Sunday, January 4, 2009

How Medicine Harms Health: Lessons from Perverse Tradeoffs in Driving

It has been well established that seat belts and other car safety equipment make people drive more recklessly than they would otherwise. The thought process is as follows. Drivers know that seat belts severely cut their risk of death and serious injury in a high-speed collision. Drivers who adopt the seat belt feel safer while wearing one.

Then, completely rationally, the drivers using seatbelts shift their "danger curve" down from the blue curve to the red one. Since for each trip your need to get somewhere is pretty much constant (excluding Sunday drives), it can be represented by a straight line, its scale on the right green axis.

This shifts the no seatbelt optimal trade off between driving speed, danger, and the need to get somewhere -- point A (the intersection between BLUE and GREEN) -- to the seatbelt optimum at point B (RED AND GREEN).

Point A corresponds to a speed of X and point B to a speed of Y. Y is greater than X, meaning people who drive with seat belts drive faster than they would without the safety restraints. If we use speed as a proxy for general recklessness, then we have proved my assertion. One would find similar results if we used percentage of brain devoted to driving instead of speed, but where the data would come from is beyond the scope of this blog.

Likewise, people face exposure to similar safety devices in other aspects of their daily lives: drugs and medicine. Consider replacing a constant "desire to live" as the GREEN line. If the horizontal axis represents the healthiness of a person's lifestyle, what effect would a similar shifting of the danger curve result in? A less healthy lifestyle filled with bad short term tradeoffs with the hope that medicine will save them in the long run.

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1 comment:

Jon Lien said...

One interesting note that is almost always forgotten in the seat belt argument, and which makes it an incorrect metaphor for almost every other example, is the physics of collisions. If you have a higher speed than the car you hit, assuming a head on collision, the other car is more severely damaged. Driving slower than others with or without a seat belt would therefor increase the expected level of injury in a collision, thus changing the rationality of the shift in the danger curve.
This does not necessarily change the core argument of the seat belt debate, as there is a "risk illusion" argument to be made, although there are arguments for it doing so, but it most certainly does make the metaphor invalid.

As Adam Smith himself once put it, "the enemy of a good theory is promiscuous analogizing" (Warsh, David. Knowledge and the Wealth of Nations, p33)

Your medical example simply avoids the potential danger to and from others, and needs a quite different metaphor.