Safety cases are prone to confirmation bias toward a predetermined conclusion of safety

The claim asserts a dispositional tendency, not that every safety case is biased, and the evidence favors it from three converging directions.

First, the structural argument: safety cases are authored by the party seeking approval to deploy or operate, and the general phenomenon that people seeking evidence for a fixed conclusion select confirming and overlook disconfirming evidence↗︎ is among the better-established findings in cognitive psychology (Nickerson 1998 is the standard survey, and Leveson cites it directly). The inference from these two premises to a tendency in the resulting documents is straightforward.

Second, documented practice. The Haddon-Cave Nimrod Review (2009) is the anchor case: it found the Nimrod safety case riddled with errors, a paperwork exercise that missed the key dangers, produced by an outsourced process that generated paper declaring the aircraft safe when it was not (www.judiciary.uk/wp-content/uploads/2017/06/mj-haddon-cave-nuclear-industry-association-speech-zen-and-safety-cases-20170620.pdf). Leveson's "Safety Cases Considered Harmful" (sunnyday.mit.edu/SafetyCases.pdf) generalizes the point, arguing that the goal structure of a safety case, starting from the presumption the system is safe, encourages phrasing questions so that affirmative answers support the hypothesis.

Third, the concession implicit in the mitigation literature. Work on assurance cases (Assurance 2.0, arxiv.org/pdf/2004.10474; defeaters and eliminative argumentation, arxiv.org/html/2405.15800) and surveys of safety-case practice (arxiv.org/html/2502.00911) treat confirmation bias as a recognized failure mode to be engineered against, not a disputed allegation.

The one contradicting consideration, that defeater analysis and independent adversarial review can substantially mitigate the bias↗︎, now stands supported (on the strength of the debiasing mechanism and demonstrations such as the external Assurance 2.0 review of DeepMind's scheming-inability safety case, with effectiveness contingent on genuine independence and lacking controlled quantification). Its being supported does not move the verdict here: it opposes only a strong reading on which the bias is unavoidable, and its own premise presupposes the tendency it mitigates. If anything, the mitigation literature's maturity is further evidence that practitioners regard the proneness as real. It does confirm that the claim should be read as a proneness finding with credible, partially demonstrated countermeasures, which the assessment now reflects.

The verdict remains supported rather than verified because the direct empirical base is thin: one thoroughly documented failure (Nimrod), a general psychological mechanism applied by analogy, and expert consensus, rather than systematic study of safety cases as a population. Controlled or comparative evidence on how often safety cases exhibit biased evidence selection, and how much structured mitigation reduces it, would move this verdict in either direction.