Carl Jung, Friedrich Nietzsche, Géza Dukes, Sándor Ferenczi and Wilhelm Stekel as well as Sigmund Freud have all examined the concept through their own works. Most of the research distinguishes between two kinds of cryptomnesia, though often both are studied together, drawing distinction in the underlying cause. The first views the bias being one of familiarity while the second posits the cause being the result of an error of authorship.