Economic and development assistance includes a range of aid to other countries such as disaster relief, infrastructure development and childhood vaccination programs.
The rich countries of the world, including the U.S., have committed themselves repeatedly to a target of spending 0.7% of their gross domestic product as development assistance. Gross domestic product is a measure of the size of an economy, so the larger a country's economy, the more it would be committed to giving in aid. Not all wealthy countries have met this self-imposed target, but the U.S. ranks second to last.*
The graph below shows the top countries and the United States and their official development assistance (ODA) as a percentage of their gross national income (GNI) in 2004. Gross national income is similar, though technically distinct, from GDP, and is currently more commonly used to measure this indicator.

*Gross national income has become used by the Organization for Economic Cooperation and Development and the United Nations in place of the gross domestic product for the measure of a country's economy. If ODA is measured as percentage of GDP, the results are similar.
Source: United Nations Development Programme, Human Development Report, 2006 [1]