Why bother? That’s the question more underwater Americans are asking themselves about their mortgage. Trapped in the abyss of negative equity, more will decide to quit paying. As they should. About a quarter of all mortgages in the United States are on houses that are worth less than the unpaid balance of the mortgage, according to real estate consultant First American CoreLogic. About half of that group, 5.3 million borrowers, are 20 percent or more underwater. For 2.2 million, the property is worth less than half the mortgage balance.