The Ising Decision Maker (IDM) is a new formal model for speeded two-choice decision making derived from the stochastic Hopfield network or dynamic Ising model. On a microscopic level, it consists of 2 pools of binary stochastic neurons with pairwise interactions. Inside each pool, neurons excite each other, whereas between pools, neurons inhibit each other. The perceptual input is represented by an external excitatory field. Using methods from statistical mechanics, the high-dimensional network of neurons (microscopic level) is reduced to a two-dimensional stochastic process, describing the evolution of the mean neural activity per pool (macroscopic level). The IDM can be seen as an abstract, analytically tractable multiple attractor network model of information accumulation. In this article, the properties of the IDM are studied, the relations to existing models are discussed, and it is shown that the most important basic aspects of two-choice response time data can be reproduced. In addition, the IDM is shown to predict a variety of observed psychophysical relations such as Piéron's law, the van der Molen-Keuss effect, and Weber's law. Using Bayesian methods, the model is fitted to both simulated and real data, and its performance is compared to the Ratcliff diffusion model. The speeded two-choice response time (RT) task is a wellestablished paradigm in experimental psychology for investigating the principles underlying simple decision making. In the psychological literature, several successful models have been proposed based on the idea of the accumulation of noisy evidence over time (Link & Heath, 1975;Ratcliff, 1978;Stone, 1960;Usher & McClelland, 2001;Vickers, 1970). An important class of accumulator models, of which the drift diffusion model is the prime example, relies on a single or a few linear stochastic differential equations (SDEs). Decades of careful research resulted in excellent fits between the best accumulator models and behavioral data from speeded twochoice RT tasks. Initially, these models were conceived as abstract representations of the decision process. In the last decade however, there has been an increasing trend of investigating their neurophysiological underpinnings (