The degree to which a study's design allows for confident conclusions about cause-and-effect relationships between variables. High internal validity means that observed changes in the dependent variable can be attributed to the independent variable rather than to confounding factors.
Formalized by Donald Campbell and Julian Stanley in the 1960s as part of their systematic approach to experimental design. 'Internal' refers to the validity within the study itself, distinguishing it from external validity which concerns generalizability beyond the study.
It's the difference between knowing something works in your specific study versus knowing why it works! A study can have perfect internal validity but tell us nothing about the real world, or vice versa.
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