Final answer:
The combination of environmental and genetic factors that influence the development of anorexia nervosa in identical twins is referred to as gene-environment interactions.
Step-by-step explanation:
The combination of environmental and genetic factors that influence the development of anorexia nervosa in identical twins is referred to as gene-environment interactions. Gene-environment interactions occur when an individual's genetic makeup interacts with their environment to determine their risk of developing a particular disorder or trait.