Dr. Sparrow is a social psychologist who studies romantic relationships. Several researchers have found that there is a link between income and marital satisfaction (e.g., Dakin & Wampler, 2008). Dr. Sparrow is curious as to whether there is a causal link between the two variables, such that having a higher income causes higher levels of marital satisfaction. What is the most important ethical issue that he must he consider when selecting a design for his study?