Equity Theory suggests that people are happiest in relationships where:

Equity Theory posits that individuals assess the fairness of their relationships based on the balance of rewards and costs experienced by both partners. According to this theory, people seek to maintain a fair ratio of what they put into a relationship (such as emotional support, time, effort, and resources) to what they receive in return. When the rewards and costs are roughly equal for both parties, individuals are more likely to feel satisfied and fulfilled, as they perceive the relationship as fair. This balance helps to foster positive feelings and commitment, leading to greater overall happiness within the relationship.

The other options do not align with the principles of Equity Theory. For instance, a significant emotional investment from one partner may create an imbalance, potentially leading to feelings of inequity and dissatisfaction. Open communication, while important for relationship maintenance, does not specifically address the balance of inputs and outputs emphasized by Equity Theory. Lastly, the presence of a significant age difference is not inherently connected to the equity in the relationship dynamics; rather, it could introduce various complexities but does not necessarily contribute to relationship satisfaction as defined by equity.