Over the past few years, research suggests that stress levels in the United States have __.

Over the past few years, research suggests that stress levels in the United States have increased.

1. Identify the subject: The subject of the sentence is "stress levels in the United States."
2. Determine the time frame: The phrase "over the past few years" indicates a recent time frame.
3. Analyze the research trend: Research suggests a trend or change in stress levels.
4. Conclude the trend: Based on various studies and reports, it is observed that stress levels have generally increased due to factors such as economic uncertainty, political climate, and global events like the COVID-19 pandemic.

Therefore, the complete sentence is: "Over the past few years, research suggests that stress levels in the United States have increased."