The study -- conducted by David Chester, Ph.D., a psychology professor at Virginia Commonwealth University, and Emily Lasko, a psychology doctoral student at VCU -- focuses on the practice of experimental manipulations, in which psychologists induce specific mental states, such as giving research participants insulting or complementary feedback to manipulate how angry they feel.
To conduct these experimental manipulations in a scientifically valid way, researchers must first establish whether their manipulations actually affect the intended psychological variable (for example: make people feel angry) and not other closely related variables (for example: make people feel sad). However, the extent to which psychologists actually examine the validity of their manipulations remains unknown.
Chester and Lasko investigated 348 psychological manipulations included in peer-reviewed studies. They found that roughly 42% of the experiments were paired with no validity evidence, and that the remaining psychological manipulations were validated in ways that were extremely limited.
Comment: There is another theory that, even if it may not explain the demise of the sloths investigated above, it may explain the sudden and near total extinction of our planet's megafauna; see Pierre Lescaudron's Volcanoes, Earthquakes And The 3,600 Year Comet Cycle for details.