Innocent until proven guilty is a fundamental norm of Western jurisprudence. This concept goes back until at least Talmudic times, when our Sages assert that all Jews have a chezkat kashrut, a presumption of honesty. Yet as human beings, we are inherently biased--"man is close to himself" (Yevamot 25b)--and, try as we may, we can never be 100% objective. Generally, such biases can be, at least from a legal perspective, ignored. However, at times the overwhelming power of self-interest can colour--consciously or not--our perception of the truth.
By dint of the fifth amendment to the Constitution, US citizens are protected from being forced to give self-incriminating testimony; pleading the fifth is a common refrain in many a courtroom. Jewish law goes one step further; it forbids the giving of self-incriminating evidence. Ein adam masshem aztmo rasha, a person cannot turn himself into an evil person (Yevamot 25b). Thus, one who admits to having killed someone cannot be convicted based on his own testimony.