Claude can't stop saying 'Marcus': AI randomness experiment reveals deterministic bias

A developer asked Claude to "pick a name at random" 37,500 times across five models and dozens of prompt variations. The most common male name was "Marcus," chosen 4,367 times (23.6%). Opus 4.5 returned "Marcus" 100 out of 100 times with a simple prompt. Nine parameter combinations produced zero entropy, meaning perfectly deterministic output. Elaborate prompts doubled unique names but introduced different biases. Random word seeds proved more effective than random noise at increasing diversity. The experiment cost $27.58 in API calls.

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