The researchers studied about 300 children with an average age of 10. The children, all Caucasian, came from six counties in the Cleveland area. As a group, males - but not females - who had the variant gene had IQ scores about 20 points lower than males who didn't. [...]

Dr. Jirtle said his assertion that the IGF2R gene affects IQ is bolstered by experiments in mice. When he and his colleagues disabled a copy of the gene in lab mice - an experiment intended to mimic humans who inherit the variant copy of the gene - they noticed that the male mice were slow learners on a maze test. Electrical recordings of the mice's brain tissue were also altered in a way that is consistent with slow learning.

Also, Dr. Jirtle said, what scientists already know about the protein produced from the IGF2R gene fits with a role in brain function. Research has suggested the protein regulates cell growth as well as the speed at which signals travel between nerve cells.

In 1998, scientists from England reported a connection between a portion of the IGF2R gene and IQ, but later retracted their work when they couldn't replicate the results. Dr. Jirtle's research concerns a different, but nearby, area of the same gene.