How Will 401 K Pension Plans Affect Retirement Income? 
401 -LRB- k -RRB- plans are defined contribution retirement accounts that accumulate tax deferred and are taxable at at distributions as ordinary income at the effective tax bracket of the plan participant . Those 401 -LRB- k -RRB- distributions are also included in the provisional income test for Social Security benefit taxation . So 401 -LRB- k -RRB- income can effect Social Security . 