et al., 2017], it is important to leverage the history of thinking about fairness from these diverse sources. Equity and other concerns, formalized as economic axioms have a long history in social choice both in allocation and voting [Sen, 2018] We hope to leverage this long history of study of the axioms, or properties, of the algorithms and aspects including fairness to improve our systems. As we build our systems we need to understand and embed the constraints of our society that can come from numerous places including morals, laws, business process, and other sources [Rossi and Mattei, 2019]. There has been extensive work in the COMSOC and preference reasoning communities to formalize new preference models for use in ethical decision making [Loreggia et al., 2018] and even use aggregation rules to find compromises between the differing view of ethics that may be present in a community [Noothigattu et al., 2017].

Kidney allocation and exchanges are perhaps one of the earliest, well known examples of use inspired systems where there has both been significant impacts on practice and significant attention paid to fairness and equity concerns [Roth et al., 2005; Dickerson et al., 2012]. There have been extensions looking at the trade-off between fairness and efficiency in these markets and work on using voting to align the outcomes of these algorithms to the values that human decision makers use in the United States [Freedman et al., 2020]. We have also worked worked with the Australian Organ and Tissue Authority to theoretically and empirically analyze the efficiency and fairness trade-offs of their proposed online organ allocation mechanisms [Mattei et al., 2017; Mattei et al., 2018].

## 5 Conclusion

Work in multi-agent systems, COMSOC, and preference reasoning has and continues to have meaningful impact in the world in which we live. As we move forward in our research I argue that it is important to incorporate empirical, data driven understanding of human behavior in our models and focus on system building as a key part of research. With this comes a responsibility to do this work for the social good and to use our knowledge of decision making to inform how actual decisions are being made. There are many avenues to explore going forward including work in social networks, voter models, and online allocation that I have not touched on here. In sum, it is an exciting time to be a researcher in these areas as we can have broad, significant impact in the real world.

## Acknowledgements

None of this would be possible without my coauthors, research partners, and mentors who have continually challenged me to do great work. I am especially grateful to Haris Aziz, John P. Dickerson, Judy Goldsmith, Omer Lev, and Elizabeth Mattei for comments on drafts.

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