University of Chicago psychologist Janellen Huttenlocher has found that the frequency with which normal parents speak to and around their child during the child's second year significantly affects the size of the child's vocabulary for the rest of his or her life. The more words a child hears during this sensitive period, whether it's "cat" or "existentialism," the stronger the basic language connections.
From "A User's Guide to the Brain: Perception, Attention, and the Four Theaters of the Brain. John J. Ratey, M.D. 2001.