What would make those decisions poor, however? Is it because they would reduce your long-term happiness?
No. (Not entirely, anyway; happiness is one of my goals.)
You're engaging in a bit of mind-body duality here
I really don't think I am. I did not claim (and I do not believe) that those things are being signalled by/in my brain to something outside my brain. If I say "the contents of address 0x1453C are the chess program's estimate of how much danger the white king is in", I'm not claiming it's using the contents of that address to signal to something other than itself. Similarly with happiness in the brain. The "I" of which I speak is, of course, implemented on and in the hardware of my brain. And yes, my brain's happiness is my happiness; I am not claiming "I should pay less attention to my brain's happiness in order to maximize my happiness" but "I should pay less attention to my brain's happiness in order to make the world more the way I want it to be".
pleasure isn't happiness [...] you appear to conflate the two somewhat in terms of what kinds of decisions you use as examples.
My apologies; I think I must have been insufficiently clear. Those weren't meant to be examples of bad decisions I would make by attending too much to happiness, but of bad decisions I would make by attending too much to similar brain-signals. Hunger, lust, fear, and so on.
I think your analogy with a car's turn signals is a good one, but it seems to me to make my point for me. If you are in your car, what you care about isn't which way the indicators are flashing but what roads you drive down. If you are on the road and looking at another car, what you care about isn't which way the indicators are flashing but whether and which way it's actually going to turn. Happiness is, indeed, like the turn signals: it's a pretty reliable but imperfect indicator. We are generally happier when the world is the way we want it to be, but the circuitry that assesses the match between the world and our purposes is imperfect and subvertible, and sometimes we can do better than maximizing our happiness. Wireheading is a useful example because it poses as starkly as possible the question: if happiness and actually getting the world the way you want it are forcibly separated, which one is it you actually care about? And, as I think we are agreed, almost everyone chooses the latter and wants other people to do the same.
So, how does all this apply to the discussion a few comments upthread? The question was whether religious belief is effective (specifically in terms of practical predictions about the world, but I think a strawmanned version of your argument would be less specific on that point) and you suggested that "are religious people happier or unhappier than non-religious people?" is an equivalent question, or maybe (you weren't explicit) just a very good proxy for it.
I think it's not that great a proxy, for reasons comparable to those involving wireheading: there are mechanisms by which religious belief or unbelief could affect a person's happiness that are basically unconnected with whether the world is the way they want it to be. You might be happy or unhappy about your expected fate in the afterlife, even if there is in fact no afterlife. You might be happy or unhappy because of "religious experiences" that you interpret as coming from the gods or ancestral spirits or whatever, even if in fact their origin is inside your head. You might also be happy or unhappy about how well God's purposes are being fulfilled in the world, which is unsatisfactory in a different way, kinda parallel to the way that if you were given 5 seconds' wireheading you might then want it to continue for 25 years: the furtherance of God's purposes would indeed be one of your goals, but it might be a bad goal if you have it only because of factually incorrect beliefs about God.
What is happiness, to you?
I grew up in an atheistic household.
Almost needless to say, I was relatively hostile towards religion for most of my early life. A few things changed that.
First, the apology of a pastor. A friend of mine was proselytizing at me, and apparently discussed it with his pastor; the pastor apologized to my parents, and explained to my friend he shouldn't be trying to convert people. My friend apologized to me after considering the matter. We stayed friends for a little while afterwards, although I left that school, and we lost contact.
I think that was around the time that I realized that religion is, in addition to being a belief system, a way of life, and not necessarily a bad one.
The next was actually South Park's Mormonism episode, which pointed out that a belief system could be desirable on the merits of the way of life it represented, even if the beliefs themselves are stupid. This tied into Douglas Adam's comment on Feng Shui, that "...if you disregard for a moment the explanation that's actually offered for it, it may be there is something interesting going on" - which is to say, the explanation for the belief is not necessarily the -reason- for the belief, and that stupid beliefs may actually have something useful to offer - which then requires us to ask whether the beliefs are, in fact, stupid.
Which is to say, beliefs may be epistemically irrational while being instrumentally rational.
The next peace I made with belief actually came from quantum physics, and reading about how there were several disparate and apparently contradictory mathematical systems, which all predicted the same thing. It later transpired that they could all be generalized into the same mathematical system, but I hadn't read that far before the isomorphic nature of truth occurred to me; you can have multiple contradictory interpretations of the same evidence that all predict the same thing.
Up to this point, however, I still regarded beliefs as irrational, at least on an epistemological basis.
The next peace came from experiences living in a house that would have convinced most people that ghosts are real, which I have previously written about here. I think there are probably good explanations for every individual experience even if I don't know them, but am still somewhat flummoxed by the fact that almost all the bizarre experiences of my life all revolve around the same physical location. I don't know if I would accept money to live in that house again, which I guess means that I wouldn't put money on the bet that there wasn't something fundamentally odd about the house itself - a quality of the house which I think the term "haunted" accurately conveys, even if its implications are incorrect.
If an AI in a first person shooter dies every time it walks into a green room, and experiences great disutility for death, how many times must it walk into a green room before it decides not to do that anymore? I'm reasonably confident on a rational level that there was nothing inherently unnatural about that house, nothing beyond explanation, but I still won't "walk into the green room."
That was the point at which I concluded that beliefs can be -rational-. Disregard for a moment the explanation that's actually offered for them, and just accept the notion that there may be something interesting going on underneath the surface.
If we were to hold scientific beliefs to the same standard we hold religious beliefs - holding the explanation responsible rather than the predictions - scientific beliefs really don't come off looking that good. The sun isn't the center of the universe; some have called this theory "less wrong" than an earth-centric model of the universe, but that's because the -predictions- are better; the explanation itself is still completely, 100% wrong.
Likewise, if we hold religious beliefs to the same standard we hold scientific beliefs - holding the predictions responsible rather than the explanations - religious beliefs might just come off better than we'd expect.