Kuhl (2004) has offered recent research in the area of Second language acquisition (SLA) which supports the claim that very small children, prior to the age of three, have an inherent skill that enables them to differentiate between two language systems when they have been consistently exposed to them. By the age of three, the child's primary speech productions already begin to display evidence of code-switching and language discrimination during specific situations. These findings support the fact that two languages can, indeed, be acquired simultaneously.
A sequential process of bilingualism occurs when the natural (L1) language of the learner is mastered, and then a secondary language is introduced afterwards. Research shows that this process is interconnected to simultaneous language processing and that, again, children at age 3, show the ability to select the language which they hear the most over the secondary one. However, throughout time, they continue to use both if they are exposed to both systems equally (Genesee, 2007).
This being said, the common denominator of simultaneous and sequential bilingualism is that they are completely dependent on exposure. Whether the exposure is consistent prior to age three, or after the L1 has been mastered, it is the consistency in application, exposure, and usage what will ultimately determine language ability. Within the safest and most developmentally appropriate setting, L1 and L2 exposure have the same potential to be acquired accurately and efficiently. The variables are key as to how this process is to take place. (Pearson, 2008)