It is my understanding that police must first have a "reasonable suspicion" in order to stop you, and only then under the new law, if they have "reason to suspect" that you are an illegal immigrant, that they are then required to ask for papers.
I wasn't really concerned with that law so much as the one enforcing it. Everyone who tangles with him ends up with legal trouble for some reason. It's a very strange coincidence because it happens regularly.
First of all, I think you are conflating prior and posterior probabilities. P(brown skin | illegal) != P(illegal | brown skin). The standard for a police stop is reasonable suspicion. Do you think having brown skin in Arizona is reasonably suspicious?
But much more importantly, the statistical evidence (which you haven't shown) would only be a consideration if justice were a statistical property. But if justice were a statistical property, there should be no objection to giving police arbitrary discretion to make stops and searches, since statistically speaking it is overwhelmingly likely that a citizen chosen at random has committed some illegal act for which they have not yet been convicted. But somehow that reasoning does not seem as appealing when we are the ones staring down the barrel of it.
The problem with the law is that people who are US Citizens get pestered frequently based on a trait they cannot change. In order to be useful, a test must be sensitive enough to match the trait you're looking for.
Suppose you have a test that could detect something with a 1% false positive rate and which never produces false negatives. Further suppose that the trait you're looking for occurs in 0.001% of the population tested.
Using that test, you would flag 99 innocent people for every guilty person.
I agree that we want to avoid false positives as much as possible, even to the point where we miss true positives.
But the law we're talking about is for after police have already stopped a suspect for a different reason, a reason that is not allowed to have anything to do with skin color.
I think it could still be an issue, however, because officers have been known to stop people for silly reasons, such as "rolling through a stop sign," when they really want to stop them because their skin color looks suspicious. I think this is a problem, it's really hard to fight against. So this is an argument against that law.
There's also a perception problem. If both parties expect to have problems, they probably will have problems. If you get stopped by cops frequently, especially if they're under pressure to justify why they stopped you, well, they can probably find something. And if you get stopped for small stuff (like the walking a dog without a leash) thing enough times, it'll get on your nerves. Doubly so, because there's nothing you can do to avoid it. So it's hard for people to be calm, which doesn't help anyone.
Please don't misunderstand, though. I came here from a small town and, due to unfortunate circumstances, we ended up getting frequent friendly visits from the police. The word "friendly" is not a euphemism. I mean that Darwin and the other guys on the force cared about us and wanted to make sure that everything was okay and that we weren't having any problems. They were great guys. They do a hard job, so when they have a bad day, things can really go to hell fast. So there are a lot of problems and the cops do have to take a lot of crap, both directly and from people second-guessing them. I do understand and respect that, but that's why I care to see that more attention is paid to avoiding perverse incentives in the law itself.
I think you've nipped it in the bud. There are always going to be good guys and bad guys. The best thing we can do (which is actually pretty effective) is set up the system such that the people are rewarded for the good and not for the bad.
Oops. The effect is real, but I typed one of the numbers wrong and the math of my example is incorrect. It's called the "Base Rate Fallacy" if you want to look it up, though.
