Year 2nd generation 1st generation
1900 20.8 13.7
1910 20.1 14.6
1920 20.6 12.7
1930 20.1 11.3
1940 18.8 8.5
1950 16.2 6.9
1960 13.8 5.6
1970 12.0 4.8
1980 10.3 6.2
1990 9.6 8.2
2000 10.0 11.4
2006 10.5 12.1
2010 11.3 12.7
2015 12.1 13.3
2016 12.0 13.5
2017 12.0 13.7
2018 12.3 14.1