Found it through Google Scholar:
http://psychology.uchicago.edu/people/faculty/ForeignLanguageEffect.pdf
It'd be interesting to see if this effect still held in circumstances where the vocabulary of the problem in the second language is mostly loaned from the first language.
Or: people who are more rational, people who can create psychological distance, people who encourage systematic rather than automatic thinking, and people with reduce emotional impact of decisions - those are people who can and do learn additional languages.
Multilingualism is more common than monolingualism in the population at large; it's impossible to narrow down perfectly since degree matters (does this person use multiple languages but favor one, how do you count languages used in specific contexts only, etc) but the world total is generally taken to be between 60 and 70 percent.
I'm not sure, it seems obvious to me - words have connotations for those familiar with them that go beyond their literal meanings, and I tend to go with the hypothesis that language defines our thought process. (I'd call it cognitive linguistics, because that's the sensible thing to call it, but I think that somebody else already defined that phrase to refer to something less sensible.)
But I have one caveat. Less biased doesn't necessarily mean either better or more rational. Our decision making processes aren't optimized, nor should they be, for laboratory environments.
I believe linguistic relativity and Sapir-Whorf Hypothesis are the usual names for this idea. I also believe it's been mostly discredited in linguistic circles, but it's been a while since I talked about it with a linguist, so you should do your own research.
Thank you! I tend to remember ideas better than their names. (And linguistic relativity is a horrible name for that idea, IMHO.)
A quick glance around the internet suggests that Sapir and Whorf's versions of the theory - that language affects thought, but doesn't strictly determine it - enjoys moderate empirical support and continuing professional support. The stronger variants, broadly linguistic determinism, seem to be largely discredited.
Paywalled article (can someone with access throw a PDF up on dropbox or something?): http://pss.sagepub.com/content/early/2012/04/18/0956797611432178
Blog summary: http://bps-research-digest.blogspot.co.uk/2012/06/we-think-more-rationally-in-foreign.html