401(k) Plan

A 401(k) plan is a defined contribution retirement plan, named for the section of the Internal Revenue Code that authorizes it. Employees elect to defer a portion of their compensation into an individual account inside a plan trust, and choose from the investment options the plan makes available.

It is defined contribution rather than defined benefit: what goes in is defined, what comes out depends on contributions and investment performance. The investment risk sits with the participant, not with the employer.

From paycheck to plan account

The employee makes a deferral election, expressed as a percentage of pay or a flat amount per paycheck. Payroll withholds that amount and the employer remits it to the plan trust, where it is invested according to the participant's elections or to the plan's default investment if they made none.

Deferrals come in two tax flavors, and a plan may offer either or both. Pre-tax deferrals reduce current taxable income, grow tax deferred, and are taxed on distribution. Roth deferrals are made from after-tax pay, and qualified distributions of both the contributions and the earnings come out tax free. The choice belongs to the employee, though many participants never revisit it after enrollment.

Automatic enrollment changes the default. Instead of requiring an affirmative election, the plan enrolls eligible employees at a stated deferral rate unless they opt out, frequently with automatic escalation that increases the rate each year up to a cap. Participation rates under automatic enrollment are dramatically higher than under opt-in designs, which is why it has become standard in new plans.

How employers contribute

A plan can use any of these, and many use more than one.

• Matching contribution: the employer contributes based on what the employee defers, using a stated formula such as a full match up to a percentage of pay, or a partial match on a larger percentage. The formula, not the headline number, determines the real value.
• Non-elective contribution: the employer contributes for eligible employees whether or not they defer anything. This reaches employees who cannot afford to defer, which a match by definition does not.
• Profit sharing contribution: a discretionary employer contribution, often decided annually, allocated under a formula in the plan document.
• Safe harbor contribution: a match or non-elective contribution meeting specific requirements that allows the plan to bypass certain annual nondiscrimination tests. It generally must be fully vested when made, with a limited exception for automatic enrollment safe harbor designs.
• True-up: a year-end correction that fixes the under-match an employee receives when a per-paycheck match formula penalizes uneven deferral timing. Whether a plan includes one is a real design decision, not a detail.

Vesting and annual testing

Two mechanics that shape both employee outcomes and employer risk.

• Employee deferrals are always immediately and fully vested. That money is the employee's from the moment it is withheld and cannot be forfeited.
• Employer contributions may be subject to a vesting schedule, either cliff vesting where the participant becomes fully vested at a stated service milestone, or graded vesting where ownership increases in steps. ERISA sets maximum schedules that a plan cannot be less generous than.
• Unvested employer money is forfeited when a participant leaves before vesting, and the plan document controls how forfeitures may be used, typically to offset future employer contributions or to pay plan expenses.
• Nondiscrimination testing compares deferral and matching rates of highly compensated employees against everyone else. A plan that fails must correct, commonly by refunding excess deferrals to highly compensated participants or making additional contributions for others.
• Top-heavy testing looks at whether key employees hold a disproportionate share of plan assets and can require a minimum employer contribution.
• Safe harbor plan designs exist to avoid this testing cycle entirely, at the cost of a committed employer contribution that vests quickly.

What the employer is on the hook for

Sponsoring a plan makes the employer a fiduciary with respect to plan administration and investment selection, which is a higher standard than ordinary business judgment.

• Remitting employee deferrals to the trust as soon as they can reasonably be segregated from the employer's general assets. Late deposits are among the most common findings in plan examinations, and correcting them requires restoring lost earnings.
• Selecting and monitoring the plan investment lineup and the service providers, with a documented process rather than a one-time decision.
• Following the plan document exactly, including its definition of compensation. Using the wrong compensation base for deferrals or match is one of the most frequent operational errors and it compounds every pay period until caught.
• Providing required participant disclosures, including the summary plan description, fee disclosures, and notices tied to automatic enrollment or safe harbor status.
• Filing the annual return required for the plan, with an independent audit once the plan crosses the participant count threshold that requires one.
• Enrolling newly eligible employees on time, including under the rules that extend eligibility to long-term part-time employees.

When participants can reach the money

Plan assets are generally locked until a distributable event: separation from service, reaching the plan's retirement age, disability, or death. Distributions before age 59 and a half are typically subject to an additional tax on top of ordinary income tax, with exceptions defined by the tax code.

A plan may, but is not required to, permit loans and hardship distributions. Loans are repaid through payroll with interest credited to the participant's own account, and an unpaid balance at separation can become a taxable distribution. Hardship distributions require an immediate and heavy financial need as defined by the plan and the tax rules, and are not repaid.

On separation, participants generally choose among leaving the balance in the plan if it exceeds the plan's cash-out threshold, rolling it to an individual retirement account or a new employer's plan, or taking a distribution and accepting the tax consequences. Small balances may be automatically distributed or rolled over under the plan's terms.

Why it matters operationally

The 401(k) is the benefit with the tightest coupling to payroll and the least tolerance for a data error. A wrong deferral percentage, a missed eligibility date, or a compensation definition mismatch does not surface as a complaint, it surfaces as a correction with lost earnings attached, sometimes years later.

The controls that matter are unglamorous: eligibility dates calculated from the plan document rather than by hand, deferral changes flowing automatically from the recordkeeper to payroll, deposits made on a consistent and documented schedule, and an annual reconciliation of what payroll withheld against what the trust received.