Best language-learning environment for kids: Two parents chattering away with fancy words

An article in the New York Times entitled Birth Order: Fun to Debate, but How Important? discusses language learning and birth order. Here's the money quote:

Frank J. Sulloway, a visiting scholar at the University of California, Berkeley, and the author of “Born to Rebel: Birth Order, Family Dynamics, and Creative Lives”(Pantheon, 1996), points out that second-born children tend to be exposed to less language than eldest children. “The best environment to grow up in is basically two parents who are chattering away at you with fancy words,” Dr. Sulloway said.

That seems fairly obvious to me—expose them to more words and they learn more words—but the good doctor lends some authority to it.