Dr. Hoff works in South Florida, where there is a very educated and affluent population raising children in Spanish and English. “The children start out as baby bilinguals, but the older they get, the more English overtakes Spanish,” she said. “The ones who are successful bilinguals as adults are still much better in English than they are in Spanish — they didn’t go to school in Spanish, they don’t read books in Spanish, and when you actually measure the size of their vocabularies, or the grammar they understand, or the coherence of the narrative they produce, they are not as proficient as they are in English.”

Gigliana Melzi, a developmental psychologist and associate professor of applied psychology at New York University who studies language in Spanish- and English-speaking Latino families, agreed. “Parents will need to be mindful about introducing the child to literacy in that language,” she said. “They will need to be thoughtful about ways they will encourage the child to maintain the language.”

It’s also important, she said, to watch the individual child and make sure the child is not overloaded with demands because of parental expectations and ambitions; maybe three languages on top of a musical instrument and a serious sport is just too much.

The languages you learn as a child are important, but so are the languages you learn later in life. “We all know people who make great contributions and do great science in English and are not native speakers,” Dr. Hoff said. “The human brain is amazing, and the human capacity to acquire language is amazing.”

So what should parents do if they want to give their children a bilingual boost? “Find a native speaker and have that native speaker have fun, interesting conversations with your child, and your child will learn something,” Dr. Hoff said. “Don’t expect it will turn your child into a perfect balanced bilingual, but that’s O.K.” Whatever you do is an advantage.

Dr. Melzi said that often, a child who has been fluent in two languages in the preschool years goes to school where English is spoken, and starts using English to describe what happens there.

“There is a push worldwide where English becomes like the lingua franca, so it’s important that the child be exposed to the other language early, and the younger you are, the more nativelike you’re going to sound,” she said. On the other hand, older children may learn more easily: “The younger you are, the more head start you have,” she said. “The older you are, the more efficient learner you are, you have a first language you can use as a bootstrap.”

So true bilingualism may be rare, but parents shouldn’t be discouraged on that account, since all the skills that children acquire along the way are very valuable, Dr. Melzi said. “It’s worth it, but it’s a lot of work.”