Update: Firaxis has confirmed that the spelling errors are a mistake. In a brief statement sent to PC Gamer, the developer wrote:
“We’re aware of a community-reported bug that has a minor impact on AI behavior. We’ve also made sure that everyone knows that I goes before E except after C… or other weird exceptions. Thanks to all who helped bring this to our attention and there will be a fix included in our next update.”
Original story: There's a story going around that sounds too improbable to be true: that Civilization 6's AI leaders have been acting strangely all this time because of five misspellings in a data file. The likelihood that Firaxis wouldn't have noticed such an obvious error for so long seems minuscule. And yet, inside one of Civ 6's loose data files, Leaders.xml, are the following five lines:
<Row Item="YEILD_PRODUCTION" ListType="DefaultYieldBias" Value="25"/>
<Row Item="YEILD_SCIENCE" ListType="DefaultYieldBias" Value="10"/>
<Row Item="YEILD_CULTURE" ListType="DefaultYieldBias" Value="10"/>
<Row Item="YEILD_GOLD" ListType="DefaultYieldBias" Value="20"/>
<Row Item="YEILD_FAITH" ListType="DefaultYieldBias" Value="-25"/>
In each line, 'YEILD' is a misspelling of 'YIELD.' That wouldn't matter if it were misspelled everywhere, but 'yield' is written with the correct spelling in every other instance across all of Civ 6's data files.
The misspelling was noticed by Something Awful forum user Straight White Shark (though they say in the thread that they weren't the first to pick up on it), and I've verified that the 'YEILD' lines can be found in a fresh Civ 6 download, though it isn't clear whether the mistake was always there, or if it was introduced in an update.
Testing with and without the typos
Shark and other Civ players believe these table entries are meant to set the AI leaders' default priorities, which are then modified by their various preferences and agendas. The effect should be that, by default, each leader prioritizes production and gold over everything else, and gives the least priority to faith. Popular theory has it that these misspellings are causing the default values to be ignored, explaining why some AI civs seem to put too much energy into religion.
By running two automated 151-turn games using the same Civilizations, each starting in the same spot on the 'true start' Earth map, Shark did find that fixing the misspellings created a noticeable difference in the AI leaders' priorities—they produced less faith overall, and more buildings and science.
I have been able to create reproducible results with the same basic methodology. By using the true start Earth map, I can run identical games using the Autoplay mod and produce identical end-of-game graphs, provided I don't change anything between tests. When I fix the typos, though, the graphs change. For example, Pedro II produces less faith-per-turn at the end of 100 turns when the typos are fixed.
To confirm that I was definitely making a difference, I set faith to 1,000 and all other yields to 0. When I did, Pedro II's faith-per-turn skyrocketed within 50 turns, and the other civs in my test trended up too. Even Queen Victoria built a Holy Site, which she didn't do in any of my other tests. I tried the same thing with the typos uncorrected, and the graphs reverted back to 'normal,' as if I hadn't changed anything from my first baseline test. With the misspellings in place, the lines don't do anything.
It's possible that the misspellings were introduced in a recent patch, and weren't there all along. It's also possible that Civ 6 is actually behaving as intended despite the misspellings, as different default values might be set elsewhere, making these five lines obsolete. But who would take these lines out of commission with typos, rather than by commenting them out?
I've reached out to 2K to find out if Firaxis is aware of the misspellings, and whether or not AI leaders are truly supposed to deprioritize faith, and just haven't been.
If you don't want to go editing the xml file yourself, you can download this mod on the Steam Workshop to try playing Civ 6 with the typos corrected. For more data, a few players are testing variations of the typo-corrected values on Reddit.