Washington, Aug 31 (IANS): Driven by economic imperatives, the United States is experiencing a surge in multigenerational joint families, with Hispanics and Asians driving the trend.

The number of such households, defined as those with three or more generations living under one roof, grew to almost 5.1 million in 2010, a 30 percent increase from 3.9 million in 2000, according to US Census Bureau data released this month.

They hit 2.9 million in 1950 and didn't top that again until four decades later, according to the Washington-based Pew Research Centre. At the 1980 low, multiple-generation homes represented just 2.9 percent of all US households, down from 7.8 percent in 1900.

Among large cities, the one with the highest percentage of multigenerational households, at 16 percent, is Norwalk, California, a collection of largely single- family homes 15 miles south of Los Angeles.

Job losses and the difficulty of purchasing a home make young people more likely to live with their parents, according to D'Vera Cohn, a senior writer with Pew. Longer life spans and growth in the Hispanic and Asian populations keep older folks in the house.

The nation's two fastest-growing ethnic groups are 50 percent more likely to live in multigenerational families than are whites, according to Pew research.

The state with the largest percentage of multigenerational households was Hawaii at 8.8 percent, twice the national average of 4.4 percent.