Poor health, substance abuse, and discrimination are the main causes of poverty in the United States. At least 13 percent of the current U.S. population lives in poverty. That's more than 38 million people.
Poverty thresholds do not vary from state to state, and the exact dollar amount of income they call "the poverty line" is determined through a rather complex equation.
The equation considers things like the total number of people in a household, the number of people in the household who are under 18, annual household income, and inflation. But it seems to presume that there IS a household, and it's unclear if and how the homeless are included in the data. To give some idea of poverty thresholds, the federal poverty line was set at $18,100 a year for a family of 4, as of 2001.
Below is a breakdown of poverty levels by state. The column on the right (%BPL) indicates the percentage of that state's population living at or below the "poverty line." The number represents a three-year average for 2004 thru 2006. This info was obtained from the U.S. Census Bureau website.
Just for the record, Washington DC (if it were a state) would finish 51st on the list, with 21.3 percent of its residents living in poverty.