Most models of response time (RT) in elementary cognitive tasks implicitly assume that the speed‐accuracy trade‐off is continuous: When payoffs or instructions gradually increase the level of speed stress, people are assumed to gradually sacrifice response accuracy in exchange for gradual increases in response speed. This trade‐off presumably operates over the entire range from accurate but slow responding to fast but chance‐level responding (i.e., guessing). In this article, we challenge the assumption of continuity and propose a phase transition model for RTs and accuracy. Analogous to the fast guess model ( Ollman, 1966 ), our model postulates two modes of processing: a guess mode and a stimulus‐controlled mode. From catastrophe theory, we derive two important predictions that allow us to test our model against the fast guess model and against the popular class of sequential sampling models. The first prediction—hysteresis in the transitions between guessing and stimulus‐controlled behavior—was confirmed in an experiment that gradually changed the reward for speed versus accuracy. The second prediction—bimodal RT distributions—was confirmed in an experiment that required participants to respond in a way that is intermediate between guessing and accurate responding.

Adolescents demonstrate impaired decision-making in emotionally arousing situations, yet they appear to exhibit relatively mature decision-making skills in predominately cognitive, low-arousal situations. In this study we compared adolescents' (13–15 years) performance on matched affective and cognitive decision-making tasks, in order to determine (1) their performance level on each task and (2) whether performance on the cognitive task was associated with performance on the affective task. Both tasks required a comparison of choice dimensions characterized by frequency of loss, amount of loss, and constant gain. Results indicated that in the affective task, adolescents performed sub-optimally by considering only the frequency of loss, whereas in the cognitive task adolescents used relatively mature decision rules by considering two or all three choice dimensions. Performance on the affective task was not related to performance on the cognitive task. These results are discussed in light of neural developmental trajectories observed in adolescence.

A defining characteristic of Major Depressive Disorder (MDD) is its episodic course, which might indicate that MDD is a nonlinear dynamic phenomenon with two discrete states. We investigated this hypothesis using the symptom time series of individual patients. In 178 primary care patients with MDD, the presence of the nine DSM-IV symptoms of depression was recorded weekly for two years. For each patient, the time-series plots as well as the frequency distributions of the symptoms over 104 weeks were inspected. Furthermore, two indicators of bimodality were obtained: the bimodality coefficient (BC) and the fit of a 1- and a 2-state Hidden Markov Model (HMM). In 66 % of the sample, high bimodality coefficients (BC > .55) were found. These corresponded to relatively sudden jumps in the symptom curves and to highly skewed or bimodal frequency distributions. The results of the HMM analyses classified 90 % of the symptom distributions as bimodal. A two-state pattern can be used to describe the course of depression symptoms in many patients. The BC seems useful in differentiating between subgroups of MDD patients based on their life course data.