Interference effects on task performance in conflict situations might reflect real limitations in inhibitory capabilities or failures to fully or consistently utilize such capabilities in executive control of task performance. We propose that useful clues regarding the actual cause of interference effects may be obtained from examination of their robustness within and between experimental conditions. We illustrate this approach for two major types of interference effects that have commonly been attributed to fundamental inhibitory limitations: Stroop-type interference and residual switch costs. We present results that indicate that these effects may not be unavoidable consequences of fundamental inhibitory limitations but may stem from goal neglect, i.e., failures to fully or effectively deploy inhibitory capabilities. These results indicate that, in addition to mean performance levels, variability of task performance may provide a valuable source of evidence regarding the actual cause of performance limitations or deficits in conflict situations. (C) 1999 Elsevier Science B.V. All rights reserved. PsycINFO classification: 2300; 2346; 2360.