The testator shall express in the instrument the reason, facts, or circumstances that constitute the cause for the disinherison; otherwise, the disinherison is null. The reason, facts, or circumstances expressed in the instrument shall be presumed to be true. The presumption may be rebutted by a preponderance of the evidence, but the unsupported testimony of the disinherited heir shall not be sufficient to overcome the presumption.
La. C.C. § 1624