The traditional explanation for dual-task interference is that tasks compete for scarce processing resources. Another possible explanation is that the outcome of the processing required for one task conflicts with the processing required for the other task (e.g., cross talk). To explore the contribution of outcome conflict to task interference, we manipulated the relatedness of the tasks. In Experiment 1, subjects searched concurrently for names of boys in one channel and names of cities in another channel. Responses were significantly delayed when a nontarget on one channel belonged to, or was even just related to, the category designated as the target for the other channel. No comparable effects were found when the tasks were performed in isolation. Thus, the difficulty of the individual tasks is not the only determinant of how much they will interfere when combined, and there must be substantial interactions between processes carrying out the two tasks. In Experiment 2 subjects searched one channel for specific target letters and another channel for specific target digits. The mmtargets in a channel were either from the same alphanumeric category as the targets for that channel or from the opposite category (i.e., the category of the targets for the other channel). It was found that although between-category search was more efficient than witbin-category search in single tasks, it was less efficient in dual tasks. Thus, there appear to be significant task interactions due to the confusability emerging when the nontargeU of one task belong to the same category as the targets of the concurrent task. In addition, the congruence of target presence or absence on the two channels was found to have a sizeable effect. We suggest four potential sources of outcome conflict that may contribute to dual-task interference, and we conjecture that a great deal of the residual interference might result from other sorts of outcome conflict.When two tasks are attempted at the same time, the typical result is some cost. The cost may be manifested in two ways. First, a deficit may be observed in the performance of either or both tasks with respect to the single-task performance level. Second, task performance may exhibit a trade-off so that the performance of one task can be improved only at the expense of performance on the other task.One way to explain dual-task interference effects is to assume that the two tasks require the same sort of limited internal input for processing. When the demand for a limited input exceeds the supply, processes carrying out the two tasks must compete for what both need, and interference results.An obvious case of competition arises when two tasks require incompatible uses of a single sensory mechanism or effector; for example, a person cannot simultaneously look or reach the right hand in two different directions. However, many instances The order of authorship was decided by a coin toss.