Individual retirement accounts may cumulatively hold close to half of private retirement assets, but that doesn't mean that IRAs are as broadly used by multiple segments of the population as they could be—nor does it mean that people are actively contributing to them.

In fact, according to a brief from the Center for Retirement Research at Boston College, nearly all IRA asset growth is instead driven by rollovers from employer-sponsored retirement plans. Individuals' contributions represent just 13 percent of the new money flowing into IRAs each year.

While IRAs were intended to give those without an employer plan access to a tax-deferred savings vehicle, the report says, it hasn't quite shaken out that way. Only 14 percent of households contribute to IRAs, and they're mostly made up of higher-income dual-earners who also save in a 401(k); moderate-income singles or one-earner couples, often with a 401(k); and higher-income entrepreneurs with no current 401(k).