In the 1940s and 1950s, the study of animal learning was one of the most popular and respected areas of psychology. The major behavior theories of this time, as seen in the work of Clark L. Hull, possessed vitality, precision, and a broad scope. Moreover, based on these theories, a very useful technology and terminology were developed that allowed for the discovery and precise identification of learning principles. However, elaborate models like these gradually disappeared from the field of animal learning research because they ultimately failed to explain the countless variations of learning that have proven to be extraordinarily comprehensive and complex. The diminishing significance of the grand theoretical frameworks seemed to signal the death of learning research altogether for many.