Golden Thirds Rule - Where does it come from?

A fairly reliable rule for effective composition is to divide an image into thirds lengthways and widthways and place the most important part of the image where the lines meet.  Why does this rule work so well, so much of the time?

I think it might be because the first, most important image of our lives is a close-up of our mother's face  

When a close-up of a woman's face is divided as per the Golden Rule of Thirds, what appears at the meeting points are her eyes

The expression in those eyes provides vital information for us as newborns. That we are safe

Thanks to opera singer Giselle Allen for providing the beauty for this post