As Democratic presidential hopefuls jockey for position, candidates like Sens. Bernie Sanders and Elizabeth Warren have sought to distinguish themselves with health care plans that promise free coverage for all. Notably, both of their proposed plans include dental coverage.

Having dental and medical insurance combined is not common in America today. But why is that?

For starters, the distinction between the two fields has always existed. In modern times, dentists have never been physicians, and physicians don't do dentistry. That's how it was as they came of age through the 19th century. And that's how it was in 1929, when the American health insurance industry was born.

"It was actually not insurance companies that brought health insurance to the market in the U.S., but rather hospitals and physicians themselves," explained Melissa Thomasson, an economics professor at Miami University in Ohio.

The idea, eventually called Blue Cross, was simple. Patients paid hospitals a small annual fee. In return, they were allotted a certain number of hospital visits each year.

"It starts to be a huge hit," said Thomasson of these hospital-based insurance plans, as they spread across the country. "Hospitals like it because it’s getting their bills paid. People like it because it’s pretty cheap and it’s helping them budget."

A similar plan for primary care launched in 1939. It was known as Blue Shield. Both got a huge boost in popularity during World War II, when Congress froze wages and employers began offering health insurance as a way to attract talent.

"It becomes a lucrative way of having a bidding war for workers," said Thomasson.

And so, health insurance in America evolved with dentists on the outside looking in. It wasn’t until the 1950s that the first dental insurance plans were created, and the 1970s until they became commonplace.

But it’s not just historical precedent that keeps most dental insurance separate in 2019.

"[Dental coverage] is not catastrophic coverage," said Adam Powell, a Boston-based health care economist and consultant. "It's the exact opposite in the medical world. There's almost a flip in benefit design."

For most health insurance plans, the buyer pays a significant portion of their basic care out of pocket, until they hit some deductible, at which point the insurance company covers the rest. It’s essentially the opposite when it comes to dental coverage.

"So, what they’re doing in the dental space is they're selling a discount program," he said. "It’s almost like AAA for your mouth."

With most dental plans, you pay little or nothing for basic services, like cleaning, until you hit a cap — at which point all further costs are on you.

"It is not serious insurance where there’s all this risk being born by the insurance provider," said Powell.

Powell said there are a number of reasons for this. But the upshot, he said, is that if dental coverage was simply included in your health insurance, the overall cost of all the insurance would be considerably higher.

"Ultimately, when you smash them all together, there is a cost to these additional benefits, and people often prefer things to be un-bundled," he said.

Indeed, Powell said that in 2017, 23 percent of Americans lacked dental coverage. That’s about double the number of people who lacked health insurance.

But what if all Americans had free and full dental coverage?

"For sure, if more patients had better coverage they'd be inclined to do more treatment," said Dr. Janice Moriarty, a Winchester dentist and president of the Massachusetts Dental Society.

And, she said, most dentists — including her — would be a lot busier.

"I would probably be working 23 hours a day and six or seven days a week," she said.

Still, it’s estimated that more than 90% of people with dental coverage don’t visit their dentist enough to hit their plan’s maximum each year. So it makes you wonder, even if coverage was free and full for all, would people still be inclined to avoid the dentist?