Since being brought into the public consciousness, the "just-so story" has come to mean any kind of non-falsifiable story or useful anecdote. It is used most often in a scientific context, where an author tells a story to add emotional connection and truthiness to some vaguely empirical but still-circumstantial analysis.
I think its original usage is more useful. You see, Rudyard Kipling used to tell bedtime stories to his daughter Effie, and she was particular. Each story had to be told in a particular way, with a particular set of words and inflections and characters. It must be told "just so." A just-so story in this more traditional sense doesn't have to be non-falsifiable. This broader definition of a just-so story must be precisely what the audience expects and it must fulfill some emotional need - and it doesn't make a hill of beans' difference whether it's true.
As you might imagine, it's a common sort of story these days. It is also precisely what we mean when we've discussed the use and misuse of powerful symbols that have become sources of identity for various large groups of people. It needn't contain a big and complex narrative. Many such just-so symbols, if you will, need only include a few necessary and expected components to achieve the desired effect.
Here's one we found in the wild yesterday.

You'll forgive me, I trust, if I don't waste your time 'debunking' the sort of claim that was never posted with any expectation of anyone evaluating it on its merits. The idea that the frauds perpetrated by poors and foreigners against America's various welfare programs could make even the slightest dent in the deficit is not just a bold claim - it's mathematically impossible to an amusing degree.
But the problem with just-so symbols isn't that they're false. It's that it doesn't matter AT ALL if they're false.
It's a thing that self-anointed 'fact-checkers' and 'debunkers' and 'community notes' miss: the effect of a just-so symbol is precisely the same whether it is disproven 100 times or 1,000 times. The complex symbol which conveys the "meaning" that fraud from the poors and foreigners is to blame for all of our problems and not the profligate spending and entitlement expansions of the people we've elected again and again to spend and expand the entitlements we care about, is fully integrated to the very identity of a huge bloc of voters. In this case, this bloc also happens to overlap pretty heavily with what we might call Facebook Boomers (sorry, mom!).
The point is that you cannot fact-check the impact of this kind of symbol. It is immune to debunking. Its power to reinforce beliefs and generate emotional response is derived entirely from it being told just so. Don't forget the SNAP people buying caviar, dad! You didn't read the part with all the Mexicans not paying taxes and stealing medical care right!
It's not a right/left thing, either. Reddit Libs have their own just-so symbols and their day on these pages is coming, too. The just-so symbol has always been a useful tool for divided populations because of human susceptibility to powerful symbols with deep networks of culturally mediated meaning. But today, it is being transformed by social networks into an optimal messaging strategy for any and every social institution.
Hooray.


