It's well-known in the health-care world that we have a looming doctor shortage, with a growing gap between the medical services we need and the doctors who can provide them.

This often gets chalked up to the Affordable Care Act: Expanding insurance, the thinking goes, will hugely increase demand for doctors' services. It's an issue I've written about, as have others.

New research in the Annals of Family Medicine throws some cold water on that theory: Researchers there suggest that it's population growth and aging largely driving the demand for doctors, with the Affordable Care Act playing a more minor role.

A team of six researchers, led by Stephen Pettersen of the Graham Center, looked at how heavily various groups use medical services. They used that data to project out the disparity between medical demands and providers to meet them.

They estimate that the United States will need 52,000 additional primary care doctors by 2025. Of those, about 33,000 are due to population growth and 10,000 would be needed to meet the needs of the aging. Then, there would be 8,000 doctors demanded by the insurance expansion. 

In the graph below, the impact of the Affordable Care Act is the very top, very skinny bar: 

How, exactly, does a country expand insurance to about 30 million Americans without driving huge demand for doctors? Pettersen and his colleagues offer one plausible explanation: The uninsured are already seeking out health care, albeit to a lesser degree than those with coverage. Here's one chart showing the rates of office-based doctor visits by those who lack coverage.

These numbers are near certain to grow over the next decade. This study estimates that the coverage expansion will lead to 19.5 million more doctor appointments than would have occurred without the Affordable Care Act.

The biggest growth in demand, however, has nothing to do with expanding insurance. It has everything to do with the fact that the United States is getting bigger and older.