Meet the Waltons: Christy ($21.5 billion), Jim ($19.6 billion), Alice ($19.3 billion) and S. Robson ($19 billion). Together, the three descendants and daughter-in-law of Sam Walton, founder of Wal-Mart, comprise the richest family in the world.

As four of the top 10 richest Americans, the Waltons are invited to take part in Bill Gates and Warren Buffett's "Giving Pledge," which asks U.S. billionaires to donate at least 50% of their wealth to charity. Tell the Waltons to take the "Giving Pledge" today.

Almost 40 million Americans are currently suffering from debilitating poverty, and the Walton family's acceptance of the "Giving Pledge" would directly ease their suffering, as well as set an example for their fellow billionaires.

Take a moment to sign this petition, and ask Christy, Jim, Alice and Rob Walton to donate half their wealth to charity.

Photo credit: WikiMapa