clairobscur-
It is not the case that American citizens (at least in Maine) have to sign a document to verify their statements to police. Actually, my mother recently was a witness in a trial, and she realized that the official police report mixed up some of the dates that she had given to the decetive when he interviewed her. She had no way to priove this discrepancy since she had never signed any original statement or seen what was written by the detective when she was interviewed. Luckily, this minor mix up was really not a big deal, but it shook her confidence in the accuracy of police reports.
clairobscur, the area of written statements and their evidentiary status is one of the differences in procedure between the civil law systems and the common law systems.
My understanding is that the civil law system places a great deal of emphasis on the written statement itself. Once reduced to writing in a formal situation, the document is itself evidence, and can be introduced into the subsequent proceedings on its own status as evidence. This is particularly the case in those civil law jurisdictions that retain the institution of the juge d’instruction as the preliminary fact-finding system. If I remember correctly, a statement given to a juge d’instruction is directly admissible in subsequent proceedings without calling the individual to testify.
It’s different in the common law system. Here, what counts is oral testimony, given in court, under oath, and subject to cross-examination by opposing counsel. A written statement, even given to the police, does not itself carry any legal weight and cannot be introduced into evidence on its own weight. What counts is the oral evidence that the witness subsequently gives in court. The written statement made by the police is not itself evidence unless the witness adopts it in oral testimony under oath.
(Note that if the witness does not adopt it and gives a different account, counsel can use the earlier written statement to try to impeach the witness’s credibility, but that does not mean that the statement itself becomes admissible.)
This is just a summary of the traditional civil law/common law positions; there can be some variation in each country/state/province, depending on local statutes and judicial decisions.