The Bias toward the Right Side of Others Is Stronger for Hands than for Feet

Round 1

Reviewer 1 Report

This is an interesting and original paper. It is also well written. I have one main criticism (1), and some other suggestions.

1. What I think is necessary to interpret the clockwise or anticlockwise bias is a baseline that has not to do with the human body. That is, a measure of the preference to see clockwise or counter-clockwise motion for an object. Maybe this data exist, or it can be collected.
2. Figure 3 5 and 7, please add a label to the y axis.
3. With respect to the phenomenon of noticing left and right handedness changes, there is an interesting study in which entire films were shown left-right reversed: Bertamini, M., Bode, C., & Bruno, N. (2011). The effect of left-right reversal on film: Watching Kurosawa reversed. I-Perception, 2(6), 528–540.
4. Today's technology makes it possible to share stimuli, including videos. Please upload the images as stills or as videos in a way that allow the reader to see them. For example, with a link to a project on Open Science Framework (www.osf.io)

 

 

 

 

Author Response

We thank the Reviewer for her/his positive evaluations and precious comments, and we hope to have fully addressed her/his requests.

 

1. What I think is necessary to interpret the clockwise or anticlockwise bias is a baseline that has not to do with the human body. That is, a measure of the preference to see clockwise or counter-clockwise motion for an object. Maybe this data exist, or it can be collected.

As indicated in the revised version of the manuscript (p. 3, ll. 118-126), we adopted the expedient of requiring participants to indicate the rotating direction rather than the perceived limb laterality of stimuli both because the former can be reported more easily by observers compared with the latter and because we wanted to prevent participants from focusing overtly on our dependent variable of interest (the latter rather than the former). Moreover, we point out that the perceived rotating direction of ambiguous human figures such as those used in the present study did not exhibit any population bias in favor of either clockwise or counterclockwise rotation in any of our published (see References 40 and 43 in the manuscript) or yet unpublished studies (i.e., one with participants aged 9-11 years; one with stimuli consisting of both female and male figures; one with stimuli consisting of both human and canine stimuli; one with stimuli presented in peripheral rather than central vision). Importantly, Troje and McAdam (see Reference 68 in the manuscript) showed that the clockwise bias observed for the well-known “Spinning Dancer Illusion” is due to a confounding variable (i.e., a viewing-from-above bias) and we replicated this finding and demonstrated a further confound (i.e., a bias toward the right leg; see Reference 40 in the manuscript). On the contrary, clockwise and counterclockwise biases, consistent with the common experience of optical flow, emerge when stimuli are presented in the left and right visual field, respectively (as we have found in a yet-unpublished study with stimuli presented in peripheral vision; Lucafò et al., in preparation). Besides the fact that perceived spinning rotation was not a central variable in the present study, we can assume that it should also be absent for object stimuli (in this regard, to our knowledge there is no evidence in literature of any clockwise or counterclockwise bias for several kinds of ambiguous stimuli, including structure-from-motion figures), similarly to what occurs for canine stimuli (Lucafò et al., in preparation; noteworthy, in this case participants neither exhibited a bias toward the right paw of the canine figure, suggesting that such a bias is specific for human figures).

 

1. Figure 3 5 and 7, please add a label to the y axis.

Done.

 

1. With respect to the phenomenon of noticing left and right handedness changes, there is an interesting study in which entire films were shown left-right reversed: Bertamini, M., Bode, C., & Bruno, N. (2011). The effect of left-right reversal on film: Watching Kurosawa reversed. I-Perception, 2(6), 528–540.

We thank the Reviewer for this suggestion. We have mentioned this paper in the revised version of the manuscript (p. 3, ll. 126-128).

 

1. Today's technology makes it possible to share stimuli, including videos. Please upload the images as stills or as videos in a way that allow the reader to see them. For example, with a link to a project on Open Science Framework (www.osf.io).

We thank the Reviewer for her/his thoughtfulness, but she/he probably did not notice that we had already met this point in the original version of the manuscript (see Data Availability Statement at the end of the main text).

