Jonassohn and Björnson postulate that the major reason why no single generally accepted genocide definition has emerged is because academics have adjusted their focus to emphasise different periods and have found it expedient to use slightly different definitions to help them interpret events. For example, Frank Chalk and Kurt Jonassohn studied the whole of human history, while Leo Kuper and R. J. Rummel in their more recent works concentrated on the 20th century, and Helen Fein, Barbara Harff and Ted Gurr have looked at post World War II events. Jonassohn and Björnson are critical of some of these studies, arguing that they are too expansive, and conclude that the academic discipline of genocide studies is too young to have a canon of work on which to build an academic paradigm.