Homelessness in the U.S. appears to be far more pervasive than previously believed, with about 13.5 million Americans spending at least a few days during their lives on the streets or in a shelter, a new survey concludes. What's more, Columbia University researchers said that an additional 12.5 million have staved off the threat of homelessness only by moving in with friends or family. The problem has always been tough to quantify and high estimates (by advocates) pegged the number of homeless people at 3 million, although the Columbia study doesn't offer a total homeless population. And the 1990 Census counted only 400,000 homeless Americans. Among those who said they had experienced homelessness, 46 percent said they had been homeless between a month and a year, 33 percent between a week and a month, 13 percent for more than a year and 8 percent for less than a week. The leading cause of homelessness, the survey found, was poverty -- not mental illness or drug abuse as some argue.