word choice - Is じゃないです equally correct as じゃありません?

Text books normally teach that the negative of です is じゃありません. However it seems very common to hear native Japanese use じゃないです.

Is this slang or somehow less correct than じゃありません? Would it be marked as incorrect in a JLPT exam?

If they are both equally correct why is じゃありません taught to beginners? (To me じゃないです seems simpler for a beginner to remember.)

Answer

I read an interesting paper on this very topic a few months ago. Let's see…ah, here it is:

A Discussion of the Polite Negative Verb Forms masen and nai desu (PDF, Japanese)

This paper by Kayoko Tanaka was presented at the eighth annual conference on Japanese language education research at Nagoya University in 2010. Ms. Tanaka, using sentences drawn from a survey of spoken Japanese, draws the following contrast:

～ません (in spoken Japanese)

「ません」はフォーマルであるという意識【いしき】が強【つよ】く、話者【わしゃ】の強【つよ】い否定【ひてい】が現【あらわ】れやすい。また、断定【だんてい】の意味【いみ】や、言【い】い切【き】りの形【かたち】が多【おお】い。それは、丁寧【ていねい】さより否定【ひてい】の意志【いし】が強【つよ】く現【あらわ】れる（丁寧【ていねい】（ませ）＋否定【ひてい】（ん）の順序【じゅんじょ】で現【あらわ】れる）という言語【げんご】形式【けいしき】が影響【えいきょう】していると考【かんが】える。これにより、話【はな】し手【て】は聞【き】き手【て】に否定【ひてい】の機能【きのう】を強調【きょうちょう】できる。より文末【ぶんまつ】に近【ちか】い位置【いち】に来【く】る表現【ひょうげん】が強【つよ】い印象【いんしょう】を与【あた】えるとすると、「ません」は否定【ひてい】の機能【きのう】が強【つよ】い印象【いんしょう】を与【あた】えている。

masen has a strong sense of formality, and often expresses the speaker's firm denial. Also, it is often used to add a sense of assertion or in emphatic constructions. This is thought to arise from the influence of the linguistic form (where the order is polite mase + negation n), in which the intent of negation appears stronger than the politeness. From this the speaker can draw the listener's attention to the negating function. If we assume that expressions which appear nearer to the end of the sentence leave stronger impressions, the negating function of masen leaves a strong impression. (emphasis mine)

～ないです (in spoken Japanese)

「ないです」は、口語【こうご】で多【おお】く使用【しよう】される。野田【のだ】（2004）の指摘【してき】通【どお】り、終助詞【しゅうじょし】を伴【ともな】いやすく、動詞【どうし】＋「シテイル形【けい】」と結【むす】びつきやすいという結果【こうか】を得【え】た。それは、否定【ひてい】の機能【きのう】より、丁寧【ていねい】さが強【つよ】く現【あらわ】れる（否定【ひてい】（ない）＋丁寧【ていねい】（です）の順序【じゅんじょ】で現【あらわ】れる）という言語【げんご】形式【けいしき】が影響【えいきょう】していると考【かんが】える。これにより、話【はな】し手【て】は聞【き】き手【て】に丁寧【ていねい】さを強調【きょうちょう】できる。より文末【ぶんまつ】に近【ちか】い位置【いち】に来【く】る表現【ひょうげん】が強【つよ】い印象【いんしょう】を与【あた】えるとすると、「ないです」は丁寧【ていねい】さが強【つよ】い印象【いんしょう】を与【あた】えている。また、さらに否定【ひてい】表現【ひょうげん】という話者【わしゃ】本人【ほんにん】の強【つよ】い主張【しゅちょう】を和【やわ】らげる効果【こうか】があるのではないかと考【かんが】える。

nai desu is often used in spoken Japanese. As indicated by Noda (2004), the effects of this construction are such that it is often used in concert with sentence-ending particles, and that it is easily joined with the (verb) + shiteiru form. This is thought to arise from the influence of the linguistic form (where the order is negation nai + polite desu), in which the level of politeness appears stronger than the negating function. From this the speaker can draw the listener's attention to the level of politeness. If we assume that expressions which appear nearer to the end of the sentence leave stronger impressions, the politeness of nai desu leaves a stronger impression. Also, it is further thought that this may have the effect of softening the speaker's strong assertion of a negative expression. (emphasis mine)

So basically…

～ません and ～ないです are semantically equivalent, but ～ないです is softer and less insistent. If you need to give a firm denial with no wiggle room, go with ～ません. But since this level of bluntness can be inappropriate in some situations, ～ないです is there if you need it.

Also, this applies to every negative sentence (not just those ending in a form of です):

好【す】きじゃないです／好【す】きじゃありません

楽【たの】しくないです／楽【たの】しくありません

飲【の】まないです／飲【の】みません