Final answer:
Gene-environment interaction in an experimental study on monkeys involves manipulating the levels of MAOA activity and exposing them to different environmental conditions to observe the effect on aggression levels.
Step-by-step explanation:
Gene-environment interaction refers to the combined effect of genetic factors and environmental influences on a certain trait or behavior. In an experimental study on monkeys with varying levels of MAOA activity and aggression, researchers would manipulate the levels of MAOA activity in different groups of monkeys and expose them to different environmental conditions. For example, some monkeys with high MAOA activity may be raised in an enriched environment with social interactions and mental stimulation, while others with low MAOA activity may be raised in a deprived environment. By comparing the behavior of different groups of monkeys, researchers can observe the interaction between MAOA activity and the environment in shaping aggression levels.