In ancient times, debates around free will centered on the concept of destiny. Stories like Oedipus Rex asked whether humans could escape the fate the gods had bestowed upon them. But as science has taught us more about our brains, bodies, and environment, thinkers have shifted from theological questions to scientific ones like: If our actions are determined by a combination of genes and upbringing, does that leave any room for free will? Does someone have free will if their brain functioning is compromised? And does quantum physics, with its nondeterministic forms of causation, provide an explanation for how conscious choice could occur? Neuroscientists, physicists, and even legal experts are now in on the debate, inspiring the rest of us to question and refine our own definitions of free will.

A conversation about free will broke out within the field of neuroscience after neurologist Benjamin Libet conducted a famous experiment addressing the issue in the 1980s. The studys participants were instructed to press a button with their fingers whenever they wanted and to note exactly what time they saw on a stopwatch in front of them when they made the decision to press it. Libet found that the neural activity initiating this action, as indicated by brain scanners, started about half a second before people consciously made the choice.

This finding brought up the question of whether conscious thought plays a role in our behavior, or whether were essentially biological automatons acting according to predetermined patterns of neural firing. Perhaps we think were choosing an action before we perform it, but that choice is simply an illusion that emerges once that actions already being performed.

The idea that peoples behavior is determined by their brain activity, which is in turn determined by other physical processes outside our control, is known as determinism. Determinists argue that if you could go all the way back to the Big Bang with an extremely advanced computer that could track the states of every particle and follow the effects of all these states, you could predict everything thats happening today, including everything that happens in our minds.

One counterargument to this theory is that, if something could actually determine every position of every particle at the time of the Big Bang, whatever is capable of doing that would be so powerful, it may be able to interfere with that system, says mathematician Jonathan Farley. And if that powerful entity were human consciousness, that might mean we do have free will.

The merits of determinism have also been called into question by some interpretations of quantum physics, which has shown that, down at the level of subatomic particles, things look more like waves of possibilities than definite events. For example, the famous double-slit experiment showed that electrons can exist in multiple places at once. Its results suggested that human observation is what collapses quantum particles into one state: The interference pattern of electrons on a wall after they were fired through a slit changed based on whether they were measured. This seems to suggest that there may be something special about the human brain that allows it to exert control over the physical universe, says Farley.

But because its unclear whether the outcomes of quantum events are the products of conscious choice or simply random, quantum physics doesnt necessarily prove that free will exists. In fact, many scientists concerned with free will actually consider questions about determinism to be irrelevant.

Those who believe free will and determinism can coexist are known as compatibilists, and studies show that most people actually have a compatibilist view of free will. That is, people appear to be less concerned with the behavior of small particles and more concerned with the constraints placed on people as a whole, such as whether someone else has forced them to do anything or whether some physical or psychological limitation is affecting their behavior.

This is reflected in the legal system, which tends to consider someones free will constrained only if they have a relevant brain injury, disease, or disability, says James Giordano, professor of neurology and biochemistry and chief of neuroethics studies at Georgetown University Medical Center. Legal debates tend not to touch on physics because that would be a moot point: Everyones actions stem from an amalgamation of physical particles, so if physical causation precluded moral responsibility, it would for everyone.

The way Giordano puts it is that free will is less important than free wont. It matters less, for example, whether the brain activity leading someone to push a button starts before they make the decision, and more whether they can decide against pushing that button if there is a disadvantage to doing so. In a legal context, this would involve someones ability to resist the temptation of committing a crime. This is likely related to the connectivity between brain areas involved in things like memory, emotion, and rational thought, says Giordano.

Many physical traits have actually been shown to correlate with someones likelihood of committing a crime, such as their genes, deficits in brain areas that inhibit impulsive behavior, IQ, health conditions, and even heart rate (the theory being that people with low resting heart rates are more prone to risk-taking because its harder for them to feel stimulated), says criminologist Margit Averdijk.

Some of these factors actually are taken into consideration in court. Typically, evidence that someones capacity to make ethical decisions is compromised wont absolve them of guilt, but it may make them more likely to be put in a psychiatric institution rather than prison, says Giordano. A 2016 analysis found that brain scans had been used as mitigating evidence in 5% of appellate murder cases and 25% of death penalty trials.

Social scientists have also posed questions about free will, often in the broader context of asking how much society is influencing human behavior. For example, some argue that someones culture and even the physical environment they grow up in will have a major effect on their lives.

A slightly different way to approach the question of how much free will society affords people is to ask how much external factors constrain our decisions. In that vein, its actually rather difficult to sway peoples choices, says MD-technologist and behavioral economics consultant Drea Burbank. Changes to things like prices and stores that attempt to get people to buy fewer cigarettes, for example, only account for around 10%-20% of variance in human behavior.

So, in short, theres no proven answer to the question of whether free will in any metaphysical sense exists. Clearer working definitions of free will have been developed for more pragmatic purposes like determining legal sentences and getting people to make healthier decisions. Under these definitions, people do appear to have free will, but the extent of the freedom varies from person to person.

While more abstract definitions of free will are interesting to ponder, its difficult to make them the lens through which we make moral judgments or attempt to understand human behavior. Still, questions about quantum states and physical determinism, like those about ethereal souls and deities, are fascinating enough that scientists, philosophers, and artists alike will unlikely stop thinking about them any time soon.

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What science teaches us about free will - Science 101