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In the era of Pay Pal Mobile, Google Wallet and near field communication, one unstoppable is keeping cash alive: poor people.

Poor people use cash even though it tends to be more expensive, writes WSJ's David Wolman:

The poorer you are, the higher the costs and risks of cash become. Anyone you know can beg you for a few bucks or steal the hard-earned money that you're trying to save to pay your children's school fees. A fire or natural disaster can obliterate your meager savings. And you may have to spend days riding buses and walking to the countryside to deliver cash to, or retrieve it from, a relative. Even if a wire service is accessible, that means paying steep service fees.

The poor rely on analog currency because many are going unbanked. As long as banks keep up monthly fees and balance minimums, don't expect this to change.

Wolman explains the bank's side: "It has never been profitable to put bank branches in the slums and rural villages where poor people live, while balance minimums and the time required to get to and from a branch make old-school banking unrealistic and unattractive."

Now take a walkthrough of Pay Pal's cool mobile wallet tool >