Nearly two decades ago, a landmark study found that by age 3, the children of wealthier professionals have heard words millions more times than those of less educated parents, giving them a distinct advantage in school and suggesting the need for increased investment in prekindergarten programs.

Now a follow-up study has found a language gap as early as 18 months, heightening the policy debate.

The new research by Anne Fernald, a psychologist at Stanford University, which was published in Developmental Science this year, showed that at 18 months children from wealthier homes could identify pictures of simple words they knew — “dog” or “ball” — much faster than children from low-income families. By age 2, the study found, affluent children had learned 30 percent more words in the intervening months than the children from low-income homes.

The new findings, although based on a small sample, reinforced the earlier research showing that because professional parents speak so much more to their children, the children hear 30 million more words by age 3 than children from low-income households, early literacy experts, preschool directors and pediatricians said. In the new study, the children of affluent households came from communities where the median income per capita was $69,000; the low-income children came from communities with a median income per capita of $23,900.