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Harold Dwight Lasswell (February 13, 1902 – December 18, 1978) was a leading American political scientist and communications theorist. He was a member of the Chicago school of sociology, and he was a professor of law at Yale University. He served as president both of the American Political Science Association (APSA) and of the World Academy of Art and Science (WAAS).
According to a biographical memorial written by Gabriel Almond at the time of Lasswell's death and published by the National Academies of Sciences in 1987, Lasswell "ranked among the half dozen creative innovators in the social sciences in the twentieth century." At the time, Almond asserted that "few would question that he was the most original and productive political scientist of his time." Areas of research in which Lasswell worked included the importance of personality, social structure, and culture in the explanation of political phenomena. He was noted to be ahead of his time in employing a variety of methodological approaches that later became standards across a variety of intellectual traditions, including interviewing techniques, content analysis, para-experimental techniques, and statistical measurement.
Harold Lasswell, a political scientist by training, was a multidisciplinary academic. He could not pigeonhole his work into a single discipline, but instead celebrated the multi-faceted nature of his products. Lasswell was well associated with the disciplines of political science, psychology, and sociology – however he did not adhere to the distinction between these boundaries but instead erased the lines drawn to divide these disciplines. 
Lasswell is well known for his model of communication, which focuses on "Who (says) What (to) Whom (in) What Channel (with) What Effect".
He is also known for his book on aberrant psychological attributes of leaders in politics and business, Psychopathology and Politics, as well as for another book on politics, Politics: Who Gets What, When, and How.
He published his first formula in 1948.
Lasswell studied at the University of Chicago in the 1920s, and was highly influenced by the pragmatism taught there, especially as propounded by John Dewey and George Herbert Mead. However, more influential on him was Freudian philosophy, which informed much of his analysis of propaganda and communication in general. During World War II, Lasswell held the position of Chief of the Experimental Division for the Study of War Time Communications at the Library of Congress. He analyzed Nazi propaganda films to identify mechanisms of persuasion used to secure the acquiescence and support of the German populace for Hitler and his wartime atrocities. Always forward-looking, late in his life, Lasswell experimented with questions concerning astropolitics, the political consequences of colonization of other planets, and the "machinehood of humanity".
In his presidential address to the American Political Science Association, he raised the famous question, demanded by the expulsion of essences from the sciences, of whether or not we should give human rights to robots.
Lasswell's work was important in the post-World War II development of behavioralism. Similarly, his definition of propaganda was also viewed as an important development to understanding the goal of propaganda. Laswell's studies on propaganda produced breakthroughs on the subject which broadened current views on the means and stated objectives that could be achieved through propaganda to include not only the change of opinions but also change in actions. He inspired the definition given by the Institute for Propaganda Analysis: "Propaganda is the expression of opinions or actions carried out deliberately by individuals or groups with a view to influence the opinions or actions of other individuals or groups for predetermined ends through psychological manipulations."
Lasswell utilized Sigmund Freud’s methodology. Lasswell, upon studying with Theodor Reik – a devotee of Freud – in Vienna and Berlin, Lasswell was able to appropriate Freud’s methods.  Lasswell built a laboratory in his social science office. It was here that he conducted experiments on volunteers, students, at the University of Chicago  Using this instrument, he was able to measure the participants’ emotional state to their spoken words.  Lasswell was furthermore able to use psychoanalytical interviewing and recording methods that he appropriated from his time of studying with Elton Mayo at Harvard University. 
Lasswell was a “behavioral revolution” proponent.  Lasswell was credited with being the founder of the field of political psychology and was the man at which the concepts of psychology and political science intersected.  By utilizing psychoanalytic biographies of political leaders, he expanded the base from which potential evidence could be garnered. The benefit of this contribution is that he was able to engage in another method of research – content analysis. By being able to use preexisting data, he was in a position to show that his work was not purely positivist but also stepped into the realm of interpretivist as well – helping him to come together in studies of personality and culture in tandem with his political behavior research.
Content analysis is the “investigation of communication messages by categorizes message content into classifications in order to measure certain variables”  While the data existed to Lasswell in the form of analyzing the messages that Allied and Axis armies disseminated within warfare, it may not have been the most accurate of methodologies for analyzing the data. “Content analysts usually seeks to infer the effects of the messages that they have analyzed, although actual data about such communication effects are seldom available to the content analyst”  While Lasswell was able to perform this particular type of analysis, the weakness to this was that Lasswell could not verify his data due to communication effects not actually being available. This is because content analysis cannot study effects. While this was a weakness, he did develop content analysis as a communication tool that is still utilized today 
Lasswell made these contributions to the field of communication study:
- His five-questions model of communication led to the emphasis in communication study on determining effects. Lasswell’s contemporary, Paul F. Lazarsfeld, did even more to crystallize this focus on communication effects.
- He pioneered in content analysis methods, virtually inventing the methodology of qualitative and quantitative measurement of communication messages (propaganda messages and newspaper editorials, for example).
- His study of political and wartime propaganda represented an important early type of communication study. The word propaganda later gained a negative connotation and is not used much today, although there is even more political propaganda. Propaganda analysis has been absorbed into the general body of communication research.
- He introduced Freudian psychoanalytic theory to the social sciences in America. Lasswell integrated Freudian theory with political analysis, as in his psychoanalytic study of political leaders. He applied Freud's id-ego-superego via content analysis to political science problems. In essence, he utilized intraindividual Freudian theory at the societal level.
- He helped create the policy sciences, an interdisciplinary movement to integrate social science knowledge with public action. The social sciences, however, generally resisted this attempt at integration and application to public policy problem.
- Though he may have done critical work, it was primarily administrative research in the service of the government and other sponsors of his research.
- He looked at communication as an act rather than as an interactive process. His model was obsessed with effects as if every communication act should have a discrete effect.
- He looked more at the individual as a loose atom rather than as a member of a group or collective, thus ignoring the dynamics of reception of messages by individuals. He paved the way for the powerful effects idea.
- Though he employed quantitative analysis in his later work, he did not employ statistical significance tests and therefore his effects studies could not be generalized beyond the narrow sample
- His work, like others of this time, followed the functionalistic paradigm, which was dominant in sociology.
In terms of other contributions, he moved the field of political science firmly toward studying political behavior.
- Propaganda Technique in the World War (1927; Reprinted with a new introduction, 1971)
- Psychopathology and Politics, (1930; reprinted, 1986)
- World Politics and Personal Insecurity (1935; Reprinted with a new introduction, 1965)
- Politics: Who Gets What, When, How (1936)
- "The Garrison State" (1941)
- Power and Personality (1948)
- Political Communication: Public Language of Political Elites in India and the US (1969)
- Rogers, Everett (1994). A History of Communication Study: A Biological Approach. NY: The Free Press. p. 3.
- Feminist Jurisprudence, Women and the Law:, Wm. S. Hein Publishing, 1999, p. 370.
- Ellul, Jacques (1965). Propaganda: The Formation of Men’s Attitudes, p. xii. Trans. Konrad Kellen & Jean Lerner. Vintage Books, New York. ISBN 978-0-394-71874-3.
- Copyright 1970 by Leo Rosten. McGraw-Hill Book Company. Library of Congress Catalog Number: 79-132099 First Edition 07-053976-6
- Rogers, Everett M. (1994). A History of Communication Study: A Biographical Approach. NY: The Free Press.