Advocates of Wellhausen's hypothesis contend it accounts well for the differences and duplication found in the Pentateuchal books. Furthermore, they argue, it provides an explanation for the peculiar character of the material labeled P, which reflects the perspective and concerns of Israel's priests. Wellhausen's theory went virtually unchallenged until the 1970s, when it began to be heavily criticized. By the end of the 1970s and into the 1990s, "one major study after another, like a series of hammer blows, has rejected the main claims of the Documentary theory, and the criteria on the basis of which they were argued". It has been criticized for its dating of the sources, and for assuming that the original sources were coherent or complete documents. Studies of the literary structure of the Pentateuch have shown J and P used the same structure, and that motifs and themes cross the boundaries of the various sources, which undermines arguments for their separate origins.