I’ve always liked animation since I was a kid. One thing I am interested in is where did the common “Shrinking Iris Trope” come from? I know it has been around for decades, but where did it originally come from, and who started it?
It’s an exaggeration and stylization of reality (like all good animation). Pupils dilate when you’re feeling fear - that is to say, they increase in size to maximize the amount of light entering. Pupils shrinking to a pinpoint is called myosis; it’s often a response to severe trauma, poisoning, or blunt force trauma.
Shouldn’t pupils dilate in animation to show fear? Well, they often do:
Using myosis instead is a deliberate stylistic choice designed to create a sense of horror and discomfort. Since we instinctually know as viewers that pupils shouldn’t shrink like that, our reaction is to be unsettled, much like if you see a leg bent the wrong way at a knee.
By subverting the viewer’s expectations- strong emotion causes the pupil to dilate- you’re achieving the same effect as any (effective) subversion; the viewer is drawn in and pays closer attention as they try to (sub-consciously) process what is wrong. At the same time, their fight-or-flight reflex kicks in- whenever we are confronted with something we instinctually know is not “correct” (in a purely physical sense), our natural response is anxiety.
It’s a classic trick to get people to pay attention, by showing them something that is wrong, they’re going to very attentively seek what is right, or at least expected. This is the science behind how horror movies work.
It’s also a classic technique used by con artists and in interrogations. By exposing someone to something that feels out of place and unnatural, and then presenting yourself as a highly normal person that meets expectations, they’re naturally inclined to latch on to you as a source of comfort. Subversion of expectations is a very powerful thing when done correctly. Done poorly and it comes off as cliche, tawdry, and obnoxious.
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Additionally the original links states:
A very minor case of Artistic License – Biology: while the pupil inside the iris can indeed shrink due to fear, the iris itself does not. Regardless, most artists consider it an Acceptable Break from Reality
… but see also wikipedia pupillary response:
constriction response (miosis): … controlled by the parasympathetic nervous system (PSNS)
And also wikipedia.org human eye:
Pupil constriction … increase depth of field.
to see what is attacking…
Or just simlpy 
:
Just an overexaggerating of “some” eye changes which when seen in your counterpart face instinctivly leads to strress reaction…
I also recall an early psychology study when men were asked to decide between two pictures which one of the (same …) woman was “more sexy.” It turned out be the one in which the woman’s pupils were dialated. It turns out that we all respond to various non-verbal clues when “love is(?) in the air.”