“…Past oddity studies (e.g., Koronakos and Arnold 1957;Thomas and Noble 1988;Wodinsky and Bitterman 1953) could not show evidence of oddity learning in rats due to the use of one-odd tasks or two-odd tasks or serial presentation of multiple oddity tasks. Conversely, recent oddity studies (e.g., April et al 2011;Lazarowski et al 2019;Taniuchi et al 2017) were able to show evidence of oddity concept learning in rats by employing multiple concurrent oddity tasks. In this entry, the focus is on the improvement in rats' oddity performances from the original oddity studies (e.g., Koronakos and Arnold 1957;Thomas and Noble 1988;Wodinsky and Bitterman 1953) until today, and some critical parameters (e.g., concurrent presentation, the use of a large number of stimuli) that enabled various species (e.g., rats, monkeys, and pigeons) to demonstrate oddity concept learning.…”