It is nothing more than a DNA typo, but a single mutation in a gene called IGF2R can put a serious dent in a man's IQ.
Biologist Randy Jirtle of Duke University suspected that IGF2R might be one of the long-sought "intelligence" genes because it facilitates the fast release of neurotransmitters, chemicals that pass impulses from cell to cell in the brain. He likens a brain under the influence of IGF2R to a fast computer: "It's going to crunch through data a lot more rapidly." After finding that genetically engineered male mice who express only one copy of the gene are slower than others at learning mazes, Jirtle wondered whether humans would be similarly affected.
To find out, he and his collaborators tested 300 subjects. Males who carried a particular variant of the IGF2R gene had significantly lower IQs overall, although Jirtle could not predict any individual's score based on genes. "It's not all-or-nothing—it's an average effect," he stresses.
The persistence of this kind of genetically rooted male stupidity is an evolutionary puzzle, since natural selection seems to have weeded out this mutant form of IGF2R from just about every other species. "You go all the way down to zebra fish, and you can look at the same base in the gene," Jirtle says. "The only place that there's a mutation in this gene at this position is in humans."