A costly signal is a cunning evolutionary device, and the classic example is the male peacock's tail. The elaborate tail imposes a cost, but (so the theory goes), it also demonstrates to potential mates the male's genetic fitness. So the guys with the big tails get the girls, and the investment in the tail pays off.
The crucial feature of a costly signal is that it's hard to fake. Keep that in mind...
There is a theory that religious rituals evolved because they're a costly signal. To understand how this works, first you have to accept that religious beliefs encourage people to be honest. (This isn't really backed up by the evidence - the evidence is that environmental primes are effective but not supernatural beliefs in themselves. But anyway...)
So, the theory goes that being altruistic is a potentially a good thing, because people will treat you better. But they can only do that if they can trust you. And that's where costly signalling comes in.
The idea is that all the rituals involved with religion are actually a kind of costly signal. Only people who truly have supernatural beliefs will devote the time and energy to religious rituals, and so you can tell the true believers by their outward show of devotion.
Anybody spot the flaw in that one?
OK, so the obvious problem is that it's a signal that's easy to fake. If going to religious services and pretending to be pious gets you and advantage, then that's what cheats will do.
So, says, Jeff Schloss, we can move up a level. He suggests that deep-seated, involuntary actions are the true costly signals:
Another way—sometimes attending ritual but often contrasted with it—is the widespread, varied, and in many respects distinctive existence of highly visible, involuntary, dramatic manifestations of religious experience: Glossalalia (“speaking in tongues”), convulsive weeping (“veil of tears”), contagious laughing or singing (“holy laughter” or “singing in the spirit”), fainting (“slain in the spirit”), trembling/shaking (“under the power”), religious trances, spontaneous bleeding, etc.
The existence of these ecstatic human behaviors, especially in the religious context, warrants both proximal (neurophysiological) and ultimate (evolutionary) explanation. Unlike involuntary displays such as blushing or piloerection, which merely signal emotional arousal, or vasomotor fainting/epileptic seizures, which are not associated with particular cognitions—these autonomic manifestations are taken to reflect the experience of a very specific (and sublime) reality.
To put it bluntly, when religious people freak out they are giving a hard-to-fake religious signal. The analogy is with smiling - a smile is hard to fake.
Frankly, I'm skeptical. Firstly, these kinds of behaviours are relatively rare, and there's no evidence that people who act like this are regarded as more trustworthy. There's certainly no evidence that they are more trustworthy.
Secondly, smiling might be hard to fake but people who have an incentive can certainly do it. There's no shortage of con men out there who can do it. If you trust people because they 'have an honest smile' then you are a ready-made dupe.
Lastly, and more fundamentally, I think the whole concept is fundamentally flawed because there is no evidence that religious beliefs are linked to altruistic behaviour. Religious priming is, certainly. If you put religious images up, or prompt people with religious messages, then they behave better. But this works with atheists just as well as with the religious. Beliefs have nothing to do with it (Shariff & Norenzayan showed this back in 2007).
What's more, what's more, people tend to justify and rationalize their bad behaviour. Since they also tend to create God in their own image, they can easily co-opt their God into their own rationalizations.
If they can do that, then the whole idea of costly signalling is fatally skewered. If religious beliefs are not linked to altruistic behaviour, then engaging in religious rituals can't possibly be a signal of good intent.
Schloss himself makes this point, and I think I'll leave the last word to him:
It is possible that these highly contagious religious displays are not adaptations for human flourishing at all, but are viral memes parasitizing reward systems that have been selected for other purposes or distorted by various deprivations. Although I have been arguing that this is not the case and that religious affections along with their distinctive manifestations play an important role in promoting cooperative commitment, still, they are notoriously vulnerable to a final, quinternary level of cheating: self-deception. Unlike intentional hypocrisy or consciously manipulative employment of costly signals (Cronk 1994), the best way to fake a hard-to-fake signal is to be sincerely, though inauthentically persuaded of ones own commitment.
This work by Tom Rees is licensed under a Creative Commons Attribution-Share Alike 2.0 UK: England & Wales License.