It's a little known trivia fact that the creator of Wonder Woman (and her Lasso of Truth) also invented a type of lie detector (I think I read it on cracked.com a while back).
Is it known which of the two ideas preceded one another in the creator's mind?
No, if you read the history of the Polygraph:
Early devices for lie detection include an 1895 invention of Cesare Lombroso used to measure changes in blood pressure for police cases, a 1904 device by Vittorio Benussi used to measure breathing, and an abandoned project by American William Marston which used blood pressure to examine German prisoners of war (POWs).[5]
but the first book of the wonder woman was in 1942
The polygraph came much earlier. William Marston had the idea in 1915, and a polygraph-testing laboratory in 1920:
He began working on his blood pressure approach to deception in 1915 as a graduate student under the direction of Hugo Munsterberg in the Harvard Psychological Laboratory. According to Marston’s son, it was his mother Elizabeth, Marston’s wife, who suggested to him that “When she got mad or excited, her blood pressure seemed to climb” Although Elizabeth is not listed as Marston’s collaborator in his early work, Lamb, Matte, and others refer directly and indirectly to Elizabeth’s work on her husband’s deception research. She also appears in a picture taken in his polygraph laboratory in the 1920s.
Since the modern idea of a "superhero comic" wouldn't exist until 1932 Wonder Woman was certainly not fully formed as such in his head.
The way I heard the story, from my psychology professor, is that Marston channeled some frustration with the academic and legal establishment's response to his work (as well as his essentialist feminism and uncommon sexual practices) into the creation of Wonder Woman.
