About 47 percent will pay no federal income taxes at all for 2009. Either their incomes were too low, or they qualified for enough credits, deductions and exemptions to eliminate their liability.

That's according to projections by the Tax Policy Center, a Washington research organization.

Most people still are required to file returns by the April 15 deadline. The penalty for skipping it is limited to the amount of taxes owed, but it's still almost always better to file: That's the only way to get a refund of all the income taxes withheld by employers.

In recent years, credits for low- and middle-income families have grown so much that a family of four making as much as $50,000 will owe no federal income tax for 2009, as long as there are two children younger than 17, according to a separate analysis by the consulting firm Deloitte Tax.

Tax cuts enacted in the past decade have been generous to wealthy taxpayers, too, making them a target for President Barack Obama and Democrats in Congress. Less noticed were tax cuts for low- and middle-income families, which were expanded when Obama signed the massive economic recovery package last year.

The result is a tax system that exempts almost half the country from paying for programs that benefit everyone, including national defense, public safety, infrastructure and education. It is a system in which the top 10 percent of earners—households making an average of $366,400 in 2006—paid about 73 percent of the income taxes collected by the federal government.

The bottom 40 percent, on average, make a profit from the federal income tax, meaning they get more money in tax credits than they would otherwise owe in taxes. For those people, the government sends them a payment.

"We have 50 percent of people who are getting something for nothing," said Curtis Dubay, senior tax policy analyst at the Heritage Foundation.

The vast majority of people who escape federal income taxes still pay other taxes, including federal payroll taxes that fund Social Security and Medicare, and excise taxes on gasoline, aviation, alcohol and cigarettes. Many also pay state or local taxes on sales, income and property.

That helps explain the country's aversion to taxes, said Clint Stretch, a tax policy expert Deloitte Tax. He said many people simply look at the difference between their gross pay and their take-home pay and blame the government for the disparity.

"It's not uncommon for people to think that their Social Security taxes, their 401(k) contributions, their share of employer health premiums, all of that stuff in their mind gets lumped into income taxes," Stretch said.

The federal income tax is the government's largest source of revenue, raising more than $900 billion—or a little less than half of all government receipts—in the budget year that ended last Sept. 30. But with deductions and credits, especially for families with children, there have long been people who don't pay it, mainly lower-income families.