Old, poorly designed studies done primarily in the United States claimed to show that bilinguals had lower intelligence than monolinguals. Newer research has revealed several flaws in the studies. The most obvious flaw is that the bilingual children were recent immigrants, with poorer knowledge of English and more stressful life situations than their monolingual counterparts. Newer studies with more careful controls have shown that bilinguals are better at some specific tasks, such as language games, but that otherwise the differences between bilinguals and monolinguals are negligible.
Myth: A child should learn one language properly first; then you can start teaching the other.
As in the myth above, this is an old belief based on flawed research. Children who learn two languages in a loving, supportive environment learn them both well. Children who learn two languages in a stressful environment may have language development problems - but so will children learning only one language in that same sort of environment.
Myth: A child who learns two languages won't feel at home in either of them. She'll always feel caught between two cultures.
Relatives, friends and strangers will often caution about the "identity problems" children may develop if their parents insist on maintaining a bilingual home. The children, they believe, will grow up without strongly identifying with either of the languages and, therefore, the groups that speak them. Adults who have themselves grown up bilingual, however, generally report when asked that they never had problems knowing what groups they were a part of. Some even find this concern to be rather bizarre.
Children who feel accepted by both their cultures will identify with both. Unfortunately it happens that two cultures have such unfriendly relations that a child who should belong to both is instead shunned by both. This is not however a specifically bilingual issue.
Myth: Bilinguals have to translate from their weaker to their stronger language.
The overwhelming majority of bilinguals can think in either of their two languages. They do not, as some monolinguals assume, think in one language only and immediately translate into the other language when necessary.
Myth: Children who grow up bilingual will make great translators when they grow up.
By no means all bilinguals are good at translating. Nor have any studies shown that growing up bilingual gives one an advantage or a disadvantage over those who became bilingual as adults when it comes to translating. There are many other skills involved, and bilinguals, just like monolinguals, are too different to allow for easy generalizations.
There is one important exception here, however. The sign language interpreters you may have seen on television or at public events are most often hearing children of Deaf parents, who grew up bilingual.
Myth: Real bilinguals never mix their languages. Those who do are confused 'semi-linguals'.
Bilinguals sometimes "mix" their languages, leading monolinguals to wonder if they are really able to tell them apart. Usually, the problem is not genuine confusion - that is, inability to tell the languages apart. Far more common problems are interference, when words or grammar from the one language "leak" into the other language without the speaker being aware of it - analogous to a slip of the tongue - or "code-switching", when the speaker more or less intentionally switches languages for effect - analogous to mixing jargon or slang in with standard speech.
Many, if not most, bilingual children will use both languages at once during the early stages of their language development. Semi-lingualism is a far more serious, and relatively rare, situation that occurs when a child in a stressful environment is trying to learn two or more languages with very little input in any of them.
Myth: Bilinguals have split personalities.
Some bilinguals do report feeling that they have a different "personality" for each language. However, this may be because they are acting according to different cultural norms when speaking each of their languages. When speaking English, they assume the cultural role expected of them in English-speaking society. This is different than the cultural role expected of them in German-speaking society, which they assume when speaking German. The change in language cues a change in cultural expectations.
Myth: Bilingualism is a charming exception, but monolingualism is of course the rule.
No accurate survey of the number of bilinguals in the world has ever been taken; for fairly obvious practical reasons, it is likely none ever will be. But it is very reasonable to guess that over half the world's population is bilingual. Most of those who will read this live in countries where monolingualism is the rule, but are seeing a very unrepresentative sample of the world.
Myth: Be very careful; if you don't follow the rules exactly, your children will never manage to learn both languages!
Some people maintain that "the only way" to raise bilingual children is to follow one specific pattern, usually by speaking both languages in the home. Practical experience, on the other hand, has shown that children learn both languages regardless of the pattern of exposure, as long as that pattern is reasonably consistent (and perhaps even that is not a requirement!).
Myth: You'll never manage to make him bilingual now. People really can't learn a language after age X.
Language learning is easier the younger you are when you start, and there are biological reasons why very few adults can learn to speak a new language with a native accent. However, people can learn valuable language skills at any age. Establishing a bilingual home when your first child is born, if not before, is the easiest for all, but it can be done later if you for some reason must do so.