From the article: " Over the course of his working life, a college-educated white male .... will have earned about $2.3 million. His high-school-educated white-male partner will have earned $1.2 million." Let's say college costs $100,000 (tuition plus opportunity cost of income, etc). Now let's say instead of spending that $100,000 on college we invested the money instead at 6% per year returns. That will give you $1…
If the high school graduate is contributing some monthly amount to an investment account, then the college-educated graduate could also invest at the same rate, making the difference in gains greater again. On top of that, a college graduate has the potential to invest an greater amount than the high school graduate based on the difference in income.