"The use of human adoptees to separate nature from nurture was first suggested by (1912–1913). During the early decades of the century, interest in adoptees first centered around the question of nature and nurture in human intelligence, which was stimulated in part by the development at that time of tests to measure intelligence. The adoption strategy involved a study of adoptees, their biologic parents, and their adoptive parents. As adoptions developed in the early years of this century, children generally separated at birth from biologic parents and birth environment were placed with nonrelatives who legally adopted and raised the children. In nature-nurture studies the focus of the investigation was usually a trait, behavior, or other characteristic—as in the early studies during the teens and twenties of this century when the focus was on intelligence. The crux of the technique involves comparisons between adoptees and both sets of parents, biologic and adoptive."