Overconfidence Effect

One of the earliest studies was conducted by psychologist Stuart Oskamp in 1965. He asked clinicians and students to read a case study in four parts, answering questions after each section with confidence ratings. As participants received more information, their confidence increased from 33% to 53%, while accuracy remained under 30%. This "Overconfidence in Case-Study Judgments" study, published in the Journal of Consulting Psychology, demonstrated that more information can inflate confidence without improving accuracy.