What is the largest cost to the federal government?

Answer 1

Let's take a look at one representative year, 2011:

The US federal government spent $3.6 trillion (that's $3,600,000,000,000). This is fully 24% of the GDP (Gross Domestic Product).

Of that 21% was paid out by Medicaid, Medicare, and CHIP (Children's health Insurance Program). This is the largest piece of the pie.

Additionally, 20% was paid out for each Defense and Security and Social Security.

Around 13% was used to continue so called Safety Net Programs such as the refundable portion of the earned-income and child tax credits, which assist low- and moderate-income working families through the tax code; programs that provide cash payments to eligible individuals or households, including Supplemental Security Income for the elderly or disabled poor and unemployment insurance; various forms of in-kind assistance for low-income families and individuals, including food stamps, school meals, low-income housing assistance, child-care assistance, and assistance in meeting home energy bills; and various other programs such as those that aid abused and neglected children.

Benefits for federal pensions and veterans made up 7%. Interest on the national debt took 6%. The transportation infrastructure took 3%. Education and Science and other research account for 2% each. Non-security international affairs programs spoke for 1%. The remaining 5% was disbursed among all other unspecified federal programs and expenditures.