The environmental assessment literature has neglected the distorting effect of cognitive and unconscious motivational biases (CUMB) in environmental assessment processes. This is problematic because CUMB are present in most, if not all, decision-making situations and can significantly distort decision-making processes. This article assesses how debiasing techniques are, or should be, incorporated in (supra)national environmental assessment legislation. The Dutch case study undertaken for this article shows that EU and Dutch legislation do not sufficiently incorporate debiasing techniques to ensure sound environmental decision making. Furthermore, the extent to which Dutch legislation incorporates debiasing techniques was found to be decreasing. Based on these findings, the article presents ways to incorporate debiasing techniques in environmental assessment legislation more generally, and in EU and Dutch legislation in particular.