Why not let investors fund a student's college education in return for part of his future income?

Investors could finance students’ education with equity rather than debt. In exchange for their capital, the investors would receive a fraction of a student’s future income — or, even better, a fraction of the increase in her income that derives from college attendance. (This increase can be easily calculated as the difference between the actual income and the average income of high school graduates in the same area.)

This is not a modern form of indentured servitude, but a voluntary form of taxation, one that would make only the beneficiaries of a college education — not all taxpayers — pay for the costs of it.

The cost of enforcing contracts contingent on future income is very large, but there is an effective solution: piggybacking on the tax collection system. The Internal Revenue Service could perform collection services on behalf of private lenders, and at no cost to taxpayers. (In Australia, such a system has been in place since the 1980s. The national tax agency enforces repayment of loans contingent on income, though the payments of the wealthiest graduates are capped, and therefore less affluent graduates need to be charged more to make the program viable than in the system I am proposing.)