What was the primary reason for the drop in the number of uninsured Americans in 2011?

The primary reason for the drop in the number of uninsured Americans in 2011 was indeed related to young adults staying on their parents' health insurance plans. This provision was part of the Affordable Care Act (ACA), which allowed individuals up to the age of 26 to remain on their parents' employer-sponsored health insurance. This change significantly increased coverage among young adults, a group that historically had high rates of uninsurance due to lower income and health care needs. By enabling them to remain on their parents’ plans, many young adults gained access to health care services, leading to a notable decline in the overall number of uninsured individuals in the country.

In this context, while other factors such as employer health insurance offerings and the eventual implementation of the Health Insurance Marketplace contributed to changes in insurance coverage, they did not have the immediate impact on reducing uninsured rates in 2011 as the policy allowing young adults to stay on their parents' plans did.