What are some of the examples of “unnaturalness” that threaten external validity?

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Thank you for your explanation.

In social research, human subjects' values, attitudes, and behavior constitute the main areas of research. If the research conditions are artificial, human beings tend to react to the conditions and the treatment. Human subjects often search for clues, hints, and information to enable them to develop an understanding of the experiment and, thereby, often arrive at a definition of their situation, which affects their attitudes, ideas, reactions, and behaviors during the program. If so, findings may not be accurately generalized to the unprotested population.