The change of behaviour in people after they come to know that they are participating in a research project, producing results that do not have validity, is known as the

When people change their behaviour merely because they know they are being studied, it is the Hawthorne effect.

Such reactive change distorts results and undermines their validity.

It is named after studies at the Hawthorne works of Western Electric in the 1920s and 1930s.

Blind effect is not a standard term for this reactivity.

Evaluation effect is not the recognised label for behaviour change from being observed.

Demand effect refers to participants guessing the hypothesis and acting to fulfil it, a related but distinct bias.