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The question of how courts assess expert evidence - especially when mental disability is an issue - raises the corollary question of whether courts adequately evaluate the content of the expert testimony or whether judicial decision making may be influenced by teleology (‘cherry picking’ evidence), pretextuality (accepting experts who distort evidence to achieve socially desirable aims), and/or sanism (allowing prejudicial and stereotyped evidence). Such threats occur despite professional standards in forensic psychology and other mental health disciplines that require ethical expert testimony. The result is expert testimony that, in many instances, is at best incompetent and at worst biased. The paper details threats to competent expert testimony in a comparative law context - in both the common law (involuntary civil commitment laws and risk assessment criminal laws) and, more briefly, civil law. We conclude that teleology, pretextuality, and sanism have an impact upon judicial decision making in both the common law and civil law. Finally, we speculate on whether the new United Nations Convention on the Rights of Persons with Disabilities is likely to have any impact on practices in this area.