Final answer:
Dr. Ivanov, who focuses on family and adoption studies, and Dr. Jefferson, who conducts twin studies, both work in the field of behavioral genetics, as their research involves the study of genetic and environmental influences on behavior.
Step-by-step explanation:
Given the provided information regarding Dr. Ivanov's and Dr. Jefferson's lines of study, we can deduce their fields of expertise. Dr. Ivanov, who conducts family and adoption studies, is focused on understanding how genetic and environmental factors contribute to individual differences, which aligns with the work of a behavioral geneticist. On the other hand, Dr. Jefferson's work with twin studies is also indicative of someone in the field of behavioral genetics as he compares the similarities between identical and fraternal twins to parse out the influence of genetics versus the environment on certain traits. Consequently, the correct completion of the sentence would be: Dr. Ivanov is a behavioral geneticist. Dr. Jefferson is also a behavioral geneticist.