Why do doctors run so many tests? Is this purely a construct of defensive medicine, trying to protect ourselves from a litigious society? I would suggest that concern about being sued is only a small part of the equation. A much larger part is an unintended consequence of how physicians are paid and the time constraints of practice.
Physicians (and other "providers") are evaluated via many different metrics. My favorite one is RVUs per clinical FTE. For those of you who are not familiar with such jargon, this is a measure of how much "work" a given MD is doing per the equivalent of a full time clinical position. It is interesting that actually talking to patients is not really considered work. Thus, the feedback we receive from such metrics is that if we invest our time in talking to patients, we are not really working and thus not really paid to do so.
Why should we think that talking to patients is of any value? Medicine should be practiced one patient at a time. Recommendations for interventions (or the wisdom of no intervention) need to be assessed on a patient by patient basis, since the suitability of any particular decision depends heavily upon patient specific circumstances. Even the most pedestrian of encounters may involve multiple different options where ideally patients should be at least reasonably informed regarding their options.
Unfortunately this is very difficult to undertake in our present ambulatory practice environment. As medicine becomes more factory like, the resource which is most short supply is time. Patients are stacked up like jet planes waiting to land at O'Hare. The way to deal with time and throughput pressures is to create a one sized fits all certainty to our encounters. Ordering tests are great for this. Whether it be biopsies, tests for Lupus, allergies, there is a false finality and certainty of a test. It also serves as a great way to force an end to the face to face encounter, thus freeing up the runway for the next plane to land.
Thus, while a CAT scan for a headache may not put money directly in the pocket of the evaluating provider, it spares them the need to discuss the uncertainty of the clinical evaluation of headaches, an activity which can be viewed through the lens of the payment system as not actually being work and thus monetarily almost useless. When faced with uncertainty and a choice to discuss this uncertainty with the patient vs ordering additional tests (even ones we do not directly benefit from financially), testing will always win hands down.