It has been suggested that the fraternal birth-order effect is a result of

A) erotic plasticity.
B) refractory periods.
C) a maternal immune response.
D) mass-media norms.

The fraternal birth-order effect, which refers to the increasing likelihood of a male being gay or bisexual with each older brother he has, has been found to be related to a maternal immune response to a male-specific protein. This response is thought to occur when a woman becomes pregnant with a male fetus, and her immune system reacts to certain proteins that are produced only by males. This immune response leaves antibodies in the woman's body, which can then affect the development of subsequent male fetuses. Hence, choice C is the best answer.