Effort estimation is a complex area in decision-making, and is influenced by a diversity of factors that could increase the estimation error. The effects on effort estimation accuracy of having obsolete requirements in specifications have not yet been studied. This study aims at filling that gap. A total of 150 students were asked to provide effort estimates for different amounts of requirements, and one group was explicitly told to disregard some of the given requirements. The results show that even the extra text instructing participants to exclude requirements in the estimation task, had the subjects give higher estimates. The effect of having obsolete requirements in requirements specifications and backlogs in software effort estimation is not taken into account enough today, and this study provides empirical evidence that it possibly should. We also suggest different psychological explanations to the found effect. © 2017 IEEE.