I believe this comes from a social sciences experiment that was (mis)interpreted through a lens of extreme bias. The poor children chose the instant gratification even with the knowledge that this would deprive them of a bigger reward down the line, while the children who didn't come from poor households chose to hold out for a greater reward later. The researchers decided that this meant poor people were poor because they lacked self-control. Realistically, the poor kids were being "wise" to their own environments; they hadn't had occasion to learn to trust in the notion of future security, so rather than gamble a "now" reward for a "future" hypothetical, they capitalized on the now.

Too many people, unfortunately, are still hung up on the old, biased, inaccurate interpretation, because it confirms their own preconceptions. It caught on, and it sticks. :(