The ‘beads task’ is used to measure the cognitive basis of delusions, namely the ‘Jumping to Conclusions’ (JTC) reasoning bias. However, it is not clear whether the task merely taps executive dysfunction – known to be impaired in patients with schizophrenia – such as planning and resistance to impulse. To study this, 19 individuals with neurosurgical excisions to the prefrontal cortex, 21 unmedicated adults with Attention Deficit Hyperactivity Disorder (ADHD), and 25 healthy controls completed two conditions of the beads task, in addition to tests of memory and executive function as well as control tests of probabilistic reasoning ability. The results indicated that the prefrontal lobe group (in particular, those with left-sided lesions) demonstrated a JTC bias relative to the ADHD and control groups. Further exploratory analyses indicated that JTC on the beads task was associated with poorer performance in certain executive domains. The results are discussed in terms of the executive demands of the beads task and possible implications for the model of psychotic delusions based on the JTC bias.