At least 40 studies have been conducted of the linguistic and conversational adjustments made by native speakers of a language using it for communication with non-native speakers. The modifications sometimes result in ungrammatical speech. Generally, however, they serve to provide input that is well-formed, a sort of linguistic and conversational cocoon for the neophyte second language acquirer. Most of the findings hold across age groups, social classes and settings, although some differences, both qualitative and quantitative, have been noted in these areas, too.In making the adjustments described, native speakers appear to be reacting not to one, but to a combination of factors. These include the linguistic characteristics and comprehen-sibility of the non-native's interlanguage, but particularly his or her apparent comprehension of what the native speaker is saying. The adjustments appear to be necessary for second language acquisition, in that beginners seem unable to acquire from unmodified native speaker input. There is some doubt as to their sufficiency in this regard.