This map shows U.S. counties that diverged from an equal (or nearly equal) sex ratio. Counties shown in white had a sex ratio of no more than 51 percent male or female. Counties shown in green had a higher proportion of males; counties shown in brown had a higher proportion of females.
I wrote a brief blog post about the map.
If you use this map in your research, I would appreciate a citation. Here is the suggested form:
Lincoln Mullen, "Divergence in U.S. Sex Ratios by County, 1820–2010," interactive map, http://lincolnmullen.com/projects/sex-ratios/.
You should also cite the NHGIS, who provided the data:
Minnesota Population Center, National Historical Geographic Information System: Version 2.0 (Minneapolis, MN: University of Minnesota, 2011), http://www.nhgis.org.