A. Quite on the contrary, I think that when you are aware of the full dimension of peer pressure around you, your immunisation has extinguished.
B. As long as you are unaware of that there is peer pressure and do not feel the effects it has on you (meaning: really understand it), you will not bend to it. Why would you... you cannot recognise its meaning.
C. Just knowing that there is something called peer pressure, but not knowing how it feels like is not the same as understanding what peer pressure all is.
Autism usually automatically puts you in B. Maybe in C. if you're socially less impaired in some areas.
Yet, peer pressure or not, the result can often be the same.
Alcohol, other drugs... what really gets you into that stuff isn't the peer pressure, but whatever issue your yourself has.
The peer pressure is just a catalyst. If you just try drugs because your friend has tried them but have no reason to take them, have a wonderful awesome time at life, then there's usually only a small chance you get stuck with them.
But if you have big enough of an issue, peer pressure isn't needed to get you into drugs. May take a bit longer, if there's a difference at all.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett