More bad news for Obamacare and its proponents. A new study from Oregon shows that better access to health care increased—rather than decreased—both the number of folks crowding emergency rooms and the subset of those who sought care for conditions that clearly were not emergencies. Many health experts who supported the Affordable Care Act had predicted the opposite outcome.

After all, when Massachusetts—under then-Governor Mitt Romney—first introduced what you might call Romneycare, data showed that the number of emergency room visits either remained the same or declined. Emergency rooms are just about the most expensive place in the world to get treated for anything, which is why anyone who cares about burgeoning health care costs tries to reserve their use to true emergencies.

The Oregon study, which was published online January 2 by Science, is particularly compelling because it took advantage of a natural experiment that happened in 2008 when the state started expanding medical coverage to working poor households through the use of a lottery system. The random selection of participants allowed investigators to conduct a more scientifically rigorous study than would otherwise normally be possible.

When the investigators compared households that were newly eligible for Medicaid insurance coverage with their counterparts who “lost out” on the lottery, they found that the number of emergency room visits increased by an average of 40 percent. Indeed, nearly every category of emergency room visit saw an increase—from visits during normal daytime hours during the week to visits for things that could just as easily been taken care of in a doctor’s office. The one category that did not increase was visits to the emergency room for true emergencies.

Presumably, the lower cost of emergency room visits for the newly insured made it easier for them to choose the service. Still unknown is whether this was a temporary effect that will even out over time.

While no doubt confounding to some health care experts, the study shows how important it is to check assumptions about what people will do against data about what they actually do.

Another takeaway: there are no magic bullets in health care. If you want to reduce the use of emergency room visits for non-emergencies, you have to do more than just give more people health insurance. For example, if a parent is worried about having to take a day off from work to see the doctor about a child’s sore throat, an emergency room visit in the early evening might seem like just the ticket to solve the problem.

A few pilot programs around the U.S. are taking a more active approach to decreasing emergency room visits for conditions that could be handled outside the emergency room. One of them, called the Voices of Detroit Initiative, has found that reducing emergency room visits requires expanding the hours and locations of primary care clinics as well as coordinating and following up with individuals who would otherwise wind up being regular customers in the E.R.