This comes from research published in 1980 done by Richard
Zweigenhaft and his colleagues showing that college men with "Jr." after their names scored significantly lower than those with "II" after their names on scales of "Capacity for Status", "Well-Being", and "Intellectual Efficiency."
The problem seems to be in the word "junior" itself. "Junior
" always sounds inferior to "Senior", and especially if the child is called "Junior
" as his nickname, that gives the impression that he's someone who's never really going to "grow up" psychologically. The Roman
numerals have no such connotation.