Studies that compare adopted children to their parents and comparative studies between fraternal and identical twins are very helpful to delineate the genetic and the environmental influences on an individual. One has that in families with adopted children, the characteristics that lie with genetic traits are usually common with the biological parents, while traits that are mostly affected by the environment are mostly correlated with the adoption parents. This helps us ascertain the influence of each factor to various traits ranging from social behavior to reflexes.
Hence, the gap-filling is: "the environment" "genes"
