The aim in foreign or second-language attrition research, more specifically, is to find out why, after an active learning process, the language competence changes or even stops (Gleason 1982).
The theoretical grounding of the language attrition research derives primarily from cognitive and psychological theories.
Though these two proposed models offering different perspectives, it is impossible to have total separation because both languages exist in the same mind.
Finally, all of the models function together to create the integration continuum, an illustration that shows the possible relationships in "multi-competence" (Cook 2003: 9).
They discussed how "travelers immersed in a new language often experience a surprising phenomenon: Words in their native tongue grow more difficult to recall over time" (Levy 2007: 29).
They suggest that the lapses in native-language words can possibly be attributed to "an adaptive role of inhibitory control in hastening second-language acquisition" (Levy 2007: 29).
This is supported by how novice foreign-language speakers immediately access native-language vocabulary for things, although the foreign word is wanted.
The aforementioned researchers conducted studies on retrieval-induced forgetting, and examined "whether inhibitory control mechanisms resolve interference from one's native language during foreign-language production" (Levy 2007: 30).
To provide an answer as to how second-language attrition happens, it is necessary to have a glance at the findings of the research of memory.
Today, the retrieval-failure hypothesis, concerning the function of the memory, is more widely accepted and popularized (Schöpper-Grabe 1998:237).
Cohen states, evidence for knowing that a learner is not able to "find" something, is the use of the so-called progressive retrieval (1986).
Time is considered the decisive factor to measure how far the attrition has proceeded already (de Bot & Weltens, 1995).
It supported the idea that some things, which are learned last, are the first to be forgotten when the learner has no input of the target language anymore.
Another point is the attrition of components, which are less "functional", "marked" or "frequent" compared to other elements (Weltens & Cohen 1989: 130).
According to these findings, the forgetting curve begins with an initial plateau, a period where the language competence is not affected at all.
Weltens (1989), who studied foreign language learners, identifies an increase in reading and listening comprehension.
Similar to this theory it is claimed, that according to conducted studies, the higher the level of competence of the learner at the end of the incubation period, the fewer will be lost.
Even though children are regarded as the better foreign language learner, their cognitive development is less progressed compared to adults.
Tomiyama, suggested on the basis of her findings, that these children might not lose their knowledge of the L2 completely, moreover the access to such information is inaccessible and may vanish with time passing by.
Longer and more frequently occurring speech pauses, under which the fluency is suffering, are observable as well (Gardner 1987).
Olshtain (1986) observed "[…] reduced accessibility in vocabulary retrieval in all situations of attrition where there is a reduction of language loss over longer periods of time."
Further, gaps concerning grammatical knowledge, especially tenses and conjunction of verbs occur quite frequently.
Nevertheless, it can be said that productive skills are more affected than receptive ones, which mainly remain constantly stable (Cohen 1989) and if the learner shows already signs of language attrition it is more likely that transfer from L1 will happen (Berman & Olshtain 1983).
Müller (1995) states that on many levels of speaking the learner falls back on a mixture between different languages.
Still, it is important to mention, that, as with almost every study that has been conducted in the different sub-fields of second-language acquisition research, several problems arise.
According to Gardner and Lambert (1972) a learner is instrumental orientated if learning a foreign language has a function, e.g. for success in career terms.
Mainly it is self-determined and the learner is eager to learn a foreign language because he wants to achieve a certain level of competence.
Moreover, several other factors, which are settled within the learner, as well as in the environment, influence motivation and are responsible for its intensity and variability.
In fact, the nature of language acquisition is still so complex and so much is still unknown, not all students will have the same experiences during the incubation period.
Fujita (2002), in a study evaluating attrition among bilingual Japanese children, says that a number of factors are seen as necessary to maintain the two languages in the returnee child.