XiXiDu's comment made me realize what the really important implication of this result is: it's not that learning a foreign language is a way to improve rationality (though maybe it is); it's that if you want other people to be rational, don't speak to them in their native language.

Kind of makes me wonder whether the Sequence translations are such a good idea after all...

Anything that activates System 2 (for instance, writing questions in a difficult-to-read font) has the same effect.

As a trilingual, I would still advise against doing learning a newer language for the sole sake of gaining rationality skills. There are much better and cost-efficient ways to improve rationality besides learning a newer language, which would likely take years, or depending on the language, decades, to fully master.

To answer your question, strangely enough, now that this topic has been brought up, I do have a slight change in personality whenever I switch to a different language conversationally, although I might have to experiment further if it affects whether I am more or less rational.

Speakers of multiple languages: have you noticed a similar pattern in your own lives?

The most obvious difference to me is that I can tolerate and even enjoy some things in English that I am unable to tolerate in German. For example, when I hear certain kinds of Hip hop or gangster rap in German I am instantly disgusted. When I hear similar lyrics in English then I am merely amused.

I am also much quicker to dislike people who speak German than people who speak English. For example, if I listen to some of what American politicians say I am just amused. If people utter similar things in German then my emotional reaction is much stronger.

Someone here will probably know this better than I do (unfortunately :( ), but I believe there have been a series of studies done on the interplay between language and culture within certain countries--that certain constructions within the language allow for the development of differing types of cultural standards. The example that is coming to my mind is a language like German when compared to English. In English, you tend to be able to interrupt people more often, because you know where a sentence is going (You can go fuck--), whereas in German (I believe), since verbs come at the end (you yourself can go fuck--I know this isn't how it would be set up exactly), you can't interrupt nearly as much. Perhaps there is some ground in the idea that since learning languages forces you to subconsciously accept new modes of communicating, your way of looking at rational choice changes.

Of course, there's probably also an effect of simply needing to be more rigorous in a language that's not your own. My ability to be lazy in English is in part because I've spent a lot of time in this language and feel comfortable assuming things that I probably should not be. Whereas if I began to learn Mandarin, I would have to think about everything I say and do.

I have two native languages which I speak regularly and fluently. I also understand basic Spanish, but I haven't tried thinking about bets or probabilities in that language.

I think knowing two different languages so intimately, especially because my second language, Telugu, is from an Eastern culture, helps me see how values shape vocabulary. Group thinking, applause lights, and tribal politics are more salient when I switch languages, and especially when I try to find the exact word in one language to match to an important concept in the other.

For example, in Telugu, "My son" is used as an insult on par with "Bastard" or even "Motherfucker," something I can't even explain to monolingual English speakers. The idea is that parents have absolute authority over their children, so the speaker is claiming a huge amount of status over the other person, essentially asserting the right to do anything and everything to them, as one would to property. "You're like dirt to me." "Your only value is your use to me, and you have displeased me." (Interesting parallels to "Who's your daddy?", although in the West they tend to emphasize dominance through greater skill rather than parental ownership.)

A very obvious example of tribalism: "Westerner" is used as an insult with the weight that "heretic" or "apostate" would have had 100 years ago in the West. Noticing that people in India spoke this way allowed me to notice the alarming frequency with which people in the US accuse others of being "un-American" or "un-democratic." Looking back, this was probably part of what helped me admit that vague notions of "democracy" may not be the pinnacle of human government, though Enlightenment republics have worked out better than any system so far. I don't think India achieved the peak of cultural perfection back when they codified the word "Westerner" to mean "Bad Person," and it's only a few hundred years later, so I doubt the US has achieved it, either.

Other things I've noticed: "homely" has a very positive connotation, meaning essentially this, which is considered the picture of female perfection. "I" tends to disappear when I'm speaking in Telugu; there is less need for it. "I'm sleepy" becomes "Sleep is coming"; "I'm hungry" becomes, roughly, "Hunger came/was applied to me." Saying "I feel" and "I think" is awkward. I noticed that it might enhance the Mind Projection Fallacy, although I haven't tried. (It could also encourage less selfish thinking.)

I feel a step removed from both cultures as a result of being steeped in them both, which is exactly the place I want to be as an aspiring rationalist. It's a great exercise trying to translate connotations and associations across languages.

It's not clear from this description what level of fluency is assumed.