NEW YORK—A new study offers more compelling evidence that life expectancy for some U.S. women is actually falling, a disturbing trend that experts can't explain.

The latest research found that women age 75 and younger are dying at higher rates than previous years in nearly half of the nation's counties—many of them rural and in the South and West. Curiously, for men, life expectancy has held steady or improved in nearly all counties.

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