International climate negotiations occur against the backdrop of increasing collective risk: the likelihood of catastrophic economic loss due to climate change will continue to increase unless and until global mitigation efforts are sufficient to prevent it. We introduce a novel alternating-offers bargaining model that incorporates this characteristic feature of climate change. We test the model using an incentivized experiment. We manipulate two important distributional equity principles: capacity to pay for mitigation of climate change and vulnerability to its potentially catastrophic effects. Our results show that less vulnerable parties do not exploit the greater vulnerability of their bargaining partners. They are, rather, more generous. Conversely, parties with greater capacity are less generous in their offers. Both collective risk itself and its importance in light of the recent Intergovernmental Panel on Climate Change report make it all the more urgent to better understand this crucial strategic feature of climate change bargaining.
Do legislative voting rules affect the diversity of policies observed across structurally similar political economies, and if so, to what extent? To what degree do these rules affect legislative compromise and the stability of the social optimum? Using a spatial model of political competition with single-peaked preferences, we examine these questions in static and dynamic political economies where changing proposed policies requires supermajority consensus. We develop three findings pertaining to equilibrium policies that are immune to change by any supermajority coalition. First, we find the number of equilibrium policies that exist as a function of the supermajority’s size. This result implies that under supermajority rules, structurally identical political economies may implement very different policies. Second, we find the optimal level of compromise needed by a leader to ensure that her proposed policy is not defeated and establish that compromise decreases in the supermajority’s size. Third, we identify the minimal supermajority rule that ensures the stability of the social optimum. We derive implications for political design, policy diversity, and political inclusiveness.