A cancer epidemiologist and a researcher discusses the question in The Conversation: How does one prove that shift work causes breast cancer, as the authors of the new study claim?

For determining the causes of cancer, the primary evidence is from epidemiological studies in people and experimental studies in animals, called toxicology. However, “proof” is in the eye of the beholder, and therein lies the tricky part. Who decides what constitutes “proof”? Can “proof” come only from experiments? If so, we’re in trouble, because we can never do an experiment in humans for any exposure that is suspected to be harmful.