There is a Halachic principle called "Shavya A’nafsheh Haticha De’isura," which literally means, "He makes himself like a forbidden piece of meat." This principle establishes that a person who testifies to the forbidden status of an object or a person (including himself) is believed with respect to himself. If he claims that something is forbidden, his word makes it forbidden for him, even though there is no other evidence of its forbidden status.
A classic example of this Halacha is the Shulhan Aruch’s ruling in the Eben Ha’ezer section (Siman 4) regarding a person who arrives and declares that he is a Mamzer, the product of a forbidden relationship such as adultery. A Mamzer, of course, is forbidden from marrying. If a person declares himself a Mamzer, his declaration has the effect of making it forbidden for him to marry. Even though a person’s testimony about himself is not generally accepted, and nobody else affirms this person’s status as a Mamzer, his claim that he is a Mamzer results in a prohibition for him to marry, by force of the rule of "Shavya A’nafsheh Haticha De’isura."
Is this person allowed to marry a Mamzeret – a female Mamzer? Halacha allows a Mamzer and Mamzeret to marry one another. Seemingly, then, if we believe his declaration about his status as a Mamzer, he should be allowed to marry a Mamzeret.
In truth, however, such a person would not be allowed to marry a Mamzeret. The principle of "Shavya A’nafsheh Haticha De’isura" enables one to create a prohibition through a declaration of status, but not to allow something which is presumed forbidden. If we have no evidence of a person’s being a Mamzer, we cannot allow him to marry a Mamzeret, even though he professes to be a Mamzer. He therefore cannot marry anyone; his only option is to produce evidence that he is a Mamzer, whereupon he would be allowed to marry a Mamzeret. This is the ruling of the Bet Shemuel (commentary to Eben Ha’ezer by Rabbi Shemuel Ben Or Shraga Feivish, 17th century), and of Rav Yechiel Michel Epstein (1829-1908), in his Aruch Ha’shulhan (Eben Ha’ezer 4:42; listen to audio recording for precise citation).
This situation is comparable to a case of a person who is presumed to be a Kohen, but one day claims that he is not a Kohen. His claim is effective in denying him the privileges of the Kehuna, such as the first Aliya in the synagogue. However, his claim is not believed with respect to the special restrictions that apply to the Kohanim. Thus, for example, if a person is presumed to be a Kohen and then declares that he is not a Kohen and wishes to marry a divorcee, he is not allowed to marry the divorcee, unless he can produce evidence that he is not a Kohen. Since he had been presumed to be a Kohen, we cannot allow him to violate the laws of the Kohanim without proof that he is in fact not a Kohen.
Summary: If a person claims that he is a Mamzer, he may not marry anybody, even a Mamzeret, unless he can produce evidence of this status, whereupon he is allowed to marry a Mamzeret.
