I had a crash course of bullies since birth all the way to the end of high school to help me keep my guard up, and a 'healthy' dose of people trying to take advantage of me (as a perceived NT) in one way or another any time I've been exposed to a large enough sample of people.
I don't know about aspy skepticism being a congenital matter, but I can see it being something that would naturally emerge in those with a strong focus on details and don't suffer from too much of a lack of social awareness; In the big barrel of apples that is humanity, there will always be some bad ones. It'd be strange never to encounter them, so of course those with better abilities to perceive, remember and/or think would develop some caution.
There's a lot of ways people can take advantage of you, so there's a lot of ways to more efficiently deal with them. Could be as simple as a deadpan "No thanks", as complicated as being disingenuously evasive, or as direct as "f**k off, I know what you're trying to do".
The 'small loan' approach is alright, but I would only take it as proof of how they deal with 'small loans'. And then, too, people's behaviors change depending on their current state and what's at stake.
Personally, I try gauge their ethical integrity and give them as much trust as they've shown able to handle. Otherwise, I try not to give anything that I don't have the ability to forcefully get back. If I'm in a situation I can't just brush off, I only give as much as I can stand to lose, and count it as a loss (and a debt) right away. If they don't return the favor, then I forever treat them as a discredited debtor (like the plague).
I'm not always the best at sticking to my logical judgement. I've noticed over time that I tend to bend to social pressure/harassment from people I'm invested in in some way. For better or for worse, I've had to culture in myself a stronger sense of pre-emptive anger and being offended to deal with that. Sometimes it's just fake acting, sometimes it's real. It's really complicated because, well, social situations can get complicated.
I don't think people specifically target aspies. It's all about differences in power, which comes in all kinds of different forms. Physical, social, economic, environmental, direct, indirect, observational, intellectual, etc.. Taking advantage of differences in power has existed since single-celled organisms were the only living things on Earth. IMO it's just an unfortunate consequence that autists tend to have more and/or stronger vulnerabilities in certain areas.
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Thank you deeply for sharing your experiences. I don't feel so alone anymore.