According to a recent analysis of its customers by Fidelity Investments, those people who had both a 401(k) and an IRA had more money saved for retirement than those people who only had either a 401(k) or an IRA. How big of a difference are we talking? Those with both types of accounts had saved 2-4 times as much money as those with only one type of tax-advantaged retirement account.
While this correlation might be explained by simply realizing that with more funded retirement accounts comes more saved money, I suspect there's more to it than that.
One possible explanation that I find plausible is that it stands to reason that people with both types of accounts may have spent more time thinking about retirement (such as figuring out that there are different tax implications for a 401(k) than a Roth IRA, for instance). If these people have thought more about retirement, it stands to reason that saving for retirement is a higher priority than those with just one account, and as a result, these people stick more away for their golden years.
What do you think? Why do you think there's such a marked difference between the two groups? Let me know in the comments.