401(k) plans – the main retirement savings vehicle for millions of workers – allow participants to save on a tax-deferred basis. This tax incentive is more valu­able to workers in high-income families than work­ers in low-income families because they face higher marginal income tax rates....

Many workers eligible for 401(k) plans fail to par­ticipate and those who do participate often save too little. In response, policy experts have advocated auto-enrollment, in which employees are signed up at a default contribution rate unless they opt out. Over the past decade, a number of...