Question: explain how the study of psychology and propaganda are linked? 

Answer: Propoganda plays on human emotions with bias, persuasion of thought, and with with-holding certain information (which links back to bias).  To know how to do this properly and to do it effectively to get the preferred results, those pushing the propoganda have to know what to do, thus they use psychological tactics.  If you want a more elaborate answer, just ask...
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