Bilingual Language Acquisition

A lot of people in the world speak more than one language to a bilingual standard (meaning that they speak both languages to the standard of which you speak your mother tongue): It is estimated that two in three children worldwide speak more than one language!

"There is a widespread popular impression that the children of bilingual parents are linguistically at risk because their brains will not be able to cope, and they will grow up confused and 'semilingual'.
There is no justification for this pessimism: There are millions of bilingual children worldwide. Most of these children will have the same linguistic abilities as a monolingual child upon starting school. But the process of learning two languages is not exactly the same as the process of learning one..." [1] 

A, B, C... It's easy as 1, 2, 3: The Three Main Stages of Acquisition in a Bilingual Child[1]
The child builds a list of words, as a monolingual does, but the single list is comprised of words from both languages. When sentences begin to have 2+ elements, words from both languages are used.  This mixing declines rapidly: By age 3, less than 5% of sentences contain words from both languages.  As vocabulary grows in both languages, translation equivalents develop.The acquisition of grammatical rules takes longer because there are two separate systems which can become confused at the early stages but are eventually easily separated by the child. 

What is bilingualism?
Bilingualism is often hard to define because some linguists see a fine line between second language acquisition and the acquisition of language to a bilingual state: Second language acquisition is the acquisition of a non-native language which usually happens through some exposure to the second language in childhood, adolescence or adulthood. Bilingualism, on the other hand, is the native-like control of two languages [2] when the child is exposed to both languages from birth or at least before the end of the 'critical period' for language learning (for more on critical period, click here). 

When is the acquisition of two languages studied?
The acquisition of more than one language has only been studied in the last 100 years or so. It was first studied by linguists such as Ronjat (1913) and Leopold (1939-49). (For more information on Ronjat and Leopold, click here).
Where is bilingual acquisition studied?
It is estimated that roughly 5000 languages are spoken in 200 countries worldwide so it can be studied in a wide variety of places.[3] It is often studied in the home to allow for natural productions of speech in both languages from the child.
Who studies the acquisition of more than one language and how do they go about it?
Research in this area is normally carried out in a case study format of children who are brought up in bilingual environments. A linguist named Leopold, for example, brought up his daughter, Hildegard, in America.  Hildegard was spoken to in English by her mother and German by her father.  Leopold showed her development in both languages in four books, focusing on different aspects of Hildegard's development: 1) Vocabulary development, 2) Pronunciation development, 3) Word formation and sentence structure and 4) his extra notes on her development.
Why is the acquisition of two languages studied?
Learning language is one of the most complex things humans undertake so learning two languages simultaneously brings another level to the process. Exposure to multiple languages at a young age allows for them to be acquired more easily and to a greater proficiency, even though it may take longer for both to be acquired to the same standard as a monolingual and the bilingual may never acquire as many words in a single language. Difficulties bilingual children encounter are also interesting for linguists, for example the use of a grammatical construction from one of the languages when speaking the other could suggest that the construction is easier in the other language.