One small but significant way that autistic people often stand out as different socially is our avoidance of eye contact. The best explanation of it that I’ve seen is Amythest’s video here.
As far as I know, an Official Medical Explanation of why autistic people avoid eye contact doesn’t really exist. It’s just one of those inexplicable behaviors (or else it’s taken as proof of a lack of empathy or interest in other people.)
Based on my own experience, I agree with Amythest. It’s fairly normal for non-autistic people to look away from what they’re focusing on when thinking or trying to remember. Since autistic people are often prone to sensory overload and likely to have executive function problems, it makes sense that we’d find maintaining eye contact even more distracting than the average person does. The discomfort and even pain that some autistic people describe having when they make eye contact makes sense considering the existence of sensory sensitivites that can make other inoccuous things feel painful.
Personally, I can usually make eye contact comfortably when I’m listening to someone else talk, but not when I’m thinking hard or while I’m talking. When I’m tired or otherwise stressed, making eye contact is more difficult– it feels like an extra burden on top of all the other things I have to think about or pay attention to. Sometimes when I’m tired, people’s voices seem very loud and hard to ignore. I look away because I want to make things somehow less overwhelming, and looking away is the best I can do, especially if I do also want to hear what they have to say.
I also avoid eye contact when it comes to people who I’m neither listening nor talking to, because of social anxiety. The prospect of having a conversation with a random stranger in a public place is stressful, and I’m not confident that I can do it “correctly”/politely, so I want to avoid catching people’s attention.