Mechanisms and models of human dynamics

Arising from: J. G. Oliveira & A.-L. Barabási Nature 437, 1251 (2005); Barabási & Oliveira reply . A stochastic queue model of human behaviour developed on the basis of the distribution of timing of tasks, as studied in a sample of e-mail messages at a university1 and in the written correspondence of Albert Einstein2, may not be as simple as it seems. Although this model reproduces the apparently non-Poisson distribution of correspondence delays, its interpretation is more complicated than suggested by Barabási, who claims that humans execute their tasks based on some perceived priority, setting up queues that generate very uneven waiting-time distributions for different tasks1. Such an explanation is intuitively appealing in the context of the queue metaphor, but does not exclude other mechanisms. By attributing delays in the correspondence to task priorities, this explanation ignores two important classes of mechanism that also contribute to the apparent distributions of task timings: the semantic content of an individual's correspondence and the social context in which this correspondence occurs.