Warning, not for the politically correct ability egalitarians who think anyone can play basketball like Michael Jordan or compose music like Mozart if they just try hard enough.
If you don't like his continued use of IQ as a measure of intelligence, just pretend you have your own measure (or even measures) and substitute it each time he writes "IQ", keeping in mind your measure will have an average - let's call it "100" for simplicity - and probably be roughly normally distributed, as most human traits are.
1) What fraction of the population is capable of absorbing a university education or mastering college-level material? It might be smaller than the proportion attending college today.
To have an IQ of 100 means that a tough high-school course pushes you about as far as your academic talents will take you. If you are average in math ability, you may struggle with algebra and probably fail a calculus course. If you are average in verbal skills, you often misinterpret complex text and make errors in logic.
These are not devastating shortcomings. You are smart enough to engage in any of hundreds of occupations. You can acquire more knowledge if it is presented in a format commensurate with your intellectual skills. But a genuine college education in the arts and sciences begins where your skills leave off.
In engineering and most of the natural sciences, the demarcation between high-school material and college-level material is brutally obvious. If you cannot handle the math, you cannot pass the courses. In the humanities and social sciences, the demarcation is fuzzier. It is possible for someone with an IQ of 100 to sit in the lectures of Economics 1, read the textbook, and write answers in an examination book. But students who cannot follow complex arguments accurately are not really learning economics. They are taking away a mishmash of half-understood information and outright misunderstandings that probably leave them under the illusion that they know something they do not. (A depressing research literature documents one's inability to recognize one's own incompetence.) Traditionally and properly understood, a four-year college education teaches advanced analytic skills and information at a level that exceeds the intellectual capacity of most people.
There is no magic point at which a genuine college-level education becomes an option, but anything below an IQ of 110 is problematic. If you want to do well, you should have an IQ of 115 or higher. Put another way, it makes sense for only about 15% of the population, 25% if one stretches it, to get a college education. And yet more than 45% of recent high school graduates enroll in four-year colleges. Adjust that percentage to account for high-school dropouts, and more than 40% of all persons in their late teens are trying to go to a four-year college--enough people to absorb everyone down through an IQ of 104.
2) On the highly intelligent fraction and their contributions to society.
If "intellectually gifted" is defined to mean people who can become theoretical physicists, then we're talking about no more than a few people per thousand and perhaps many fewer. They are cognitive curiosities, too rare to have that much impact on the functioning of society from day to day. But if "intellectually gifted" is defined to mean people who can stand out in almost any profession short of theoretical physics, then research about IQ and job performance indicates that an IQ of at least 120 is usually needed. That number demarcates the top 10% of the IQ distribution, or about 15 million people in today's labor force--a lot of people.
In professions screened for IQ by educational requirements--medicine, engineering, law, the sciences and academia--the great majority of people must, by the nature of the selection process, have IQs over 120. Evidence about who enters occupations where the screening is not directly linked to IQ indicates that people with IQs of 120 or higher also occupy large proportions of positions in the upper reaches of corporate America and the senior ranks of government. People in the top 10% of intelligence produce most of the books and newspaper articles we read and the television programs and movies we watch. They are the people in the laboratories and at workstations who invent our new pharmaceuticals, computer chips, software and every other form of advanced technology.
Combine these groups, and the top 10% of the intelligence distribution has a huge influence on whether our economy is vital or stagnant, our culture healthy or sick, our institutions secure or endangered. Of the simple truths about intelligence and its relationship to education, this is the most important and least acknowledged: Our future depends crucially on how we educate the next generation of people gifted with unusually high intelligence.
How assiduously does our federal government work to see that this precious raw material is properly developed? In 2006, the Department of Education spent about $84 billion. The only program to improve the education of the gifted got $9.6 million, one-hundredth of 1% of expenditures. In the 2007 budget, President Bush zeroed it out.
Just to clarify for Murray: he's not proposing a hard cutoff in IQ for any particular achievement (becoming a chip designer, writing a senior thesis in French Literature), but merely that the fraction of people capable of that achievement with IQ below the value he gives is very small (e.g., very few medical researchers with IQ less than 120). Therefore, we can use the estimated value as a way to guess what percentage of the general population is sufficiently capable. There are of course other factors involved in success at a particular task than raw cognitive ability (motivation, organization, communication skills, ...). See earler post on success vs ability.
For more on theoretical physicists, see here. This will sound terribly arrogant (so shoot me), but Murray's estimate of few per thousand is way too high. That's about the ability level of the average Caltech undergrad, and I would guess only the top 5-10% of students there could be theoretical physicists.