it may be assumed that it is a simpler task to learn totally foreign sounds than sounds which bear a resemblance to sounds found in the L1.
phenomenon of universals and their significance to interlanguage transfer
unmarked, (common and regularly occurring phenomena), or marked (distinctive and unique phenomena).
implicational universals, the presence of a marked segment /θ/ in a language implies the presence of an unmarked segment /s/, but the reverse is not true.
Those less marked phonetic or phonological characteristics of L1 are harder to unlearn.
the syllable structure of English is more problematical for Japanese than for Koreans.
Avoidance is a general tendency for learners to avoid those aspects of production that they know to be problematic for them.
Other reasons for L2 phonology: