25 YEARS LATER For the most part, the validity of the Fagan test holds up. Indeed, Dr. Fagan (who died last August) and Dr. Holland revisited infants they had tested in the 1980s, and found that the Fagan scores were predictive of the I.Q. and academic achievement two decades later when these babies turned 21.

“It’s really good science,” said Scott Barry Kaufman, scientific director of the Imagination Institute at the University of Pennsylvania and the author of “Ungifted: Intelligence Redefined.”

But Dr. Fagan’s hope for widespread screening of infants has not come to pass. “There are some centers that have it,” Dr. Holland said. “It never came to be the kind of thing where it’s widely available.”

The trend is perhaps in the other direction, away from dividing young children by I.Q. and its surrogates out of concerns that the labels become self-fulfilling prophecies. Private schools in New York City, for example, have agreed to abandon intelligence tests for 4- and 5-year-old applicants.

Dr. Kaufman said that because the Fagan test was only “moderately predictive” of later academic success, it was not accurate enough to forecast the intellectual trajectory of a particular child. “From a practical standpoint, it’s not valid,” he said. “It’s not random, but it’s not enough for individual prediction.”

But the numbers become more reliable in aggregate, and the test is widely used in the academic world to quantify the effects of, for example, toxic chemicals on young children.

For the last decade of his life, Dr. Fagan was unexpectedly drawn into the “genes versus environment” debate over intelligence after he found that babies from widely different cultural backgrounds performed equally well on his test. That, he argued, undercut the argument for a biological basis for the stark “achievement gap” between white and black children, or rich and poor.

In a chapter in “The Cambridge Handbook of Intelligence”, co-edited by Dr. Kaufman and published in 2011, Dr. Fagan wrote, “A parsimonious explanation for the findings is that later differences in I.Q. between different racial-ethnic groups may spring from differences in cultural exposure to information past infancy, not from group differences in the basic ability to process information.”