Monotonic Mixing of Decision Strategies for Agent-Based Bargaining

In automated bargaining a common method to obtain complex concession behaviour is to mix individual tactics, or decision functions, by a linear weighted combination. In such systems, the negotiation process between agents using mixed strategies with imitative and non-imitative tactics is highly dynamic, and non-monotonicity in the sequence of utilities of proposed offers can emerge at any time even in cases of individual cooperative behaviour and static strategy settings of both agents. This can result in a number of undesirable effects, such as delayed agreements, significant variation of outcomes with lower utilities, or a partial loss of control over the strategy settings. We propose two alternatives of mixing to avoid these problems, one based on individual imitative negotiation threads and one based on single concessions of each tactic involved. We prove that both produce monotonic sequences of utilities over time for mixed multi-tactic strategies with static and dynamically changing weights thereby avoiding such dynamic effects, and show with a comparative evaluation that they can provide utility gains for each agent in many multi-issue negotiation scenarios.