Whenever Julie played with toy vehicles or blocks, her mother would be very disapproving. Whenever she played with dolls and domestic toys, her mother would smile and praise her. As Julie grew up, she played less and less with "boy's" toys and more and more with dolls, dollhouses, and domestic accessories. This description of gender-role development exemplifies _____ theory.

a. social learning
b. evolutionary
c. gender schema
d. gender identity