A large portion of the nation's housing units in 1990 were built before 1960. Despite the large proportion of older homes, about 21 percent of all housing units in the United States were built during the 1980s. The 1970s saw the most housing units constructed, just over 22 million, while the 1940s had the least, about 8.5 million, primarily because of the lack of homebuilding during World War II.