> A university acquaintance went acute schizophrenic shortly after developing a pot habit, that could be a coincidence but the timing is suspicious.

Early adulthood is when mental illness often sets in, especially illness like Schizophrenia. You're implying cause one way without considering the other likely alternatives:

1) Your acquaintance started smoking a lot of pot because of the onset of early schizophrenia symptoms as a coping mechanism.

2) They are unrelated and early adulthood is just when people experiment more with drugs, and also when conditions like Schizophrenia start to manifest.

> A second cousin became mentally retarded immediately after smoking a joint. He's now a 30 year old who can't tie his own shoes.

I don't know what to say to this other than nobody's ever established an even remotely plausible link between these two things, assuming you aren't being hyperbolic.