Final answer:
The incorrect statement about the nuclear family is the claim that they accounted for just 21% of households in the US in 2010; the actual figure was around 66% for households with two married parents.
Step-by-step explanation:
The statement about the nuclear family that is not true is: 'In the United States, nuclear families accounted for just 21% of households in 2010'. In reality, according to the 2010 census data, 66 percent of children under seventeen years old lived in a household with two married parents, a decrease from 77 percent in 1980, indicating that the prevalence of nuclear families was significantly higher than 21%. Other variations include single-parent households and same-sex families, further diversifying the definition of a nuclear family. Also, it is important to note that while the nuclear family is a cultural universal, the composition and function of families can vary widely across different cultures and societies.