Most Helpful Guy
The stereotype that blond girls are ditzy could arise from the following:
1. Girls who dye their hair a different color may very well be ditzier on average than girls who don't. Not saying all are ditzy, but something like this could be happening: 70% of ditzy girls dye their hair blond, only 50% of not ditzy girls do. Therefore, girls with dyed hair are, on average, ditzier than girls with not-dyed hair.
2. Majority of blondes in the USA are not naturally blond. So a higher proportion of blondes are girls who dye their hair.
3. Therefore, because most blondes in US are girls who dye their hair, and that girls who dye their hair are ditzier than average, blondes in the US are ditzier than average.
And thus a stereotype is born.