it's real easy to look at a behavior that doesn't seem to make sense otherwise and say "oh, duh, signalling". The key is that the behavior doesn't make sense otherwise: it's costly, and that's an indication that, if people are doing it, there's a benefit you're not seeing.
People do all sorts of insane things for reasons other than signaling, though. Like because their parents did it, or because the behavior was rewarded at some point.
Of course, signaling behavior is often rewarded, due to it being successful signaling... which means it might be more accurate to say that people do things because they've been rewarded at some point for doing them, and it just so happens that signaling behavior is often rewarded.
(Which is just the sort of detail we would want to see from a good theory of signaling -- or anything else about human behavior.)
Unfortunately, the search for a Big Idea in human behavior is kind of dangerous. Not just because a big-enough idea gets close to being tautological, but also because it's a bad idea to assume that people are sane or do things for sane reasons!
If you view people as stupid robots that latch onto and imitate the first patterns they see that produce some sort of reward (as well as freezing out anything that produces pain early on) and then stubbornly refusing to change despite all reason, then that's definitely a Big Idea enough to explain nearly everything important about human behavior.
We just don't like that idea because it's not beautiful and elegant, the way Big Ideas like evolution and relativity are.
(It's also not the sort of idea we're looking for, because we want Big Ideas about psychology to help us bypass any need to understand individual human beings and their tortured histories, or even look at what their current programming is. Unfortunately, this is like expecting a Theory of Computing to let us equally predict obscure problems in Vista and OS X, without ever looking at their source code or development history of either one.)
Signaling behavior is often rewarded, due to it being successful signaling... which means it might be more accurate to say that people do things because they've been rewarded at some point for doing them, and it just so happens that signaling behavior is often rewarded.
The evolutionary/signaling explanation is distinct from the rewards/conditioning explanation, because the former says that people are predisposed to engage in behaviors that were good signaling in the ancestral environment whether or not they are rewarded today.
Response to Man-with-a-hammer syndrome.
It's been claimed that there is no way to spot Affective Death Spirals, or cultish obsession with the One Big Idea of Everything. I'd like to posit a simple way to spot such error, with the caveat that it may not work for every case.
There's an old game called Two Truths and a Lie. I'd bet almost everyone's heard of it, but I'll summarize it just in case. A person makes three statements, and the other players must guess which of those statements is false. The statement-maker gets points for fooling people, people get points for not being fooled. That's it. I'd like to propose a rationalist's version of this game that should serve as a nifty check on certain Affective Death Spirals, runaway Theory-Of-Everythings, and Perfectly General Explanations. It's almost as simple.
Say you have a theory about human behaviour. Get a friend to do a little research and assert three factual claims about how people behave that your theory would realistically apply to. At least one of these claims must be false. See if you can explain every claim using your theory before learning which one's false.
If you can come up with a convincing explanation for all three statements, you must be very cautious when using your One Theory. If it can explain falsehoods, there's a very high risk you're going to use it to justify whatever prior beliefs you have. Even worse, you may use it to infer facts about the world, even though it is clearly not consistent enough to do so reliably. You must exercise the utmost caution in applying your One Theory, if not abandon reliance on it altogether. If, on the other hand, you can't come up with a convincing way to explain some of the statements, and those turn out to be the false ones, then there's at least a chance you're on to something.
Come to think of it, this is an excellent challenge to any proponent of a Big Idea. Give them three facts, some of which are false, and see if their Idea can discriminate. Just remember to be ruthless when they get it wrong; it doesn't prove their idea is totally wrong, only that reliance upon it would be.
Edited to clarify: My argument is not that one should simply abandon a theory altogether. In some cases, this may be justified, if all the theory has going for it is its predictive power, and you show it lacks that, toss it. But in the case of broad, complex theories that actually can explain many divergent outcomes, this exercise should teach you not to rely on that theory as a means of inference. Yes, you should believe in evolution. No, you shouldn't make broad inferences about human behaviour without any data because they are consistent with evolution, unless your application of the theory of evolution is so precise and well-informed that you can consistently pass the Two-Truths-and-a-Lie Test.