The 47 million Americans who lack health insurance are the reason emergency rooms are crowded all the time -- right? And only the uninsured visit the emergency department for minor complaints, because it's easier than going to a doctor -- right?

Not so fast. According to a new study by a University of Michigan team, published in the Journal of the American Medical Association, the reality of what causes ER overcrowding is a lot more complicated. And some widely repeated perceptions about the uninsured and emergency care may be rooted more in assumptions than in solid fact.

Those faulty perceptions, they conclude, may be getting in the way of real efforts to solve both the uninsurance problem and the problem in American ERs.

In truth, the uninsured do not make up a disproportionate share of ER patients, because they are the only group that faces the full cost of care, the study shows. It also demonstrates that people who have insurance are more likely to contribute to ER overcrowding and to use the ER for minor complaints or in place of a primary care doctor's visit, because primary care offices are also overcrowded.

The study is based on an exhaustive review of 127 medical research papers, and on detective work to find out whether often-repeated statements about the uninsured and emergency care were actually based in fact.

Most of the papers were published in the last decade, when both the plight of the uninsured and the state of the nation's ERs captured the national spotlight.

Although it challenges some of the most-repeated mantras about the uninsured and ER care, the study does confirm that solid evidence exists for many of the things that Americans have come to believe about the uninsured and emergency care.

For instance, the study shows, the number of people without insurance visiting American ERs is rising -- but less quickly than the numbers of uninsured are rising. Meanwhile, patients with insurance are going to the ER more frequently.

There is also solid evidence that caring for patients -- insured and uninsured -- in an ER is more expensive than treating the same complaint in a doctor's office. Uninsured people definitely have a hard time finding primary care doctors who will see them as outpatients, but even insured patients have difficulty finding primary care.

"What we found is that there is a perception that -- because one of the roles of the emergency room is a safety net for the uninsured -- it is the uninsured who must be causing all the problems in ER care," says first author and emergency physician Manya Newton, M.D., MPH, M.S., a Robert Wood Johnson Clinical Scholar at the U-M Medical School.

"The crisis in emergency medicine and the problems of the growing uninsured population have been conflated," she said. "While there's excellent research out there on both issues, the myths about how the uninsured use the emergency department threaten to interfere with the policy-making process. The rise in ER use has much more to do with the aging of the population, the increase in chronic diseases, and the decrease in available primary care than with the uninsured. Policies based on false assumptions risk diverting energy and money from confronting the true drivers of emergency department crowding."

At the least, Newton and her co-authors conclude from their review, ER policy solutions will need to address the lack of timely access to primary care by the uninsured and insured alike.

The uninsured have a nearly impossible task in finding primary care. But both insured and uninsured have trouble getting appointments in less than two to three weeks, or finding primary care after regular business hours and on weekends -- which leads them to the always-open ER.

A reluctance by some physicians to take on the legal liability of counseling a patient over the phone, instead of instructing them to go to the ER, may also contribute, Newton says.

Meanwhile, Newton says the evidence is very strong that the overall cause of ER overcrowding is an "input-throughput-output" problem at American hospitals.

Patients come to the ER for treatment, and under federal law the ER cannot turn them away. Some of them need at least an overnight stay in one of the hospital beds upstairs from the ER. But those beds are often full because of a lack of safe and appropriate places to discharge current patients to -- so patients get backed up down in the ER, making it more crowded.

The closure of hospitals, ERs, and long-term skilled nursing facilities around the country makes the situation worse and worse, the researchers say. Fewer beds plus more patients equals an ER calamity.

Newton and her colleagues began the study after noting a curious phenomenon in the medical literature: many papers whose introductory passages included phrases like "It is well understood that..." and other statements of conventional wisdom about the uninsured and ERs. They often appeared without direct citations of studies that could support such statements.