We know that physicians (and their pens/keyboards) are some of the main drivers in health care spending. But which ones are the biggest offenders?

A recent study from the nonprofit RAND Corporation asked this question and found that newer doctors tend to run up higher health care bills for their patients than their more seasoned colleagues. The study, published in Health Affairs earlier this month, looked at insurance claims filed by more than 12,000 doctors in Massachusetts between 2004 and 2005. The researchers found that those with fewer than 10 years of experience generated 13.2 percent higher costs for comparable “episodes of care” (say, a series of appointments to diagnose and treat a breast lump) than doctors with 40 or more years of experience.

Doctors between 10 and 40 years of experience fell somewhere in the middle. When they broke down the costs by types of care, the trend went in the opposite direction for preventative care (as opposed to care for acute or chronic illnesses): less experienced doctors spent less on prevention. Surprisingly, factors such as prior malpractice claims, practice group size, or whether or not the doctor was board certified had no significant impact on the cost profiles.

How to explain the age differential, if it’s not just artifact? It’s not that the less experienced doctors saw a larger proportion of first-time patients, the researchers found. And it’s not that they ordered proportionately more imaging or other tests. But they were more likely to have some sicker, more complex patients who were outliers on the cost curve. Less experienced doctors may also be more aware of – and more likely to use – newer, costlier treatments than their older colleagues, the authors suggested.

Another theory: lack of experience favors caution. Anecdotally, I’ve noticed that new attending doctors seem more guarded as they test the perilous waters of legal responsibility. This caution can manifest as a lower threshold for consulting a specialist doctor, ordering more tests, and keeping patients in the hospital longer. They are more careful than they were (or than we are) as residents, even months before, because the buck now stops with them.

Whatever the reason for the differences, private insurers and Medicare have started to use these cost profiles to rank doctors and penalize those who fall on the side of big spending. That’s just one reason it’s so important to teach residents about the costs of care and to make it easier to see how much we spend.

The looming question, not answered by this study, is how these costs relate to the quality of care. Many studies have found that more experienced doctors actually provide lower quality care. So what is the sweet spot, the right amount of health spending?

Ishani Ganguli is a journalist and an internal medicine-primary care resident who blogs at Short White Coat on Boston.com, where this article originally appeared.