Why would one change from using polite form (です/ます) to using the plain form in the middle of a conversation (and change back later)?


I am reading _The Curious Incident of the Dog in the Night-time_ by Mark
Haddon in Japanese.

Towards the end of chapter 67, Christopher is talking to Mrs Alexander, trying
to find out who killed Wellington the dog.

I noticed that Christopher started off by speaking politely, using です and ます,
but then he says something that ends in る. I am very confused by this.

Here is part of the conversation. Note that I removed all the other non-
dialogue text and C for Christopher and A for Mrs Alexander.

> A: あなたのうちには犬はいないでしょ？
>
> C: ええ
>
> A: あなたはきっと犬が好きなのね？
>
> C: ネズミを飼って **います**
>
> A: ネズミ？
>
> C: トビーという名前 **です**
>
> A: おや
>
> C: 大概の人はネズミを **嫌がります**
> 、なぜかというとネズミは腺ペストをうつすと思っているから。でもそれは彼らが下水管に住んでいたり、珍しい病気のある外国からやってくる船にこっそり乗り込んでくるせい
> **です** 。でもネズミはとても清潔 **です** 。トビーはいつも自分の体をきれいにして _いる_ 。それからトビーをわざわざ散歩に連れて行く必要も
> _ない_ 。ぼくの部屋にはなして走りまわらせて、運動させるだけで _いい_ 。それからときどきぼくの肩に座ったり、巣穴のつもりでぼくの服の袖に隠れたり
> _する_ 。でもネズミはかならずしも巣穴には **住みません**
>
> A: なかに入ってお茶を飲まないこと？
>
> C: 他人の家には _入らない_

I have made all the polite forms bold and all the plain forms italic.
Christopher is starting with ネズミを飼って **います** and then changes to plain form
here: トビーはいつも自分の体をきれいにして _いる_. He then continues with the plain form until
this: でもネズミはかならずしも巣穴には **住みません**. And then changes to plain form again here:
他人の家には _入らない_.

Note that I understand that plain forms should be used in a relative clause
like 珍しい病気のある外国からやってくる船, I'm talking about why he ends his sentences with
plain forms.

I have never seen this kind of thing before. Characters in other books I read
don't seem to do this. The forms they use seldom change and changes are few,
and never more than twice in a single dialogue! Why does Christopher switch
between the two politeness levels?

My guess is that this is a symptom of Christopher's autism. Because he is
autistic, he mixes up the two forms.




It's quite natural to mix forms. While we are taught as beginners that there
is just a change in register, there's a lot more going on. E.g. the plain form
could in some contexts be more formal than the desu/masu form as it
_increases_ distance between the speaker and the listener. The cat in 吾輩は猫である
sounds pompous for this reason.

However, the plain form can also be more emotive. Desu/masu implies a more
factual statement, and the plain form can be used for things closer to the
speaker.

Look at the sentences with desu/masu:  
でもネズミはとても清潔です But mice are very clean  
でもネズミはかならずしも巣穴には住みません But mice do not necessarily live in holes  
These are statements of fact not directly related to Christopher.  
In contrast, look at the plain form sentences:  
トビーはいつも自分の体をきれいにしている Toby is always cleaning himself.  
他人の家には入らない I don't go into other people's houses.  
These are all things that are closer to Christopher.

Plain and desu/masu forms carry far more nuance that just degree of
politeness.




It's quite natural to mix forms. While we are taught as beginners that there
is just a change in register, there's a lot more going on. E.g. the plain form
could in some contexts be more formal than the desu/masu form as it
_increases_ distance between the speaker and the listener. The cat in 吾輩は猫である
sounds pompous for this reason.

However, the plain form can also be more emotive. Desu/masu implies a more
factual statement, and the plain form can be used for things closer to the
speaker.

Look at the sentences with desu/masu:  
でもネズミはとても清潔です But mice are very clean  
でもネズミはかならずしも巣穴には住みません But mice do not necessarily live in holes  
These are statements of fact not directly related to Christopher.  
In contrast, look at the plain form sentences:  
トビーはいつも自分の体をきれいにしている Toby is always cleaning himself.  
他人の家には入らない I don't go into other people's houses.  
These are all things that are closer to Christopher.

Plain and desu/masu forms carry far more nuance that just degree of
politeness.

