There are several theories regarding the origin of the stereotype "dumb blond," with the earliest dating back to ancient Roman times. In ancient Rome, women who lightened their hair were regarded as not having serious intentions.
In early European times, it was common for female peasants to have lighter hair than upper class individuals due to the fact that the peasants spent long hours working in the hot sun. Peasants were believed to be less intelligent than the upper class, which led to the "dumb blond" stereotype.
There is no scientific evidence that proves light-haired individuals lack intelligence. The "dumb blond" stereotype is still common in present society, making it difficult for many blond women to be considered as equals.