The financial picture for Social Security isn’t as dire as some describe: Without any modifications to its funding, Social Security will generate enough revenue to pay for three-quarters of promised benefits.

The main reason it’ll fall short, though—the reason that that remaining one-quarter of benefits hasn’t yet materialized—is that the method of funding for Social Security was calibrated to an America with much less inequality than the nation currently has.

Since the late ‘70s, most of the growth in workers’ earnings has gone to the people who have made the most money. To be precise, the wages of the top 1 percent of workers have grown 138 percent since 1979, while the wages for the bottom 90 percent grew only 15 percent during that period.

If all of that income growth were taxed evenly, Social Security would have no shortfall. But it’s not taxed evenly: Any dollar that an American earns beyond $118,500 is, under current laws, not subject to Social Security taxes. In other words, someone who makes $118,500 this year is going to pay the same amount in Social Security taxes as someone who makes $4 million this year.

For most people, this doesn’t really matter: Less than six percent of wage-earners last year took home more than $120,000. But because lots of the last three decades’ earnings growth has been in the realm beyond $118,500 a year, much of it has escaped Social Security taxes.