"You are your neurons," she said. "There's nothing else up there. When you really think about the brain, the physical system, it becomes very difficult to think about where choice comes in, where free will comes in."
Even consciousness, Wheatley notes, doesn't guarantee free will. You can be conscious of a choice, but could you have truly made another decision, given the brain activity that led to that choice?
Others are not so convinced. No experiment done so far rules out free will, argues Alfred Mele, a philosopher at Florida State University and author of "Free: Why Science Hasn't Disproved Free Will" (Oxford University Press, 2014).
There are two ways to think about free will, Mele said. One is that if a person is sane, rational and uncoerced, and they can make a decision, then they have free will. This is the definition on which our court system is based.
"It's clear that most people, some of the time, do make such decisions, so they would have free will, according to this conception of it," Mele said.
The free will most neuroscientists are discussing is a bit more nuanced. The idea is that, given everything leading up to a conscious choice — external influences, brain activity, prior experiences — people can still make a different choice.
"It could all be shaped, and probabilities could be influenced by the environment and so on, but when you make your decision, there are different possible ways you could go," Mele said.
Even when measuring very trivial decisions with the most accurate technology (electrodes actually inserted into the brain, as part of treatment for severe epilepsy), researchers can only predict decisions based on brain activity about 80 percent of the time. It could be that the equipment isn't good enough to yield perfect predictions, Mele said. Or it could be that there is a certain amount of randomness present in the brain – randomness that represents free will.
It's clear that a decision process begins in the brain far before we're aware of it, Haynes said. But what isn't as clear is whether that process can be stopped, and thus it isn't clear that these experiments rule out free will.
"Can you interfere with this process at any point in time? What are your chances of stopping this process?" Haynes asked. He and his colleagues are in the process of submitting a paper to a peer-reviewed paper on just this subject.
"All I can say is that the data we have at the moment suggests that people can control this process all the way right until the end," Haynes said.
Why free will matters
Free will or not, we are built to assume agency over our actions. It's not surprising to feel a need to solve the mystery of whether something that feels so intertwined with our being is actually real. But all of this free will debate also matters because it turns out people behave very differently when they think they're not accountable for their behavior.
In 2008, psychologists Kathleen Vohs and Jonathan Schooler published a study in the journal Psychological Science in which they asked people to take a math test on a computer. Because of a computer glitch, the participants were told, the answers to the questions would appear onscreen unless they quickly pressed the space bar before each question. Prior to the test, some of the participants read articles telling them that science had disproven free will.
Those participants, primed to disbelieve free will, were more likely to cheat by not pressing the space bar. Similar studies showed other bad behavior. A 2009 study by Florida State University psychologist Roy Baumeister found that when people were told they had no free will, they became more aggressive towards others, giving them spicy hot sauce even after being told they disliked spicy food.
On the other hand, the belief that there is no free will can also make people less punitive and less likely to seek vengeance, according to a 2014 article published in the journal Psychological Science.
"When you confront people and say, 'Oh, you don't have free will,' they can act badly," Wheatley said. But that's not the whole story, she said. "They can also become more compassionate."
The next step for neuroscience, Wheatley said, should be to investigate ever-more complicated decisions. The criticism of work like Libet's and Haynes' tends along the lines of "so what?" she said, because the choices are so primitive. Haynes' recent work, she said, has been delving into more complex choices.
"These decisions are still no 'where to go to college' or 'who to marry,' but they're getting more interesting," she said.