This research seems to prove something I have suspected for a long time.
I read this and related papers last night and they all arrive at similar conclusions.
The 'Thal cannot cope in a theatrical setting with hundreds of potential interactions with hordes of people expressing fake, overdramatic responses they don't really feel. It completely overloads them.
In the course of everyday existence their larger, simpler amygdalas get exhausted trying to disregard one fake emotional expression after another. This is the reason that after a while, they just assume whatever the expression, it is best treated as some sort of potential hostility.
Homo Sapiens often smiles when he is angry, threatening or pretending to be amicable when in fact he is menacing. Other Homo Sapiens, themselves fake and contrived, have complex social intelligence to recognize the other guy is also completely full of sh*t. This is all unconscious and completely instinctive as an expression of the hardware inside their head which is otherwise junk technology.
Modern Neanderthals have to wade through reams of fake emotions, discard 90% of it and try to figure out which expressions are genuine. By the end of the day, their brains feel like they have run an emotional marathon.
When a Neanderthal smiled at you 100,000 years ago it meant he was friendly and in a good mood. This is the reason our amygdalas never needed to be much complex.
I also seriously doubt if we caught a Homo Habilis trespassing that he tried to win us over with charm and fake grins. I am sure the Neanderthal could see the stranger was ugly, angry and dangerous. Category = Hostile. Answer = Crush To Size of Shoebox. Problem solved. No big demands on the amygdala there.