I think all people on the spectrum CAN get meltdowns, but not all do (or not all do as frequently as others do). I have them frequently, and no, they are not like speaking loud at the cashier because she couldn't hear you (I would simply call that....frustration, maybe?) A meltdown is completely uncontrollable. You would know if you have them. For me, they could last for hours (I had about a 12 hour one today with only a scattered hour here and there as a break followed by a mental shutdown that is only partially lifted. This meltdown followed me at school, on the bus--I could not stop it). When I was a kid, my parents could punish me, threaten me, try to bribe me out of it, try to talk me out of it....anything at all--NOTHING stops a meltdown. I cannot even stop them once they start. They stop when I am so completely exhausted that I cannot meltdown anymore--and I either go into shutdown, or I just come out of the meltdown (but still exhausted). I've gotten snarky on occasion to people I've misunderstood or to who I thought were being mean to me (not often, and usually only when I am in a miserable mood anyway). This is not a meltdown. This is me being snarky and probably kind of rude even though I don't notice it at the time.
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Diagnosed with classic Autism
AQ score= 48
PDD assessment score= 170 (severe PDD)
EQ=8 SQ=93 (Extreme Systemizer)
Alexithymia Quiz=164/185 (high)