When is diversity of citizenship determined in a legal case?

Diversity of citizenship is determined at the time the lawsuit is filed. This principle stems from the requirement that for a federal court to have diversity jurisdiction, the parties involved must be citizens of different states, and this status is assessed at the initiation of the case rather than at any later point.

When a party files a complaint, the court examines the citizenship of all parties based on their domicile at that time. If the parties are from different states, this creates the necessary diversity for the case to be heard in federal court. Subsequent changes in the parties’ citizenship after the filing do not affect the court's subject matter jurisdiction based on diversity.

This principle is crucial for ensuring that the jurisdiction is appropriate, as it helps avoid any potential bias that could arise if a citizen of one state were compelled to litigate against another citizen within the same state court system. Therefore, determining diversity of citizenship at the time of filing ensures fairness and impartiality in the judicial process.