The United States has entered into agreements, called Totalization Agreements, with several nations for the purpose of avoiding double taxation of income with respect to social security taxes.

Is there a totalization agreement between US and China?

U.S. – China Social Security Totalization Agreement

As of this time, China has not entered into a Totalization Agreement with the United States thus there is opportunity to avoid double taxation of social security income for US expat tax in China.

Is there a totalization agreement between US and Mexico?

A: The Social Security Totalization Agreement that the U.S. signed with Mexico in 2004 has not gone into effect yet. It remains pending and awaits action from the President and Congress. ... The agreements help workers who divide their careers between the U.S. and their native country.