Spencer Phillips, a 39-year-old orchestral musician who plays the bass violin, loves and trusts his wife of 10 years—but he refuses to file a joint income-tax return with her. 

Instead, Mr. Phillips and his wife opt into the “married, filing separately” status, called MFS, and each spouse reports only his or her income to the Internal Revenue Service. This saves them a lot of money: By filing separately, and so reducing his stated income, Mr. Phillips’s student-loan payments on about $200,000 of debt from the prestigious Eastman School of Music and other programs come to about $240 monthly instead of more than $1,000.