Reviewer 2 Report

Title: The bias toward the right side of others is stronger for hands than for feet

 

Manuscript ID: symmetry-1077951

The study's principal aim was to examine whether the tendency to interpret ambiguous animated figures as right-handed was more robust than the bias to perceive them as right-footed. To test their hypotheses, the authors performed experiments in which participants (36 females and 38 males; age: 18-38 years) had to indicate the rotating direction of ambiguous human figures, with either one arm or one leg extended, who were either acting on an object (a ball) or not; ambiguous human figures with one arm and the contralateral leg extended; and, ambiguous human figures with either one arm or one leg extended. The three experiments confirmed the presence of a bias toward the right hand and/or foot of observed bodies, supporting the hypothesis of an attentional bias toward the right side of others' bodies.

According to the negative frequency-dependent selection hypothesis, left-handers could benefit from the limited experience and familiarity of right-handed players with left-handed ones, and this could explain the overrepresentation of left-handers in several sports. The authors confirm previous works suggesting that left-handers may have an advantage in contact sports like karate, boxing, or fencing in which success depends on winning the opponent in the fight. Specifically, a perceptual frequency effect could prompt people to preferentially pay attention to the right rather than the left side of human bodies, and thus a reduced ability in discriminating left-limbed movements compared with right-limbed trends, explaining a ‘surprise effect’ at the basis of the hypothesized advantage of left-handers in the fighting. This behavioral asymmetry in humans supposes that left-handers are more trained because they face right-handed people more often than right-handed people. If this were the case, would there not have been an evolutionary advantage of left-handers in the ongoing struggles and clashes for survival, which would have resulted, at least, in a similar percentage of left-handed and right-handed individuals?

Author Response

According to the negative frequency-dependent selection hypothesis, left-handers could benefit from the limited experience and familiarity of right-handed players with left-handed ones, and this could explain the overrepresentation of left-handers in several sports. The authors confirm previous works suggesting that left-handers may have an advantage in contact sports like karate, boxing, or fencing in which success depends on winning the opponent in the fight. Specifically, a perceptual frequency effect could prompt people to preferentially pay attention to the right rather than the left side of human bodies, and thus a reduced ability in discriminating left-limbed movements compared with right-limbed trends, explaining a ‘surprise effect’ at the basis of the hypothesized advantage of left-handers in the fighting. This behavioral asymmetry in humans supposes that left-handers are more trained because they face right-handed people more often than right-handed people. If this were the case, would there not have been an evolutionary advantage of left-handers in the ongoing struggles and clashes for survival, which would have resulted, at least, in a similar percentage of left-handed and right-handed individuals?

 

We thank the Reviewer for her/his positive evaluations, and we try to reply to her/his question here (but please feel free to require the inclusion of this part in the revised manuscript, if needed).

As suggested by Vallortigara (2006), an important outcome of behavioral asymmetries observed in many species implies an advantage in interindividual coordination when laterality is aligned at the population level, which suggests that the strength of asymmetry might have been selected under social pressure. According to this view, the extremely large proportion of right-handed individuals in humans might exert positive effects during cooperative interactions. In agreement with the negative frequency-dependent selection hypothesis, the minority of left-handers might in turn be advantaged by their behavioral deviation from the majority of right-handers during competitive interactions. The relative proportion of right- and left-handers in human population might thus represent an equilibrium point that tend to preserve itself, a larger proportion of left-handers possibly reducing both the advantage of this minority in competitive contexts and the advantage of right-handers in cooperative contexts. We hope we have managed to dispel adequately the Reviewer’s doubts.

 

Reference:

Vallortigara, G. (2006). The evolutionary psychology of left and right: Costs and benefits of lateralization. Developmental Psychobiology: The Journal of the International Society for Developmental Psychobiology, 48(6), 418-427.

Round 2

Reviewer 1 Report

I thank the authors for the changes and the answers. 

On the main point I still think it would have been nice to have data on a baseline clockwise response for a non-human object using this very procedure, however it is true that this is not critical for the interpretation of the main findings.

I was now able to see the stimuli uploaded online. Thanks.