Despite living in a world filled with uncertainty, people are generally not constantly paralyzed by doubt, but instead feel confident about their choices. Previous research often assumed that this feeling of confidence was based solely on the availability of high-caliber information. Now scientists find that how our body feels may also influence our confidence in our decisions. The researchers detailed their findings online Oct. 25 in the journal eLife.

Prior computational models of human decision-making suggested that certain properties of sensory signals such as their strengths, on which people base their decisions also determines how confident they are that those decisions are correct. However, recent experiments suggested that internal states, such heart rate, might influence how confident we are in our decisions.

To investigate the roots of confidence, study lead author Micah Allen, a cognitive neuroscientist at University College London, and his colleagues first asked 29 volunteers to decide whether clouds of moving dots on a screen were on average traveling to the left or right, and to express how confident they were in their decisions. As they increased the amount of noise in this task—the degree to which these dots moved in random directions rather than clearly left or right—the volunteers became less confident in their decisions. This is what conventional models of external impacts on confidence would predict.

But in another part of the experiment, each time the volunteers carried out their tasks, faces of people were flashed at them once for only 16 milliseconds. This span of time was too brief for the volunteers to consciously detect but long enough for them to subliminally perceive—faces with disgusted expressions caused “a small yet statistically detectable shift in both heart rate and pupil dilation, of which the participants were totally unaware,” Allen says.

The scientists find that when people were unconsciously excited by these subliminal cues, they were more confident with their decisions when confronted with highly noisy tasks. “I believe that if we want to understand the conscious mind, we need to also take into account how it is situated within a living body,” Allen says. “Even when we’re doing a boring experiment—looking at dots and reflecting on our decisions—the body is there shaping our awareness in subtle ways.”

Allen believes the relationship between internal states and confidence may help inform the brain as to how much it can trust its own sensory inputs. “Evolutionarily speaking, we think that the capacity to monitor the world and to monitor our own internal states are probably quite closely related,” he says. “Whether this is a matter of convenience or if there is some deeper benefit to this more general sense of confidence remains to be seen.”

Allen’s findings are “relevant to anyone whose job is to make difficult perceptual judgments trying to see signal in a lot of noise,” such as radiologists or baggage inspectors, says cognitive neuroscientist Rebecca Todd at the University of British Columbia in Vancouver, who did not take part in the research. Todd suggests that people who apply decision-making models to real world problems need to better account for the influence of internal or emotional states on confidence.

The fact that bodily states can influence confidence may even shed light on mental disorders, which often involve blunted or heightened signals from the body. Symptoms could result from how changes in sensory input affect perceptual decision-making, says cognitive neuroscientist and schizophrenia researcher Phil Corlett at Yale University, who did not participate in this study.

Corlett notes that some of the same ion channels involved in regulating heart rate are implicated in schizophrenia as well. “Maybe boosting heart rate might lead people with schizophrenia to see or hear things that aren’t present,” he speculates, adding that future work could analyze how people with mental disorders perform on these tasks.

For his part, Todd would also like to investigate what happens when volunteers are subjected to sustained excitement rather than transient jolts from a quickly flashed face.