Before-Tax or After-Tax Contributions
Retirement plans are funded by either "before-tax" or "after-tax" contributions.
If the taxpayer made all contributions to a plan with before-tax dollars, the entire distribution will be fully taxable. The funds are taxed at the time of the distribution because neither the contributions nor the earnings/investment gains were previously taxed. This is common in 401(k) and Thrift Savings plans.
If the taxpayer did not contribute to the retirement plan, all the distributions are fully taxable.