Who Decides?

The recent controversy about birth control and health insurance in the US raises an interesting question. The Catholic church is adament in its opposition to birth control, but almost all (98 percent is the figure that's always being thrown about) American Catholics do pratice birth control. Who determines what a church believes? Is it the doctrines preached by the clergy or the lives lived by the laity?

Before you pick one or the other, keep in mind that in many cases the laity looks to the clergy to be the sort of Christian they think they should be, whether they are or not.