Whitespace in passwords

generally, and i dont know if this is social conditioning due to the 
misnomer "passWORD" rather than passPHRASE but it seems that [most?] people 
choose passes that dont contain whitespaces, and in fact, there are some 
system implementations that wont allow whitespaces in the password.
my main question, re security, is wether the whitespace made the password 
too vulnerable? [historically] and why this constraint is introduced in many 
systems.. [but then, if myth- why propogate it?]
i'm thinking that whitespaces [if yr system can handle them, and why not?] 
would add another measure of complexity in cracking pwds? 


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