The tendency of people being observed to act differently than normal as a result of their awareness of being monitored refers to:

Subject reactivity is the tendency of observed people to change behaviour because they know they are being watched.

Observer bias is the observer's own expectations distorting what is recorded.

Observer drift is the gradual change in how an observer applies criteria over time.

Debriefing is informing participants after the study, not a change in behaviour.

This effect is classically illustrated by the Hawthorne studies.

Unobtrusive or covert methods reduce subject reactivity.