Final answer:
Klein's study showed a combination of jealousy and prosocial behavior in which relationships were involved.
Step-by-step explanation:
Klein's study likely addressed the complex nature of human emotions and relationships. The blank in the question appears to be looking for a term that is related to the combination of jealousy and some form of behavior in a relationship context.
While the information given does not specify which word fills the blank, several key concepts from social psychology might fit, including prosocial behavior, which is the voluntary behavior with the intent to help other people, or resistant attachment, characterized by clingy behavior and rejection by a child towards a parent.
However, without more context, it's not possible to provide a definitive answer to what specifically fills in the blank in the given sentence.