Read Kevin Drum at Washington Monthly.

In previous reports, the other-immigrant population was projected using assumed annual numbers of net other immigrants with a static age-sex distribution. For this year's report, the annual numbers of net other immigrants are projected by explicitly modeling other immigrants and other emigrants separately.

Translation: instead of just pulling a net number out of a hat, the [Social Security] trustees built a model that estimated the actual demographic characteristics of both immigrants and emigrants. And guess what?

Illegal immigrants tend to skew young. This benefits the system.

Young people have more children than older people. This benefits the system.

Some illegal immigrants pay taxes for a few years and then leave. This benefits the system.

Bottom line: "This year's report results in [...] a substantial increase in the number of working-age individuals contributing payroll taxes, but a relatively smaller increase in the number of retirement-age individuals receiving benefits in the latter half of the long-range period." Give or take a bit, it turns out that this shores up the Social Security system to the tune of around $13 billion per year. Thanks, illegal immigrants!