Internal validity and Experimenter bias

Internal validity

• Definition: the degree to which changes in the $DV$ are due to the manipulation of the $IV$.
• Also: results must be free from biases and logical errors that would render them suspect.

Experimenter bias

• Occurs when the experimenter's expectations influence the outcome.

Demand characteristics

• Definition: any aspects of a study that communicate to participants how the experimenter wants them to behave.

Oxytocin study example

• Initial study: participants receiving oxytocin in a nasal spray showed greater trust than controls (Mikolajezak & others, $2010$).
• Replications failed: Lane & others, $2015$, $2016$; Microp & others, $2020$.
• Reason for failed replications: the experimenter interacted with participants and knew their condition in the initial study, which can influence participant behavior even without intent.

Rosenthal 1966 maze study

• Classic study: college students were turned into experimenters.
• Procedure: participants were randomly assigned rats from the same litter; half were told their rats were "maze bright" and the other half "maze dull".
• Students then conducted maze navigation tests, and outcomes were influenced by expectancy.

Safeguards

• Experimental research requires safeguards to prevent biases and protect internal validity.