Created by a provision in a 1978 tax law and authorized by Internal Revenue Service regulations issued three years later, 401(k) plans have become by far the predominant employer-sponsored retirement savings vehicle in the United States.

That growth — from the first plan set up in 1981 by benefits consultant Johnson Cos. for its employees, to more than 500,000 plans covering more than 60 million active participants today — has been spectacular even amid legislative threats and new rules.

From the beginning, employers added the plans at a rapid clip. Just four years after the first 401(k) plan was launched, employers had set up 30,000 plans with nearly 10 million people enrolled.

But in 1985, 401(k) plans faced their first serious threat when the Reagan administration, as part of sweeping tax reform legislation, proposed to kill off the rapidly growing plans.

“Given their revenue costs,” 401(k) plans “cannot be justified as general savings vehicles,” then-Secretary of Treasury James Baker wrote in a letter sent to the chairmen of the three congressional tax writing committees.

Congress, though, under heavy pressure from employee benefits lobbying groups, didn't sign on to the administration proposal to kill the plans.

But 401(k) plans were not unscathed in what became the Tax Reform Act of 1986, which President Reagan signed that October.

Among other things, the new tax law slashed the maximum annual contribution limit to $7,000 from $30,000, while tightening nondiscrimination testing requirements.

Still, even with these new restrictions, the plans continued their rapid growth. By 1990, the number of plans neared the 100,000 mark, covering 20 million active participants.

%%BREAK%%

And soon lawmakers, who only a few years before had reduced the appeal of 401(k) plans, passed new measures to sweeten them.

Tax legislation passed in 1996 exempted the plans from nondiscrimination testing if employers offered generous matching contributions.

Then in 2001, Congress boosted the maximum 401(k) plan annual deferral limit to $11,000, with that limit increasing in $1,000 annual increments until a new $15,000 maximum was hit in 2006.

In addition, employees age 50 and older were given the green light to contribute an additional $5,000 a year in extra “catch-up” contributions.

That same 2001 law also authorized so-called Roth 401(k) plans, in which employees’ aftertax contributions could be withdrawn tax-free if certain conditions were met.

A year later, though, lawmakers imposed a new requirement on 401(k) plan sponsors: Employers were required to give employees 30 days advance notice of plan blackout periods when no transactions could be conducted.

That requirement was the result of the fallout resulting from the earlier collapse of energy giant Enron Corp. As the company’s stock was plunging, Enron imposed a blackout period on 401(k) plan transactions, resulting in employees who had used 401(k) plan salary deferrals to invest in Enron stock watch helplessly as the value of their shares held in their 401(k) accounts plunged.

In 2006, though, Congress again broadened the appeal of 401(k) plans by passing legislation removing obstacles that blocked employers from adding automatic-enrollment features to their plans.