The concept has been backed up by studies showing how the different languages a person learns affect each other.
There is also evidence that people who learn other languages gain general cognitive benefits.
To avoid implying deficiency on the part of second language speakers, Cook prefers the term L2 user to L2 learner.
He makes three main points:[2] People learning a second language rarely reach the same level of competence as native speakers.
These changes can be with any aspect of language, from pronunciation and syntax to gestures the learner makes and the things they tend to notice.
[6] This evidence suggests that language systems inside a person's mind cannot be viewed as completely separate from one another.
L2 users think more flexibly than monolinguals, are more aware of language in general, and have better attitudes towards other cultures.
[7] At the time the term "multi-competence" was coined, SLA research often relied on comparing an L2 user to native speakers of the L2 using methods of error analysis.
However, these effects are not always negative; L2 users often experience improved L1 reading and writing skills, as well as increased creativity.
If there were separate lexicons for each language, then an L2 user should not become distracted by L2 word units when accessing their L1 knowledge.