Obtaining a handwriting exemplar is not a seizure within the meaning of the Fourth Amendment. A person has no expectation of privacy in handwriting because it is a physical characteristic which is constantly exposed to the public. So long as the initial seizure of the person is reasonable, compelling production of a handwriting exemplar is permissible. See United States v. Mara, 410 U.S. 19 (1973).
Updated May 20, 2015