Expectations in the Ultimatum Game

Being treated fairly by others is an important need in everyday life. Experimentally, fairness can be studied using the Ultimatum Game, where the decision to reject a low, but non-zero offer is seen as a way to punish the other player for an unacceptable offer. The canonical explanation of such behavior is inequity aversion: people prefer equal outcomes over personal gains. However, there is abundant evidence that people’s decision to reject a low offer can be changed by contextual factors and their emotional state, which cannot be explained by the inequity aversion model. Here, we expand a recent alternative explanation: rejections are driven by deviations from expectations: the larger the difference between the actual offer and the expected offer, the more likely one is to reject the offer. Specifically, we provided participants with explicit information on what kind of offers to expect using histograms depicting distribution of offers given in a previous experiment by the same proposers. Crucially, we showed four different distributions, manipulating both the mean and the variance of these expected sets of offers. We found that 50% of our participants clearly and systematically changed their behavior as a function of their expectations (11% followed the standard-economic model of pure self-interest and 39% where not distinguishable from the inequity-aversion model). Using a logistic mixed-model analysis, we found that the mean and variance differently affect the decision to reject an offer. Specifically, the mean expected offer affected the threshold of what offers are acceptable, while the expected variance of offers changed how strict participants were about this threshold. Together, these results suggest that social expectations have a more complex nature as current theories propose.

在日常生活中，获得他人的公平对待是一项重要的心理需求。在实验研究中，公平行为可通过最后通牒博弈（Ultimatum Game）开展：参与者会选择拒绝虽非零但数额偏低的出价，以此惩罚提出不可接受出价的对手。针对此类行为的经典解释为不公平厌恶（inequity aversion）：相较于个人收益，人们更倾向于平等的结果。然而，已有大量研究证据表明，人们拒绝低价出价的决策会受到情境因素与情绪状态的影响，而这一点无法通过不公平厌恶模型加以解释。本研究针对一项新近提出的替代性解释展开拓展：拒绝出价的行为源于实际出价与预期出价之间的偏差——实际出价与预期出价的差距越大，参与者拒绝该出价的概率就越高。具体而言，本研究通过展示由先前实验中相同提议者所给出出价分布的直方图，为参与者提供了明确的预期出价信息。尤为关键的是，我们设置了四种不同的出价分布，通过操纵这些预期出价集合的均值与方差来控制实验变量。研究结果显示，50%的参与者会根据自身预期系统性地调整其决策行为：其中11%的参与者遵循纯粹自利的标准经济学模型，另有39%的参与者的行为模式与不公平厌恶模型无显著差异。通过逻辑混合模型（logistic mixed-model）分析，我们发现出价预期的均值与方差对出价拒绝决策的影响存在差异。具体而言，预期出价的均值决定了参与者可接受出价的阈值，而出价的预期方差则会改变参与者对该阈值的严格程度。综上，本研究结果表明，社会预期的本质比现有理论所提出的更为复杂。