Error Minimizing Minimax Avoiding Search Pathology in Game Trees

Game-tree pathology is a phenomenon where deeper minimax search results in worse play. It was was discovered 30 years ago (Nau 1982) and shown to exist in a large class of games. Most games of interest are not pathological so there has been little research into searching pathological trees. In this paper we show that even in non-pathological games, there likely are pathological subtrees. Further, we introduce error minimizing minimax search, an adaptation of minimax that recognizes pathological subtrees in arbitrary games, and cuts off search accordingly (shallower search is more effective than deeper search in pathological subtrees). Finally, we present experimental studies of error minimizing minimax in two different games. In our experiments, error minimizing minimax outperformed minimax, sometimes substantially, and never exhibited pathological characteristics.