Unless the study involves randomly selected groups and controls, it's not going to tell us much. It could also be that in countries where co-sleeping is the norm, mothers are socialized to do it safely, whereas the mothers who do it in places where it is not the norm have not the (unconscious, absorbed) understanding of the practice, not having been inculcated with it.

And your point about nonconformist mothers' higher risk tolerance is I believe highly relevant; your remark about autonomy is especially apropos. A mother who is convinced that The Man has no business telling her how her baby should sleep is carrying a whole other set of issues besides child welfare and may not be assessing the situation as well as she thinks she is.