UK Post Office / Fujitsu
UK Post Office Horizon Scandal
Estimated impact: 700+ wrongful prosecutions; suicides; £1B+ in compensation
The Post Office prosecuted over 700 sub-postmasters for false accounting and theft based on faulty data from Fujitsu's Horizon IT system. The software had known bugs that created phantom shortfalls. The Post Office pursued criminal prosecutions despite internal evidence that Horizon was unreliable.
Decision context
Whether to investigate potential Horizon software defects when sub-postmasters reported unexplained shortfalls, or to continue prosecuting operators based on the assumption that the software was reliable.
Biases present in the decision
Toxic combinations
- Echo Chamber
- Yes Committee
Reference class base rates
Across all 146 curated case studies in our library:
Lessons learned
- Confirmation bias led the Post Office to treat each sub-postmaster case as individual fraud rather than investigating a systemic software problem
- Authority bias toward the IT system: the Post Office maintained that Horizon was "robust" despite mounting evidence of bugs
- The most consequential UK miscarriage of justice in modern history resulted from institutions believing computers over people
Source: Sir Wyn Williams, "Post Office Horizon IT Inquiry" (ongoing, 2022-present); Court of Appeal judgment, Hamilton v Post Office [2021] (Post Mortem)
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