Threats to external validity arising from subjects' responses to being studied are known as what?

• A. Reactivity effects
• B. Order effects
• C. Participant variables
• D. Non-equivalent groups

Answer: (A)

Reactivity effects occur when people change their behavior simply because they know they’re part of a study or being observed. This matters for external validity because the behavior you measure in the study may not match how people would act outside the research setting, where there’s less scrutiny. A common example is participants putting in extra effort on a task because they know they are being watched, producing results that wouldn’t occur in everyday life—the so-called Hawthorne effect.

This differs from other issues: order effects come from the sequence of tasks rather than being observed, participant variables are the stable traits individuals bring to the study, and non-equivalent groups refer to preexisting differences between groups that can bias comparisons.