Interesting question, though difficult to answer. Here's one attempt:
https://slate.com/news-and-politics/2020/08/police-testilyin...> This tendency to lie pervades all police work, not just high-profile violence, and it has the power to ruin lives. Law enforcement officers lie so frequently—in affidavits, on post-incident paperwork, on the witness stand—that officers have coined a word for it: testilying. Judges and juries generally trust police officers, especially in the absence of footage disproving their testimony. As courts reopen and convene juries, many of the same officers now confronting protesters in the street will get back on the stand.
> Defense attorneys around the country believe the practice is ubiquitous; while that belief might seem self-serving, it is borne out by footage captured on smartphones and surveillance cameras. Yet those best positioned to crack down on testilying, police chiefs and prosecutors, have done little or nothing to stop it in most of the country. Prosecutors rely on officer testimony, true or not, to secure convictions, and merely acknowledging the problem would require the government to admit that there is almost never real punishment for police perjury.
Police and DAs, the parties who do most of the investigating of police perjury, are not exactly eager to share their internal findings for reasons we can probably guess, but the article does offer one striking anecdote:
> One NYPD officer, David Grieco—commonly known as Bullethead—has been sued at least 32 times, costing the city $343,252, for civil rights violations, including excessive force and fabrication of evidence. Yet Grieco was promoted and prosecutors continued to call him to the stand long after a slew of his victims blew the whistle on his violent and lawless behavior. Judges continued to rely on his word to lock up defendants. And Grieco’s name did not appear on Brooklyn District Attorney Eric Gonzalez’s long-secret list of officers with known credibility problems.