A psychometric consulting firm named AptiQuant asked 100,000 people to take an online IQ test, and then correlated scores with the web browsers used to take them. The blue bars represent test results from 2006 and the red bars test results from 2011. The vertical axis represents IQ points. Link -via Geekologie
UPDATE: The story was a hoax. Thanks to commenter che.
Opera (and SeaMonkey!)
First: I am skeptical about the methodology. I'm not sure this "study" would hold up to proper scientific scrutiny.
Second: The consensus in geek circles is that IE is pretty much the worst browser to choose from. In that respect it is no surprise that IE scores badly. (Yes, I am assuming the average IQ in "geek circles" is above average.)
PS: The results might be skewed because a lot of senior citizens (or others) are probably using IE because they don't have enough experience with computers or matters related to IT or just don't care. That does not mean they are more stupid.
apparently the study is bogus.
I regularly use two. Sometimes it depends on the site I'm visiting.