The given data are IQ scores from identical twins; one raised in a foster home, and the other raised by birth parents.
This data frame contains the following columns:
Social class, 1=high, 2=medium, 3=low
Burt, C. (1966). The genetic estimation of differences in intelligence: A study of monozygotic twins reared together and apart. Br. J. Psych., 57, 147-153.
Weisberg, S. (2005). Applied Linear Regression, 3rd edition. New York: Wiley, Problem 6.16.