Get paid now to take parental leave in exchange for deferring Social Security payments later. It sounds like a simple and “budget neutral” to help women earn more in their careers while getting their children off to a good start. Kristin Shapiro, Andrew Biggs, and others have argued on behalf of Marco Rubio’s proposal to divert future Social Security payments to pay for family leave today. Shapiro’s plan is elegant and straightforward.

John Cogan, my favorite entitlement curmudgeon, is back to rain on the parade with a reminder that many government programs began modestly before ballooning. Biggs thinks it would be difficult to expand this new entitlement. The program as designed and explained seems most valuable to middle class parents, and so most prone to expansion in a deficit-expanding way to lower-income mothers. Cogan expects

pressure will mount on Congress to increase the amount of assistance and add other equally worthy activities—job search during a spell of unemployment, caring for a sick family member, helping one’s children attend college, putting a down payment on a house. Pressure will also mount to raise the $70,000 annual family income eligibility limit in his current proposal. Advocates for expansion will argue first that it’s unfair to exclude families making $70,500—then $71,500 and $80,000 and so on. Congress will acquiesce to these pressures, and Mr. Rubio’s well-meaning proposal will become a large and costly entitlement program. As these expansions occur and the cost grows, so will the national debt.