The belief that Americans are "paying for Europeans' healthcare" is a mix of several real economic phenomena, political messaging, and misunderstandings. It doesn't have a single origin, but rather developed from several arguments that became popular from the 1980s onward.

Here are the main sources:

1. The pharmaceutical pricing argument (the biggest source)

This is the strongest factual basis behind the claim.

The United States generally allows pharmaceutical companies to charge much higher prices than most European countries. Many European governments negotiate or cap drug prices through national health systems.

For example:

A new cancer drug might sell for $120,000 per year in the U.S.
The same drug might be reimbursed at the equivalent of $40,000–60,000 in Germany or France.

Because drug development is extremely expensive and companies earn a disproportionate share of their profits in the U.S., economists often say that American consumers subsidize pharmaceutical R&D that benefits patients worldwide.

Over time, this was simplified in public discourse into:

"Americans pay for Europeans' healthcare."

That is not literally true. Americans are paying higher drug prices, not paying European hospital bills or insurance costs.

2. NATO burden-sharing got mixed into healthcare

Beginning especially after the Cold War and intensifying during the Trump administration, many American politicians argued:

European countries spend less on defense.
Because the U.S. provides much of NATO's military capability, Europeans can devote more tax revenue to social programs, including healthcare.

The argument becomes:

America pays for Europe's defense → Europe can afford universal healthcare.

This is a political argument rather than a direct economic one.

It does not mean U.S. tax dollars go into European healthcare systems.

3. American debates over universal healthcare

Since the 1990s, opponents of single-payer healthcare frequently argued:

Europeans get "cheap healthcare."
But someone has to pay.
America bears many innovation costs.

Think tanks, politicians, and commentators often combined:

pharmaceutical R&D,
biomedical innovation,
venture capital,
medical technology,

into one broad claim that the U.S. finances the medical advances everyone else enjoys.

There is some truth to the innovation aspect:

The U.S. accounts for a large share of global biomedical research investment.
American capital markets support many biotech startups.
U.S. consumers pay unusually high prices for drugs and some medical devices.

However, European countries also contribute substantially through public research institutions, universities, and pharmaceutical companies.

4. Misunderstanding of who pays for healthcare

Many people assume:

Europeans have free healthcare.

But healthcare is not free.

European systems are generally financed through:

taxes,
payroll contributions,
mandatory insurance premiums.

Countries like Germany, France, the Netherlands, Sweden, Denmark, and others spend significant portions of their own GDP on healthcare.

For example:

Germany spends roughly 12% of GDP on healthcare.
France spends around 12%.
The U.K. spends around 11%.

These systems are overwhelmingly financed by their own taxpayers.

5. Media simplification

Television, political speeches, and social media compressed nuanced economic ideas into memorable slogans such as:

"Europe gets a free ride."
"Americans pay for everyone else's drugs."
"We're funding European healthcare."

These statements are rhetorically powerful but blur together several distinct issues.

What economists generally say

Most health economists would distinguish between three separate claims:

Claim	Generally supported?
Americans pay much higher prices for prescription drugs than Europeans.	Yes.
Those higher prices contribute disproportionately to pharmaceutical company profits and future R&D.	Largely yes, though the magnitude is debated.
Americans directly pay for European healthcare systems.	No. European healthcare is primarily funded by European taxpayers.
Lower European defense spending may leave more fiscal room for social programs.	Plausible as a budgetary argument, but it's an indirect relationship rather than Americans funding European healthcare.
Bottom line

The belief originated from a combination of:

the fact that U.S. patients often pay much higher prices for prescription drugs, helping generate profits that fund global pharmaceutical innovation;
arguments that U.S. defense spending allows European governments to allocate more of their own budgets to healthcare and other social programs; and
political rhetoric that merged these ideas into the simpler claim that "Americans pay for Europeans' healthcare."

In reality, Europeans fund their healthcare systems primarily through their own taxes and insurance contributions. The part of the claim with the strongest empirical support is that U.S. consumers pay disproportionately high prices for many branded medicines, which contributes to the global economics of pharmaceutical innovation.