Female-Male Face Illusion

Illusion by Richard Russell of Harvard University shows two faces which are perceived as male and female. Why does our brain perceived the image on the right is a male, and the image on the left as female? What’s the difference between the images?


The difference?
Both images are off the same androgynous face. The only difference is the image on the right has a higher contrast (male), whereas the image on the left has a lower contrast (female).