One has to get into the age of photography to see the graphic evidence of this because the portraitists using oils routinely idealized their subjects. Here is a photograph of the famous French artist Eugène Delacroix from the early 19th Century.
I did not chose Delacroix for any particular reason. It was just the first photograph that I came across that portrayed what I was looking for. Such photographs are easy to find. Notice his hair.
Delacroix's hair is noticeably greasy. This is not a hair care products issue, although I am sure that played into the phenomenon. This is a bathing issue.
I am not getting on Delacroix's greasy hair in order to make fun of him. Quite the contrary. I sympathize with him. I would lay money that this photograph was taken in the winter. Just imagine the problem.
You are living in Paris in the winter with the temperature routinely at 40° F. (4.4° C.) or less. Your quarters are heated with a fireplace or a variation on the Ben Franklin stove, assuming you can afford fuel. You decide to take a bath. To do that, you must heat water over a fire in a kettle and then pour it into a copper tub. That does nothing to change the ambient air temperature, however. That is a daunting task and an easy one to put off. I will guaran-goddamn-tee you that you will not do it every day.
Now, you might say that if the bather were a person of means, he could simply order the young scullery maid to heat some water and perhaps even assist him in bathing, which could be a pleasant thing. To which I ask in reply, what does the scullery maid look like? Does she have all her teeth? How much does she weigh? Is her own hair in a big greasy pile, perhaps infested with vermin? So there you go.
And I am not such an insensitive lout that I do not appreciate that great numbers of people in this world today who live in climates where there is a winter face this same situation. Most people are poor.
All I am saying is that when it is cold outside and inside, bathing and getting your hair washed is a very easy project to put off. That's all. Greasy hair becomes less of a vital concern in daily life. I have a better subjective feel for all that now. I no longer even put it in the category of personal hygiene. Personal hygiene consists of brushing your teeth and washing your hands. Period.