Final answer:
The growth of HMOs is primarily driven by the incentive to control costs within the healthcare system, by switching from fee-for-service to a fixed payment model that reduces excessive healthcare provision and addresses the moral hazard issue.
Step-by-step explanation:
The apparent principal driving force behind the growth of health maintenance organizations (HMOs) has been the shift from a fee-for-service model to one that prioritizes cost containment and the management of resources. In the traditional fee-for-service system, medical care providers are paid based on the number of services they provide, incentivizing them to deliver more care, potentially leading to higher costs. In contrast, HMOs provide healthcare under a fixed payment system where providers receive a set amount per enrolled patient, regardless of the services rendered. This system encourages healthcare providers to manage care delivery efficiently and reduces the moral hazard issue, as they have a financial incentive to avoid unnecessary treatments while ensuring good health outcomes to prevent future higher costs.