The history of the biological atavisms, the theory that some individual animals, including humans, at times revert back to an earlier evolutionary type, was reviewed. In the case of humans, many behavioral scientists believed that the atavism response caused persons to revert in a major way, both physically and mentally, to their animal origins. Many criminologists once adopted this theory to explain crime, and partly for this reason it influenced public opinions and official policy. The "criminal physical type" stereotype is still very much with us, even though the theory of atavism as a causative factor in criminal behavior has been empirically disproved. Examples of so-called atavism are discussed, including extra fingers, nipples, and various body abnormalities such as extreme levels of body hair. It is concluded that no known biological atavism mechanism is sufficient to account for this phenomenon. The probable causes are genetic malfunctions, hormonal problems, or diseases. The research is reviewed that shows why the concept of atavisms has today, like its relatives the vestigial and nascent organ theories, now been discarded.