In many real world situations, collective decisions are made using voting and, in scenarios such as committee or board elections, employing voting rules that return multiple winners. In multi-winner approval voting (AV), an agent submits a ballot consisting of approvals for as many candidates as they wish, and winners are chosen by tallying up the votes and choosing the top-k candidates receiving the most approvals. In many scenarios, an agent may manipulate the ballot they submit in order to achieve a better outcome by voting in a way that does not reflect their true preferences. In complex and uncertain situations, agents may use heuristics instead of incurring the additional effort required to compute the manipulation which most favors them. In this paper, we examine voting behavior in single-winner and multi-winner approval voting scenarios with varying degrees of uncertainty using behavioral data obtained from Mechanical Turk. We find that people generally manipulate their vote to obtain a better outcome, but often do not identify the optimal manipulation. There are a number of predictive models of agent behavior in the social choice and psychology literature that are based on cognitively plausible heuristic strategies. We show that the existing approaches do not adequately model our real-world data. We propose a novel model that takes into account the size of the winning set and human cognitive constraints; and demonstrate that this model is more effective at capturing real-world behaviors in multi-winner approval voting scenarios.

Costly mistakes can occur when decision makers rely on intuition or learned biases to make decisions. To better understand the cognitive processes that lead to bias and develop strategies to combat it, we developed an intelligent agent using the cognitive architecture, ACT-R 7.0. The agent simulates a human participating in a decision making task designed to assess the effectiveness of bias reduction strategies. The agent's performance is compared to that of human participants completing a similar task. Similar results support the underlying cognitive theories and reveal limitations of reducing bias in human decision making. This should provide insights for designing intelligent agents that can reason about bias while supporting decision makers.