Why do humans walk in circles?

It's a trusted plot device of many a thriller. The lost protagonists stagger for hours through creepy forest only to end up back where they started. In fact the idea that humans walk in circles is no urban myth. This was confirmed by Jan Souman and colleagues in a 2009 study, in which participants walked for hours at night in a German forest and the Tunisian Sahara. But the question remains - why?

Souman's team rejected past theories, including the idea that people have one leg that's stronger or longer than the other. If that were true you'd expect people to systematically veer off in the same direction, but their participants varied in their circling direction.

Now a team in France has made a bold attempt to get to the bottom of the mystery. Emma Bestaven and her colleagues hired out a huge indoor exhibition space in Bordeaux (90m x 150m) and blindfolded their participants, thus ensuring that there was no way any environmental sounds or bumps on the ground could interfere with the results.

Another innovation was that the researchers used EMG to monitor the participants' muscle activity in their legs whilst they walked; they had them stand on a force plate to check how their balance was distributed; and they assessed their subjective sense of the vertical, via their placement of a vertical bar.

Given six attempts to walk in a straight line, the fifteen blindfolded participants (7 women) in fact frequently veered off in one direction or the other, just as expected based on past research. Half the time they deviated off to the left, 39 per cent of the time to the right, and the remainder of the walks they managed to go straight. Six of the participants always veered in the same direction, the others mixed it up. There was no evidence of participants getting straighter with each attempt, but a faster walking speed was associated with a straighter trajectory.

Many past theories for why humans walk in circles were ruled out - walking deviation was unrelated to hand, eye or leg dominance. The recordings of muscle activity drew a blank. But there was one clue. The participants' "centre of pressure" score obtained when they stood on the force plate, which reflects their postural balance, was correlated with how much they deviated from a straight line when they walked. In turn, this "centre of pressure" score was correlated with participants' subjective sense of a straight vertical line.

Taken together, Bestaven's team said this suggests that our propensity to walk in circles is related in some way to slight irregularities in the vestibular system. Located in inner ear, the vestibular system guides our balance and minor disturbances here could skew our sense of the direction of "straight ahead" just enough to make us go around in circles.

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Emma Bestaven, Etienne Guillaud, and Jean-René Cazalets (2012). Is “Circling” Behavior in Humans Related to Postural Asymmetry? PLoS ONE DOI: 10.1371/journal.pone.0043861

Post written by Christian Jarrett for the BPS Research Digest.

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