WASHINGTON (AP) — A record number of U.S. counties — more than 1 in 3 — are now dying off, hit by an aging population and weakened local economies that are spurring young adults to seek jobs and build families elsewhere.

New 2012 census estimates released Thursday highlight the population shifts as the U.S. encounters its most sluggish growth levels since the Great Depression.

The findings also reflect the increasing economic importance of foreign-born residents as the U.S. ponders an overhaul of a major 1965 federal immigration law. Without new immigrants, many metropolitan areas such as New York, Chicago, Detroit, Pittsburgh and St. Louis would have posted flat or negative population growth in the last year.