The county was formed in 1836. It was named after the Polish general Tadeusz Kościuszko, who served in the American Revolutionary War, and then returned to Poland. The county seat is named after Warsaw, the capital of Poland.
2000 74,057 1990 65,294 1980 59,555 1970 48,127 1960 40,373 1950 33,002 1940 29,561 1930 27,488 1920 27,120 1910 27,936 1900 29,109 1890 28,645 1880 26,494 1870 23,531 1860 17,418 1850 10,243 1840 4,170
As of the census of 2000, there were 74,057 people, 27,283 households, and 19,998 families residing in the county. The population density was 138 people per square mile (53/km²). There were 32,188 housing units at an average density of 60 per square mile (23/km²). The racial makeup of the county was 94.58% White, 0.60% Black or African American, 0.25% Native American, 0.55% Asian, 0.01% Pacific Islander, 2.94% from other races, and 1.06% from two or more races. 5.03% of the population were Hispanic or Latino of any race. 29.2% were of German, 20.9% American, 9.2% English and 8.3% Irish ancestry according to Census 2000.
There were 27,283 households out of which 35.60% had children under the age of 18 living with them, 60.90% were married couples living together, 8.30% had a female householder with no husband present, and 26.70% were non-families. 21.90% of all households were made up of individuals and 8.00% had someone living alone who was 65 years of age or older. The average household size was 2.66 and the average family size was 3.11.
In the county the population was spread out with 27.80% under the age of 18, 8.70% from 18 to 24, 29.00% from 25 to 44, 22.60% from 45 to 64, and 12.00% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 99.70 males. For every 100 females age 18 and over, there were 97.30 males.
The median income for a household in the county was $43,939, and the median income for a family was $49,532. Males had a median income of $36,209 versus $23,516 for females. The per capita income for the county was $19,806. About 4.40% of families and 6.40% of the population were below the poverty line, including 7.10% of those under age 18 and 7.90% of those age 65 or over.