The shadow effect occurs when two players each score less points by playing on the same team than they would playing solo. It is a direct result of the fact that every question can be answered by, at most, one player on a team.
When more than one player is on a team, each player's knowledge base overlaps, however slightly, with each other player's knowledge base. However, only one player on that team can buzz in and earn points on a tossup. Therefore, two players whose knowledge bases overlap significantly will each see a large decrease in their PPG relative to if they had been on separate teams, while two players who specialize in different areas will each see a smaller decrease. This decrease in PPG is known as the shadow effect.
The shadow effect is a function of each player's knowledge base and each player's aggressiveness. However, as a player's knowledge base cannot easily be quantified, an exact formula for the shadow effect cannot be derived.
The assumption that the shadow effect is roughly the same for all groups of two players is a fundamental flaw in PATH. Nevertheless, PATH is often considered a more accurate indicator of a player's ability than PPG, which does not account for the shadow effect.