This is one of those issues that keep a PA game designer up at night lol.
"how to bring about the fall"
"why are there mutations"
"what does a spider goat taste like"
and the big one, "why be a Pure Strain Human (PSH)"
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There are some easy answers:
mundanes outnumber the mutants and will generally hang, kill, or burn at the stake any and all mutants
PSH are the only ones who have access to pre fall technology, education, and training
Robots and AI will not recognize non-PSH via retinal scans, bio scans, etc.
because nothing quite fits when you are a mutant
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In the end I tend to grant pure strain humans advantages of stability and resistance.
I usually class them in two groups, innies and outies.
Innies are those PSH who have been isolated from the fall in domes, shelters, vaults, or even on space stations/moon...they have been screened, manipulated, and other aspects to create a pure form of humanity...good education, training, etc...a longer life span with an extended maturity span, 3 years to 1...so a 30yr old will look like a 10yr old and a 90yr old will look like a 30yr old...the average PHS innie has a life span of 120-130 with some reaching 200...so a wealth of skills, education, and related is the advantage of the PSH innie.
outies are those PSH who have made it through the worst of the fall and made it through with their humanity intact...resistant to mutation, to radiation, to disease, etc...they never get cancer for example...outies have an extended life span, 2 yrs to 1...so a 30yr old will look like a 15yr old and a 90yr old will look like a 45yr old...the average life span of an PSH outie is 120-130 with some reaching 200...outies have the advantage of living on the ground, building knowledge of the new world, etc. they are the keepers of new medicine, as well as the lands and seas.
This is how I handle PSH.
Why play a PSH? Because they know more, have vast skill bases, and live a long long time.