Psychology says people who are not able to speak properly in front of a crowd aren't defective, less intelligent or incapable. Difficulty speaking before an audience can be linked to glossophobia, also known as public speaking anxiety. Estimates cited in the supplied sources suggest that it affects about 75% to 77% of people globally. The response can involve fear, physical stress and difficulty finding words. A person may know the subject well but struggle to explain it when many people are watching. This happens because the brain can interpret being watched and judged as a threat. The reaction can affect memory, language and performance.
