Pioneering bilingual researcher Francois Grosjean has just published a new book with Harvard University Press, Bilingual: Life and Reality, which explores some of the misunderstandings held about bilinguals. He outlines some of the myths in a post for the HUP website.
Another common misconception is that bilinguals have equal knowledge of their languages. In fact, bilinguals know their languages to the level that they need them and many are dominant in one of them.
There are also the myths that real bilinguals do not have an accent in their different languages and that they are excellent all-around translators. This is far from being true. Having an accent or not does not make one more or less bilingual, and bilinguals often have difficulties translating specialized language. Then there is the misconception that all bilinguals are bicultural (they are not) and that they have double personalities (as a bilingual myself, and with a sigh of relief, I can tell you that this is not the case).
I highlight these because these myths about bilinguals are also used to discredit, or discount, hyperpolyglottism as a real, significant, and interesting phenomenon:
•hyperpolyglots can’t really be considered to speak or know a language unless they don’t know the culture
•hyperpolyglots can’t be given credit for speaking or knowing a language because their vocabulary is limited and they can use the language only in limited domains
•hyperpolyglots must know all their languages to an equal, and very high, degree of proficiency; otherwise they’re fake
Grosjean (in his post, and presumably in his book, too) elaborates on these myths and discusses their impact. In the American context, other damaging myths exist — most notably, the notion that being an English/language X bilingual is equal to not speaking English.