What kind of twins showed similar ratings for social aggression in Brendgen's study?

• A. Identical twins only
• B. Dizygotic twins only
• C. Both MZ and DZ twins
• D. None of the twins

Answer: (C)

In Brendgen's study, both monozygotic (MZ) twins, also known as identical twins, and dizygotic (DZ) twins, or fraternal twins, showed similar ratings for social aggression. This finding suggests that both genetic and environmental factors play a role in the development of social aggression.

The study highlighted the complexities of how aggressive behaviors can be influenced not just by genetic makeup, as seen with identical twins who share 100% of their genes, but also by shared environmental factors that both types of twins experience. The fact that DZ twins, who share about 50% of their genes, displayed similar rates of social aggression indicates that there are significant environmental influences at play, which can affect siblings regardless of their genetic similarities.