Today is National Doughnut Day, a day with a surprisingly long history — but there's more to doughnut day than pastry celebration alone.

At first, it sounds like the plot of a wild pastry-based conspiracy theory: over the course of a couple of decades, our nation's donuts holes dramatically decreased in size. The very shape of the donut changed, in just two decades, from a ring of dough to the pinched dough vessel we know today.

But then you see this undated chart, and it doesn't seem so crazy anymore:

Part of the Smithsonian's Sally L. Steinberg Collection of Doughnut Ephemera, the chart presents a theory long held by donut truthers: that donut hole size has gradually shrunk over the years.

So does the theory hold true? Has donut hole size changed, as the chart argues? Part of the problem might be that dunking donuts, occasionally still in use today, skew the sample. These donuts are thinner and have wider holes, so even today there are donuts that look like the ones from 1927 on the chart. The chartist has passion, but he might not have a lock on the facts.

Still, there's some decent evidence that should intrigue donut morphologists. In 1955, these sugared donuts were photographed, and their holes are positively puny:

But let's go back to the past. The year is 1918. A nation at war uses donuts to raise money and promote men at war, and so-called "Doughnut Girls" bring them to soldiers, occasionally on the battlefield. And one poster shows donuts as they were. The size of this man's donut hole may give you chills, because it's gigantic.

To be clear, it's easy to find examples of donut holes of all sizes across different time periods, but there's a definite trend toward smaller donut holes. What happened?

Our best guesses at why donut hole size decreased

Smithsonian's history of the donut provides a comprehensive look at the food, and from it we can draw a few guesses about why donut holes shrank. Donut holes are shrouded in legend, but they probably exist to help fry the donut more evenly — without a hole, the center of the donut would end up more raw than the outside.

As machines improved — starting around the 1920 invention of the first donut machine — it's possible it became easier to make donuts with smaller holes, since they no longer had to be manually dunked and plucked from the frier, but could ride along a conveyer belt instead. It could also be that chains like Krispy Kreme, founded in 1937, imposed a standardized donut vision upon the United States. It did develop its own manufacturing processes to create unique and consistent donuts. Smaller holes also could help with breakage.

It's also possible that filled donuts, inspired by European confections like the Berliner, became more popular after World War I. As donuts became a treat, their shape may have changed to accommodate that, making them less about holding coffee and more about holding sugar, jelly, and even chocolate.

But for such an iconic American snack, we may never learn the truth about the size of our donut holes. Hole size varies wildly from donut to donut, and different purposes (dunking or eating) only compound the confusion. For the time being, all we have is the historical record and one brave bespectacled man with a chart and a pointer, trying to show the truth.