That doesn't mean we can't draw inferences from their work. Here is what they say:
If the assets in the two accounts -- the Roth IRA and the 401(k) -- are identical, and a constant tax rate is assumed, the order of withdrawal does not matter. The money should last the same number of years irrespective of where it is withdrawn from first. However, if the accounts are invested in different assets, then the money might last longer if money is withdrawn from the poorer-performing account first.
An important assumption is that the Roth IRA account has been funded for more than five years. The investment earnings of Roth IRA accounts that haven't seasoned for five years may be subject to income tax.
Estate-planning issues also come into play. If you plan to leave an inheritance, taking money out of the 401(k) might make sense versus withdrawals from the Roth IRA, especially if you are not paying any income taxes or paying very low income taxes. That's because the Roth IRA beneficiary will never have to pay income taxes on distributions from the Roth IRA account.
While the income tax angle is covered for the Roth IRA, there could be estate tax implications for both of the retirement accounts.
Another thing to consider is that if the assets in both accounts are invested identically and you expect your taxes to go up in the future, then taking the money out of the 401(k) first may be beneficial. On the other hand, if at a later date you expect to be in a lower tax bracket, take the money from the Roth IRA first.
As you can see, the decision gets complicated fast, and the decision is highly dependent on individual circumstances and assumptions. Review the investment performance of these accounts, your goals for the money and your tax situation in deciding which account to take the withdrawal from first.
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