If one thing’s for sure, it’s that I decided what breakfast cereal to eat this morning. I opened the cupboard, Iperused the options, and when I ultimately chose the Honey Bunches of Oats over the Kashi Good Friends, it came from a place of considered judgment, free from external constraints and predetermined laws.

Or did it? This question—about how much people are in charge of their own actions—is among the most central to the human condition. Do we have free will? Are we in control of our destiny? Do we choose the proverbial Honey Bunches of Oats? Or does the cereal—or some other mysterious force in the vast and unknowable universe—choose us?

The Greek playwright Sophocles seemed convinced that people have no real control over their fortunes. The character Oedipus, for example, tries desperately to buck the prophesy that he will kill his father and marry his mother, only to end up doing just that. Shakespeare’s characters, on the other hand, attempt to seize control of their futures. Cassius encourages Brutus to assassinate Caesar by appealing to his sense of self-responsibility: “The fault, dear Brutus, is not in our stars, but in ourselves, that we are the underlings.”

Fortunately, though, for social scientists (and for readers of this column), the task of the experimental psychologist isn’t to settle once and for all whether we have free will, but rather to see whether people think they do. This is the study of “lay theory”—people’s convictions about the workings of the world.

The study of lay theory yields interesting insights about the factors that hold sway over our seemingly most deeply held beliefs. What if I were to tell you, for instance, that belief in free will is negatively correlated with the desire to urinate? Those are the implications of a new study published in the journal Consciousness and Cognition by Michael Ent and Roy Baumeister. They predicted—and found—that the more people felt they needed to pee, the less they believed that humans are in control of their destinies.

Whence comes such a seemingly bizarre theory about the relationship between something as mundane as bodily function and as lofty as human freedom? It’s based on a brand of psychological research known as “embodied cognition,” the primary lesson of which is that moment-to-moment states of our bodies influence how we consider about the world around us. An example of this is work by Amy Cuddy showing that “power postures” change how we come across in job interviews. Ent and Baumeister turned this approach toward the question of free will, hypothesizing that body-states could affect even abstract philosophies. When a feature of physical experience reminds subjects they are constrained by the laws of nature, Ent and Baumeister reasoned, their belief in free will should diminish.

The researchers started by contacting people with epilepsy and panic disorder. Epileptics suffer uncontrollable seizures that can come without warning; panic disorder results in recurrent attacks of intense fear and anxiety. Ent and Baumeister reasoned that, due to these daily reminders of physical limitations, these patients would have less belief in free will. Sure enough, when they compared these people’s attitudes to those of a control population, they saw significantly more skepticism among epileptics.

But this data, though intriguing, doesn’t prove unequivocally that our deepest beliefs are influenced by the states of our bodies. Instead, epileptics’ opinions could just be reflections of the fact that they actually do have less control over their actions.

To address this concern, the researchers next demonstrated that even healthy subjects have less belief in free will when they’re subtly reminded of their own physical limitations. Ent & Baumeister had people respond to a battery of questions not just about free will, but also about their current corporal desires. The desires that negatively correlated most strongly to belief in freedom were: a) the desire to urinate, b) the desire to sleep, and c) the desire to have sex. You read that correctly: when people feel stymied in their desire to sleep or to procreate, among other things, it affects their opinions on one of the most hotly debated issues of all time.

Perhaps this finding does not come as a surprise. After all, small influences in our daily lives—a complement from a friend, a criticism from a boss—have the power make our entire world seem sunny or grey. But given the ramifications that belief or distrust in free will would have on people’s life outlook (a “yea” vote gives a person carte blanche to proactively tackle life’s challenges, a “nay” stimulates a considerably more laissez-faire approach), it’s interesting to know how malleable these beliefs really are.

William James said that belief in free will comprises “the whole sting and excitement of voluntary life.” That sting and excitement may be as fleeting and ephemeral as the last bite from a bowl of Rice Crispies.