In 1908, Waddell began to learn Sumerian. Thus in his later career he turned to studying the ancient near east, especially Sumeria and dedicated his time to deciphering or translating ancient cuneiform tablets or seals, most notably including the Scheil dynastic tablet. In 1911, Waddell published two entries in the "Encyclopædia Britannica". By 1917, Waddell was fully retired and first started exclusively writing on Aryans, beginning in an article published in the "Asiatic Review" entitled "Aryan Origin of the World's Civilization". From the 1920s Waddell published several works which attempted to prove an Aryan (i.e., Indo-European) origin of the alphabet and the appearance of Indo-European myth figures in ancient Near Eastern mythologies (e.g., Hittite, Sumerian, Babylonian). The foundation of his argument is what he saw as a persistence of cult practices, religious symbols, mythological stories and figures, and god and hero names throughout Western and Near Eastern civilizations, but also based his arguments on his deciphered Sumerian and Indus-Valley seals, and other archaeological findings.