Referee 2's review:

**********
N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication, the manuscript has been revised in light of these comments and to address other editorial requirements.
**********

This is a very original paper which i would like to see published. the authors found that during biological motion perception (observing a moving hand, which varied according to its androgenicity), females showed stronger mu suppression than did males. this is taken as evidence of greater activation of the mirror neuron system, thought to underlie empathy. thus, as well as confirming previous claims of a female advantage in empathy, this study neatly pinpoints the brain basis of this to mu rhythmns.

in addition, their study found that mu suppression was correlated with both empathy measures and systemizing measures, in opposite directions, as would be predicted. this goes beyond previous studies of sex differences in empathy using either fMRI or electrophysiology in making important links with other aspects of cognition. i am not an expert in ERP so cannot comment on how the mu rhythmns were measured but i have collaborated on such studies before and from my basic knowledge of these procedures i am satisfied that that this aspect of the study was well conducted.

i was a little puzzled as to the SQ scores as these do not seem to correspond to the range usually reported in other studies (e.g by baron-cohen et al 2003 in the proceedings of the royal society of london). if the authors have used a different scoring system (eg z scores?) then they should explain what they have done, so that readers can compare their findings of sex differences on the SQ in this study to those found in earlier studies.

my only other general criticism is the concept of mirror neurons as clearly we are making big assumptions that what is being measured here are mirror neurons. unlike the single cell recording studies from monkeys, we cannot conclude that neurons are being isolated in either fMRI or ERP studies. the authors do acknowledge this point but would do well to highlight it as the mirror neuron system is currently very fashionable but may not always be, and their study has important results that are in some ways independent of any theoretical interpretation.

the study is important not only for its relevance to individual differences and sex differences in particular, but also for its relevance to autism. as the authors note, autism has been characterized as an extreme of the male brain and the prediction would therefore be that people with autism should show even less mu suppression whilst observing biological motion such as hands. has this study been done or is it underway in the authors' lab?

it is always good for science if predictions can be spelt out in print in advance of the study being reported or even conducted, so that the scientific community can see if the theory is confirmed or refuted at a later date.

if the stimuli (the manipulations of the hands) could be made available on a website (eg in online supplementary info or on the authors' website) this would be useful for other scientists who may wish to try to replicate these interesting findings.