ethology, study of animal behavior based on the systematic observation, recording, and analysis of how animals function, with special attention to physiological, ecological, and evolutionary aspects. Laboratory or field experiments designed to test a proposed explanation must be rigorous, repeatable, and show the role of natural selection. At one time, an organism's actions were classified as either instinctive or learned behavior; the former included those actions, such as common reflexes, that are not influenced by the animal's previous experience; the latter comprised those actions, such as problem solving, that are dependent on earlier experiences. Current thinking emphasizes the complex interaction of environment and genetically determined responses, especially during early development. Among the early ethologists were Herbert Spencer, Charles Darwin, G. J. Romanes, and William James. Zoologists Konrad Lorenz and Nikolaas Tinbergen are widely considered to be the founders of modern ethology. In 1973 they and zoologist Karl von Frisch were awarded the Nobel Prize in Physiology or Medicine for their work in shaping the science of comparative animal behavior. See instinct; imprinting; sociobiology.