Only a fraction of Americans contribute to an IRA, but the ones that do tend to take it to the max. That’s one of the findings from a recent study of 10 million individual retirement accounts conducted by the Investment Company Institute, a mutual fund industry trade organization. The study — which also indicates that women are more likely to contribute to an IRA than men are, that the wealthy are more likely to contribute than the poor, and that people’s contribution activity peaks in their late 50s — illustrates that IRAs can be a powerful tool for retirement, but that benefits aren’t spread evenly throughout the population. As of 2004, according to IRS figures, about 2 in 10 American taxpayers had a traditional IRA. Among working-age Americans who do have an IRA, the contribution rates are low, according to the ICI study, which examined data from 2007 and 2008. At the close of 2008, IRAs amounted to more than one-fourth of Americans’ retirement savings, but that year only 9.4% of traditional IRA owners made a contribution. If one includes Roth IRAs, the activity rate increases: In 2007, when 11.2% of traditional IRA owners made a contribution to their accounts, an additional 5.4% of them put all their new contributions into a Roth. Why is the contribution rate for traditional IRAs so small? The ICI says this could be because of competition from other retirement savings options, such as 401(k) plans and Roth IRAs, or because of limitations on the tax-deductibility of traditional IRA contributions. And yet the people who do use traditional IRAs are committed to them. In 2007, 60% of contributors in 2007 put in the legal limit. In 2008 — a rotten year for the market, and one in which legal limits increased from the prior year — about 50% maxed out. That decline in the share of people taking advantage of the legal limit points to another interesting pattern: Many contributors who don’t maximize their contribution are instead depositing a round-number amount that was the maximum allowable contribution in prior years — say, $2,000 or $3,000. It seems that the power of inertia is strong — and that people don’t often bother to check year-to-year and see whether various IRS limitations on retirement-plan contributions changed. So that you don’t unnecessarily underfund your IRA, check out the IRS rules for 2010 limits. You can also visit the IRS site for in-depth information about Roth IRAs and guidelines for 401(k) plan contributions. To shed further light on investor behavior, the ICI promises more IRA-account-behavior studies, which, like this one, will be mined from a new database it developed with the Securities Industry and Financial Markets Association. In the meantime, tell us about your own IRA behavior. Do you contribute to a traditional IRA or a Roth? Are you maxing out your contribution? Tell us why or why not. Follow MONEY on Twitter at https://twitter.com/money.