A basic — and increasingly nostalgic — assumption in the private pension world has long been that the financial payouts of defined benefit plans are much better than those of defined contribution plans, and it’s too bad that defined benefit plans seem to be heading for extinction.

Now an Employee Benefit Research Institute study has cast doubt on the conventional wisdom.

The study compared the tens of thousands of 401(k) plans in EBRI’s long-term database with two current defined benefit plan models — a standard three-year, final-average-pay pension and a cash balance plan. Institute researchers found that the 401(k) benefits were better for almost every age and income cohort. The median differences ranged from about even to 44 percent, with higher earners and those with longer tenure generally doing best with defined contribution plans.

How could that be?

“Conventional wisdom always made defined benefit plans more advantageous than they really are,” says Jack VanDerhei, EBRIs research director and the author of the study.

Not so fast, retort critics, arguing that the study makes a lot of assumptions that are biased toward 401(k)s. “In its day, a defined benefit pension was a better way of delivering benefits,” says Alicia Munnell, director of the Center for Retirement Research at Boston College. “What the paper is doing is looking at the effect of an institution in a world that no longer exists.”

One problem is that the report includes only voluntary 401(k)s, ignoring the automatic enrollment variety — even though the latter constitute more than half of all 401(k)s and are steadily gaining ground, according to the consulting firm Aon Hewitt. The study omitted them because researchers say there was not enough data available on participants who opt out of certain types.

In some ways, that methodology inherently tilts the conclusion in favor of defined contributions. That’s because the average contribution rate in auto-enrolled plans — 6.6 percent of salary, according to Aon Hewitt — is lower than the average rate of 7.9 percent in the voluntary type. (The most common auto-default rate — 3 percent — is even worse.) Thus, in the real world, most employees actually have significantly less money in their 401(k)s than the samples in this survey, reducing any comparative advantage against traditional pensions.

But VanDerhei argues that if the numbers are analyzed in terms of what really matters — how much money people are accumulating for retirement — the inclusion of auto-enrollment plans would actually boost defined contribution’s results versus defined benefits because automatic plans cover more people. “Auto-enrollment increases low-income participation and thus increases their benefits,” he says.