Warning: the amount of yapping you are about to see is horrendously long and confusing, but I have to write it because I'm a nerd
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There is, in fact, a large difference between the two. Autism is when the filter in their brain for listening to a specific sound is not formed properly. Normally you would be able to read this is a room with a few conversations going on, but for someone with autism, they would not only hear the words they are reading in their head, but they would also hear political talk, family discussions, conspiracies, people talking behind someone else's back, and the list goes on and on. It is sensory overload, which is why autistic children often wear noise-canceling headphones. For adhd, however, it is caused by hyperactivity and difficulty focusing. I actually have ADHD, and often when someone is talking to me, I have trouble listening or I have to start figeting. It also causes difficult standing still or staying on-task. In conclusion, there is a large difference between and autism and ADHD, and ADHD is, most definitely, not simply spicy autism.