Would you be surprised to find out that people didn't always wash their hair every day? In the past, shampooing ones hair wasn't a daily or even a weekly occurrence. It wasn't until the early 1900's that frequent shampooing became popular. During this time, America's Industrial Revolution was in full swing and there were many new advances in modern conveniences like indoor plumbing which made bathing and hair washing more accessible. A New York Times column in May of 1908 advised women of the day that it was perfectly OK to wash their hair once every two weeks.
In prior centuries, hair was only washed when absolutely necessary. In fact, in the Middle Ages, washing the hair was an undesirable habit and medieval cultures avoided it like the "plague".
The 1970's ushered in the lather, rinse, repeat customs that many adhere to today. Ad campaigns from that era used celebrities like Farah Fawcett to convince the general public that they could shampoo their way to shiny locks of lustrous hair.
We've learned a lot since then and, while our medieval predecessors may have taken the "no shampoo" lifestyle to the extreme, dermatologists and other health care professionals generally agree that shampooing the hair every day is not necessary and actually damages hair by stripping it of natural oils.
