It is in general easier to notice differences between freely varying segments in a foreign language than in one’s native language, where one has become accustomed not to notice such linguistically irrelevant facts. On the other hand, one may easily fail to notice slight but relevant differences in a foreign language if these differences do not occur in one’s native language, or if they occur there only between members of one phoneme and morpheme.
- Harris (1951), a pag.30