Why Appeals Courts Rarely Reverse Lower Courts: An Experimental Study to Explore Affirmation Bias

One of the most striking features of appellate courts in the United States is also one of the least understood: Appellate courts rarely reverse lower court decisions. The data reviewed in this Article indicate that roughly 90% of appellate court decisions affirm lower court rulings. Why do appeals courts so often affirm? Do trial court judges rarely make reversible errors or do appellate courts suffer from ¿affirmation bias¿ and fail to correct enough errors? These fundamental questions cannot be answered empirically, so I conducted a decision-making experiment with law students and political science majors. The results indicate that random variation in who prevailed in the lower court significantly influences who wins on appeal. Affirmation bias may inflate appellate court affirmation rates by as much as 7%. Further research is needed, but these results suggest that appellate courts fail to correct many trial court errors due to affirmation bias