In U.S. v. Stevenson, Judge Niemeyer wrote the following:
"Because we can read Stevenson’s letter and draw inferences from it just as the district court did, Stevenson argues that we should review the district court’s finding with respect to the letter de novo, and not for clear error. The assumption underlying this argument is that an appellate court reviewing documentary evidence can make a factual finding as well as a district court can, and, to the extent that an appellate court draws an inference inconsistent with a finding of the district court, the appellate court should follow its own finding. The argument, of course, recognizes that appellate courts defer to district court findings when they are based on credibility because credibility can only be determined by the district court, which is in a position to observe witnesses.
It is indeed true that only the trial court can observe "the variations in demeanor and tone of voice that bear so heavily on the listener’s understanding of and belief in what is said," which is important to the determination of whether a witness is to be believed. Anderson, 470 U.S. at 575. But any assertion that this is the governing principle for appellate court deference to a district court’s factual findings is too limited and has been explicitly rejected by the Supreme Court. . . . It is well-established that even when findings of fact are not based on observations of credibility, but rather on undisputed evidence or on entirely documentary evidence, appellate courts must nonetheless defer to the trial court’s factfinding function."