Final answer:
The top income quintile includes the highest 20% of income earners. This group has gained a larger share of income over time, revealing an increase in income inequality.
Step-by-step explanation:
The question asks about the top income quintile and which group of earners it includes. Based on the data provided, the top quintile refers to the highest 20% of income earners in a given population. The quintile distribution is an important measure that highlights economic inequality. Over the years, the share of income for the top quintile in the U.S. has increased, indicating a rise in income inequality. Specifically, Census Bureau research has found that a significant part of this increase can be attributed to the top 5% of income earners.