People frequently express moral outrage, on blogs, on social media, and other media. My conservative friends, for instance, frequently express outrage at abortions, such as selective abortions of fetuses with disabilities. My liberal friends, on the other hand, frequently post about about how appallingly refugees are treated, and about the treatment of the poor by the government.
What is the function of all this outrage? Why do we do it? A generally accepted explanation is that Internet outrage is a form of virtue-signaling. This point has recently been made by Jason Brennan at the Bleeding Heart Libertarians blog:
What’s really going on, most of the time, is that people expressing such outrage are trying to demonstrate that they are morally better than other people. Much of the time, outrage is moral masturbation. The people who express outrage do so in order to demonstrate to others that they have increased moral sensitivity and a stronger than normal concern for ethics than others do. Thus, they are better people, and should be admired.
I here want to suggest an alternative explanation, not necessarily incompatible with Jason Brennan's claims. I think that expression of outrage is not only a form of virtue-signalling, but that it is first and foremost a way of reliably signalling in-group membership.
While it's plausible people will want to be perceived as morally better, it is doubtful that moral outrage on social media would help us to accomplish this goal. This is because merely expressing that you're outraged isn't a good guide to moral exemplariness at all. It's simply not expensive enough for this, and for this reason, worthless as a reputational signal. Reputational signals tend to incur some costs in order to be viable and credible. For example, in civil rights movements, reputation is built by how people's actions within those movements are perceived, with people devoting more time and energy quickly achieving a higher reputation in the group.
If we look at signaling theory, then we often spontaneously think of signals that are super-expensive such as peacock tails and the stotting of gazelles. Stots are high jumps that gazelles make that signal to a predator that there's really no point in hunting it as it is too quick, too fit, and pursuit would be a waste of time and energy for both. In the stotting case, the signal needs to be expensive because the interests of a gazelle and a lion are barely aligned. The lion wants lunch, the gazelle wants to avoid becoming lunch. The only aligned interest they have is neither wants to waste time and energy on a fruitless pursuit through the savannah. So the gazelle needs to use an expensive signal, the high jumps, to convince the lion that there is no point pursuing it.
Many cases of signalling are not like the stotting case, however. As Cronk and Leech point out, signaling can be cheap if the interests of people are already well-aligned. For example, if you want to meet several people in town and you're all unfamiliar with the place, it's useful for to coordinate to a public place like Grand Central. This brings me to the idea that expressing certain beliefs can be a cheap, but effective and reliable, signal for expressing group membership.
Nicholson has argued that beliefs unlikely to be held by the outgroup are good signals of group membership, and may be adopted for precisely this reason. For example, the Trinity (Christianity) and no-self (Buddhism) are positively weird, counterintuitive beliefs, but holding them signals you are part of the relevant religious groups.
Factual beliefs can also act as markers of in group membership, specifically if they become politically polarized: Evangelicals tend todeny evolutionary theory, even if they do not understand the theory fully and Republicans deny that climate change is human induced, even if their understanding of it is very low. Expressing group-appropriate moral beliefs may be even better, given morality is a central feature of group identification.
When someone expresses moral outrage, she isn't necessarily saying she's morally better (though she may well desire to be perceived as morally better, there are probably more effective ways for her to accomplish this). She's signalling that she holds the right beliefs, the beliefs that are important for her ingroup. Lack of assent “Indeed! This is outrageous” shows lack of deference to group norms and thus makes you suspect as an plausible ingroup members. Precisely because the signal is so cheap there's an expectation that one should at least sometimes engage in it.
This explains also why people of different factions will express a lack of outrage at things that don't fit the moral views of their community. For example, I know plenty of liberal-leaning people who find abortion upsetting and say so in private, but they don't post about it because it does nothing to enhance their credibility as members of the group, quite the contrary. Similarly, I rarely see conservative friends post about the treatment of refugee children. This is not because they don't care. There is only so much care we can express given how many bad things happen (as Brennan points out), but it because is not a requirement of their ingroup to be morally upset about it.
To conclude, I think we hold many beliefs not for purely epistemic reasons — not merely because we believe they are true, or likely true — but for social reasons (see here about scientifically discredited ideas).**
Is it rational to attune your beliefs to your group in this way? The positives of doing this is that having similar beliefs to the group can help facilitate coordination (if you're all morally on the same page, it can help facilitate action for example). But the cost is that we sacrifice a certain moral complexity in our thinking, perhaps even in our character, as a result.
**Note, I do not thereby think that such beliefs are immune from evidence (see e.g., Van Leeuwen who defends such a view).