In 2002, a group of teenagers sued McDonald's for making them fat, charging, among other things, that McDonald's used promotional techniques to get them to eat more than they should. The suit was roundly condemned as an the erosion of the sense of free will and personal responsibility in our society. Less widely remarked upon was that the teenagers were offering an accurate account of human behavior.

Consider the phenomenon of 'super-sizing', where a restaurant patron is offered the chance to increase the portion size of their meal for some small amount of money. This presents a curious problem for the concept of free will — the patron has already made a calculation about the amount of money they are willing to pay in return for a particular amount of food. However, when the question is re-asked, — not "Would you pay $5.79 for this total amount of food?" but "Would you pay an additional 30 cents for more french fries?" — patrons often say yes, despite having answered "No" moments before to an economically identical question.

Super-sizing is expressly designed to subvert conscious judgment, and it works. By re-framing the question, fast food companies have found ways to take advantages of weaknesses in our analytical apparatus, weaknesses that are being documented daily in behavioral economics and evolutionary psychology.

This matters for more than just fat teenagers. Our legal, political, and economic systems, the mechanisms that run modern society, all assume that people are uniformly capable of consciously modulating their behaviors. As a result, we regard decisions they make as being valid, as with elections, and hold them responsible for actions they take, as in contract law or criminal trials. Then, in order to get around the fact that some people obviously aren't capable of consciously modulating their behavior, we carve out ad hoc exemptions. In U.S. criminal law, a 15 year old who commits a crime is treated differently than a 16 year old. A crime committed in the heat of the moment is treated specially. Some actions are not crimes because their perpetrator is judged mentally incapable, whether through developmental disabilities or other forms of legally defined insanity.

This theoretical divide, between the mass of people with a uniform amount of free will and a small set of exceptional individuals, has been broadly stable for centuries, in part because it was based on ignorance. As long as we were unable to locate any biological source of free will, treating the mass of people as if each of them had the same degree of control over their lives made perfect sense; no more refined judgments were possible. However, that binary notion of free will is being eroded as our understanding of the biological antecedents of behavior improves.

Consider laws concerning convicted pedophiles. Concern about their recidivism rate has led to the enactment of laws that restrict their freedom based on things they might do in the future, even though this expressly subverts the notion of free will in the judicial system. The formula here — heinousness of crime x likelihood of repeat offense — creates a new, non-insane class of criminals whose penalty is indexed to a perceived lack of control over themselves.

But pedophilia is not unique in it's measurably high recidivism rate. All rapists have higher than average recidivism rates. Thieves of all varieties are likelier to become repeat offenders if they have short time horizons or poor impulse control. Will we keep more kinds of criminals constrained after their formal sentence is served, as we become better able to measure the likely degree of control they have over their own future actions? How can we, if we are to preserve the idea of personal responsibility? How can we not, once we are able to quantify the risk?

Criminal law is just one area where our concept of free will is eroding. We know that men make more aggressive decisions after they have been shown pictures of attractive female faces. We know women are more likely to commit infidelity on days they are fertile. We know that patients committing involuntary physical actions routinely (and incorrectly) report that they decided to undertake those actions, in order to preserve their sense that they are in control. We know that people will drive across town to save $10 on a $50 appliance, but not on a $25,000 car. We know that the design of the ballot affects a voter's choices. And we are still in the early days of even understanding these effects, much less designing everything from sales strategies to drug compounds to target them.

Conscious self-modulation of behavior is a spectrum. We have treated it as a single property — you are either capable of free will, or you fall into an exceptional category — because we could not identify, measure, or manipulate the various components that go into such self-modulation. Those days are now ending, and everyone from advertisers to political consultants increasingly understands, in voluminous biological detail, how to manipulate consciousness in ways that weaken our notion of free will.

In the coming decades, our concept of free will, based as it is on ignorance of its actual mechanisms, will be destroyed by what we learn about the actual workings of the brain. We can wait for that collision, and decide what to do then, or we can begin thinking through what sort of legal, political, and economic systems we need in a world where our old conception of free will is rendered inoperable.