A lot of historical experiments would be considered unethical by today's standards because of the former treatment of those being experimented upon. In fact, up until the 1970s the rules for running experiments on humans were way different. But then scientists came up with a set of standards that would protect "human services in biomedical and behavioral sciences". These standards are called the Belmont Report and have three key points that everyone must now follow when conducting scientific experiments. The points are: 1) respect for persons (meaning the subject must give informed consent), 2) beneficence (meaning scientists should try not to harm their subjects), and 3) justice (which ensures that subjects aren't exploited). The Belmont Report also states that researchers should also be careful to distribute the burden and the benefits of the report equally, so that one group is not always benefiting while another is always burdened.
But as we know, these standards are relatively young, so many a questionable experiment has been performed on participants. One historic example of these was the case of Little Albert in 1920, where psychologist John Watson classically conditioned a baby to be afraid of animals by pairing the sight of a white rat with the unrelated stimulus of a loud noise. The horrible part about this case is that Watson never reconditioned the baby to be unafraid of animals and it's unlikely that the child's mother consented to the experiment in the first place. To learn about more examples of experiments that would be totally unacceptable in post-Belmont Report days, watch the video!