IN RECENT decades, researchers have documented a rising wage premium for college educated workers, and economists have theorised that recent increases in income inequality may be due to rising demand for skills combined with lagging supply of skilled workers. University, it would seem, is more important than ever. But some critics contend that the better earning performance of those with college degrees primarily reflects the higher skill level of those who attend and complete a college degree, and others indicate that the main benefit of university is its signalling power to employers.

So, do colleges actually teach students anything? A new book tracking 2,300 students at four-year universities includes some striking findings:

45 percent of students "did not demonstrate any significant improvement in learning" during the first two years of college.

36 percent of students "did not demonstrate any significant improvement in learning" over four years of college.

Those students who do show improvements tend to show only modest improvements. Students improved on average only 0.18 standard deviations over the first two years of college and 0.47 over four years. What this means is that a student who entered college in the 50th percentile of students in his or her cohort would move up to the 68th percentile four years later -- but that's the 68th percentile of a new group of freshmen who haven't experienced any college learning.

"Learning" in this case is determined by performance on a test called the Collegiate Learning Assessment, which gauges "critical thinking, analytic reasoning, and other 'higher level' skills". This may not be what students go to college to learn. It is possible that collegiate learning mainly involves an increase in knowledge in specific study areas. That is, a history student might not improve his critical thinking faculties while at university, but he might learn an awful lot about history. On the other hand, employers probably value general critical thinking skills far more than they do course content. The information necessary to do any particular job tends to be highly specific to that job and only loosely related to the key concepts of a degree programme.

Some other interesting results:

Students who study by themselves for more hours each week gain more knowledge -- while those who spend more time studying in peer groups see diminishing gains.

Students whose classes reflect high expectations (more than 40 pages of reading a week and more than 20 pages of writing a semester) gained more than other students.

Students who spend more time in fraternities and sororities show smaller gains than other students.

Students who engage in off-campus or extracurricular activities (including clubs and volunteer opportunities) have no notable gains or losses in learning.

Students majoring in liberal arts fields see "significantly higher gains in critical thinking, complex reasoning, and writing skills over time than students in other fields of study." Students majoring in business, education, social work and communications showed the smallest gains. (The authors note that this could be more a reflection of more-demanding reading and writing assignments, on average, in the liberal arts courses than of the substance of the material.)

It's easy to think of potential endogeneity problems in these analyses, but the authors have probably tried to control for them.

But here's the thing. If there is a good way to assess ability through examination (and it seems like we're assuming that the Collegiate Learning Assessment is a good way to assess ability), and if firms are primarily interested in this measurable ability, then why wouldn't firms just ask to see these test scores as part of an application and not worry about whether an applicant had completed a university degree? At the very least, why wouldn't firms ask for test scores alongside some documentation attesting to completion of a degree? Indeed, why don't more firms requiring skilled workers hire people without college degrees? I understand that there are issues of asymmetric information such that college is a useful signal, but given the enormous direct expense and opportunity cost of a four-year college degree, the market failure seems to large here to be realistic. Some firms should be able to find an advantage in going to top quality secondary schools and hiring graduates at some salary lower than what they'd pay a new college graduate but representing a major improvement in net financial position relative to full-time student status.

Something is amiss. And I have to believe that firms value something imparted by a college education that's not captured by these assessments of learning.