This is a follow-up to my last post where I posted a graph that surprised me. As a percentage of GDP, per-pupil expenditures have remained flat over time -- other than during WWII a little under a dollar per billion dollars of GDP per student. The first comment (by the author of the post that prompted my post) asked why we should expect to spending on education to rise over time compared to GDP. I gave a terse response in my follow-up comment, but I'd like to take more time to flesh that out.
To oversimplify economic theory, many goods can be counted as either necessities or luxuries and people (and countries) are expected to increase their spending on luxury items as income increases while simultaneously decreasing their spending on necessities relative to their income. So, for example, somebody making a million dollars per year is expected to spend a smaller portion of their budget on food and shelter than is somebody making twenty thousand dollars per year. At the same time, the wealthier person is expected to spend a greater share of their budget on wine or boats or vacation homes. They're able to do this because they have more "discretionary income" -- money beyond what's needed to pay for the bare necessities of life.
Countries work in a similar fashion. There are certain things that even the poorest country must spend money on (e.g. a basic system of defense or a basic network of roads) and some things that wealthy countries can afford to devote a larger share of income towards as their discretionary income increases (e.g. science research or museums).
So, whether you believe that education spending should increase, decrease, or remain the same relative to GDP depends on how you view education spending. If you think that it's a bare necessity, then you would expect it to decrease over time as we gain more money. If you view it as more of a luxury good, then you'd expect spending to increase over time as discretionary income increases. If you view it as somewhere in between, then you might expect spending to stay relatively flat over time.
I said at the beginning of the post that I was surprised when I saw that it was flat. I was surprised because I was under the distinct impression that countries tended to spend a larger share of GDP on education as they become wealthier. I guess further investigation is needed to find out whether I was wrong or if the US is an outlier.
Update: Actually, when I corrected the graph, it appears as though spending per pupil compared to GDP per capita has been falling rather than flat