This thesis contains three chapters in which we axiomatically analyze individual and collective decision problems in competitive and cooperative environments. In Chapter 1, we give a general introduction. In Chapter 2, we propose a theory of revealed preferences that allows both the status-quo bias and indecisiveness between any two alternatives. We extend a standard choice problem by adding a status-quo alternative and we incorporate standard choice theory as a special case. We characterize choice rules that satisfy two rationality requirements, status-quo bias, and strong SQ-irrelevance. In Chapter 3, we analyze bargaining situations where the agents' payoffs fromdisagreement depend on who among them breaks down the negotiations. We model such problems as a superset of the standard domain of Nash. On our extended domain, we analyze the implications of two central properties which, on the Nash domain, are known to be incompatible: strong monotonicity and scale invariance. We characterize bargaining rules that satisfy strong monotonicity, scale invariance, weak Pareto optimality, and continuity. In Chapter 4, we analyze markets in which the price of a traded commodity is fixed at a level where the supply and the demand are possibly unequal. The agents have single peaked preferences on their consumption and production choices. For such markets, we analyze the implications of population changes as formalized by consistency and population monotonicity properties. We characterize trade rules that satisfy Pareto optimality, no-envy, and consistency as well as population monotonicity together with Pareto optimality, no-envy, and strategy-proofness
