From personal experience I would have to answer most all are truth tellers.
Has there been a credible study with published conclusions stating otherwise??
There's probably a lot of regional variation. In an area where most cops are honest, there may be little tolerance for dishonest ones. If, however, a certain critical mass of dishonest cops is reached, however, they will be able to keep out anyone who would make life difficult.
One thing many people don't seem to appreciate about "isolated incidents" is that if people who behave a certain way get rewarded, people in future will be more likely to behave that way. If a cop who shoots a dog for no remotely-justifiable reason while conducting a search does not face any repercussions for doing so, cops conducting searches in the future will be more likely to shoot dogs. If it becomes apparent that someone who becomes a cop will be free to shoot dogs without reprisal, people with a disposition toward animal cruelty will be drawn to police work. Given that even cops who have been recorded on video going out of their way to shooting dogs that were tied up well away from the places they were searching have not been punished for their conduct, it would seem that some police departments feel that they have the right to deprive people of their canine property without due process of law, notwithstanding the fact that such conduct is expressly contrary to the Supreme Law of the Land, meaning that those who engage in it are robbers rather than legitimate government agents.
Pick a profession and we’ll see. I suggest you start with Policemen, since that’s what the thread is about. I think you’d be surprised at how many admit to lying in court.