Glad you're 'pretty sure', I, for one, am CERTAIN of what I am saying.
Don't want to keep on hitting this nail (especially since it's relative to
The Reaction Re-read Thread) but just to round it up, my rationale for the case in point goes as follows:
A human is an animal, whose mind consists of an amalgalm of complex systems, too many and complex to here state. But take emotion for the sake of argument. The average human is capable of a vast array of emotions, that discern it from the animalia.
The subject: Rachel is a human character whose persona is a catalogue of bravery, confrontational, boldness, style, eclecticism, temperamental, stubborness, zeal, etc...
The issue: Cassie, whom, let's agree is Rachel's diametrical opposite persona, inspite being her best friend, same gender and age, morphs Rachel for the sake of a mission. She gets the job done, but hints towards inner conflict to do with the 'Rachel' morph.
Result: Yes, Cassie made her little retort, but to me it was also another factor to morphing added to this stage of the series. Much like the Z-Space-mass-storage or the metamorphosis-re-news-the-2-hours-clock etc...
So we get quote
This brain of yours. It keeps trying to get me to do really dumb things unquote.
Which is indicative of some sort of a inner conflict, due to the Rachel morph on Cassie's part. Comparisons would indicate a character like Cassie would think of things a character like Rachel would do as dumb, hence we get the light-hearted retort. Remember they are still best friends.
The feedback there is indicative that Cassie didn't just get the base human emotions (remember she is a female of the same age and shouldn't be a stranger to the mind of a subject of the same sex and age). She got the conflicting ones to her own persona, which HAPPEN TO BE the ones willing to do what she judges as 'stupid things', much like the ones one may take Rachel for doing.My interpretation: That beyond the human mind, 'Rachel' (brave, bold, confrontational, action-adrenaline rush etc...) was alongside the 'Cassie' (peace-keeper, serene, intuitive and so on) mind during the morph.
My hypothesis: That scene is still significantly early in the series, (#12) so the author may have still been discerning what she wanted to 'say' in her series (nothing more evident than in the very first book).
Since I can't think of any other instances where the narrators came up with something along this, I can only think that KA decided to steer away from that. And so we get this discontinuity.
We never really delved into this through the narrator. Only through hints of the other characters. Mostly Cassie, really.
But we can't deny that the protagonists (with the possible exception of Ax) agreed that morphing humans was an ethical decision without reason.
Sure, KA did it so she wouldn't have to do the 'morph human' thing as a cheap way out of the plots, make life harder for the Anis, but the concept is feasible just the same.
I have never heard of Ax mentioning any sort of alternate awareness while in Human morph, and he was the one that did it the most.
It doesn't make sense for there to be self awareness built into DNA like that.
Of course it doesn't. That's my personal view, too.
But Cassie's comment suggests otherwise.
Ax was hardly expressive on lots of matters (ref. to his own homeworld descriptions, technology). He began a diary on humans that was discontinued throughout his narrratives. He is an Andalite (that sees himself somewhat superior, and as such, never looked at humans as any different than Hork-Bajirs, Taxxons or non-sentient creatures), an 'outsider looking in', who had enough control over his morph, to not have anything worth mentioning. He never mentioned anything towards the raw hunter-gatherer instincts, the primate offshoot physiology... and yet we know it's all there.
maybe Rachel naturally has a lot of testosterone, giving her a really quick temper. 
I laughed the first time I read it, but you know what, writing the above, the more I give
CounterInstinct's thought, the more it makes sense.
It may well be as simple as that. Chemicals are biologically coded and that we have seen active in morphs before (e.g. Marco/wolf spider making web).
