The American political class has long held that higher education is vital to individual and national success. The Obama administration has dubbed college "the ticket to the middle class," and political leaders from Education Secretary Arne Duncan to Federal Reserve Chairman Ben Bernanke have hailed higher education as the best way to improve economic opportunity. Parents and high-school guidance counselors tend to agree.

Yet despite such exhortations, total college enrollment has fallen by 1.5% since 2012. What's causing the decline? While changing demographics—specifically, a birth dearth in the mid-1990s—accounts for some of the shift, robust foreign enrollment offsets that lack. The answer is simple: The benefits of a degree are declining while costs rise.

A key measure of the benefits of a degree is the college graduate's earning potential—and on this score, their advantage over high-school graduates is deteriorating. Since 2006, the gap between what the median college graduate earned compared with the median high-school graduate has narrowed by $1,387 for men over 25 working full time, a 5% fall. Women in the same category have fared worse, losing 7% of their income advantage ($1,496).

A college degree's declining value is even more pronounced for younger Americans. According to data collected by the College Board, for those in the 25-34 age range the differential between college graduate and high school graduate earnings fell 11% for men, to $18,303 from $20,623. The decline for women was an extraordinary 19.7%, to $14,868 from $18,525.

Meanwhile, the cost of college has increased 16.5% in 2012 dollars since 2006, according to the Bureau of Labor Statistics' higher education tuition-fee index. Aggressive tuition discounting from universities has mitigated the hike, but not enough to offset the clear inflation-adjusted increase. Even worse, the lousy economy has caused household income levels to fall, limiting a family's ability to finance a degree.