It has simply disappeared.
It's almost as if the Left realizes that they've lost the nature/nurture argument, so feel it's best to just not bring up the subject.
Forty years ago the nature-nurture controversy was still a hot topic: are differences in intelligence genetic in origin, or more due to the environment? Even though political correctness had not set in during the 1970's the way it has now, the battle lines -- and arguments used -- were strikingly similar to what they are now.
On the Left were arrayed such luminaries as Richard Lewontin and Stephen Jay Gould, who would invariably try to turn what should have been a scientific argument into a moral one, and would invoke America's long history of slavery and discrimination to back up their arguments.
Gould tried to draw a parallel between geneticists and the old time phrenologists. And those on the Left would frequently try to tar their opponents by associating them with the KKK, or by saying that Hitler held similar views (sound familiar?)
On the right were the men widely regarded as heretics: Richard Herrnstein, William Shockley, and Arthur Jensen. All three were widely reviled by student activists at the time. But all stubbornly stuck to their view, knowing they had the facts on their side, even if those facts were unwelcome.
As the evidence accumulated, eventually it became plain that intelligence is largely genetic in origin. All the studies comparing IQ differences between adoptive siblings and biological siblings, or between separated identical twins and regular siblings raised together, pointed in the same direction. And every sophisticated mathematical analysis applied to studies of IQ pointed toward regression to a different mean for each race.
Environment obviously can play a role: if a child is starved as a youngster, or kept in a closet his entire life, that will obviously have a strong negative effect. But, by and large, differences in IQ are due to genes.
In 1994, Herrnstein and coauthor Charles Murray briefly reawakened the controversy with The Bell Curve, an 845 page book of which three pages were devoted to racial differences in IQ. Herrnstein and Murray were of course roundly denounced for their heresy; soon after the issue went back to sleep. (Come to think of it, you rarely hear the phrase "bell curve" these days, either.)
But after decades of affirmative action, Head Start, disparate impact lawsuits, countless movies and TV shows featuring wise blacks and foolish Anglos, and even a black President, all those racial differences stubbornly persist. The SATs, ACTs, LSATs, GMATs, MCATs, PISA scores, the various Regents exams, the Army's ASVAB test, and the various other civil service exams, all show the same rank order of finish.
All of which combine to make clear that your IQ is as genetic as the features on your face.
So, IQ never gets mentioned anymore. Because once you take it into account, the whole edifice of liberal thought comes tumbling down. Once a realistic look at IQ enters the picture, all the talk of how our schools and teachers are failing us looks misguided. All the talk of racism as the explanation for the differences in accomplishment, all the talk of white privilege, all the talk of disparate impact, just look sort of....silly.
As silly as saying that the only reason the men's weight-lifting records are better than the women's is because of sexism.
As silly as suggesting that the only reason 64 out of the last 64 finalists in the men's 100 meter dash at the Olympics have been black is because they had better coaching.
Ignore differences in natural ability, and you're left with a lot of inexplicable patterns and correlations.
And the most striking thing about all this is how obvious it is -- which is why you never hear IQ mentioned much anymore.