In the days before university ethics committees became ascendant, studies sometimes included subjects who had more than a passing acquaintance with the researchers yet were still treated in a manner that seems a bit shocking today.
In a 1949 study on pain, women in child labor consented to have their hands burned between contractions. The women, as the methods section described, were either nurses themselves or the wives of physicians and other "professional men." They undertook participation in the study in order to help others.
They were not paid for their labors.
Credit: Image courtesy of Hardy et al.