Reliability in developmental research is important to consider not only for infant research but also for researchers who wish to study individual differences at all ages. Forced‐choice behavioural tasks are popular in cognitive developmental research with preschoolers and young school‐age children because of their ease of administration and interpretation; however, the fact that guessing correctly is possible introduces noise that reduces reliability. An informal simulation is presented to illustrate this issue. Researchers wishing to investigate individual differences are encouraged to consider the reliability of the tasks they choose and not reflexively rely on familiar tasks that may be poorly suited to the study of true individual differences.