Lessing’s “The Fifth Child” and its sequel, “Ben, in the World” examine notions of humanity and animalism within the context of cognitive deficit and socio-environmental factors. 

11 – “He did like wildlife programmes, but there wasn't once night.  This was a good thing, really, because he sometimes got too excited: she knew wild instincts had been aroused.  She had understood from the start that he was controlling instincts she could only guess at.  Poor Ben - she knew he was that, but not how, or why. […]

In his sleep then ran from enemies, hunted, fought.  She knew he was not human: ‘not one of us’ as she puts it.  Perhaps he was a kind of yeti.”

85 – “Ben’s rage had blanked out his vision with red, and his fists had filled with murder - he had only just subdued this dangerousness, only just held himself in.  He must not ever let that rage loose, he knew it, but when Alex hit him like that … the unhappiness that had been deepening in him since he knew that the old woman had gone, and Johnston and Rita too, had rage as its partner.  He scarcely knew whether he wanted to bellow and howl with pain, or go berserk and kill.”