1.1 Subject Matter Jurisdiction: Diversity, Federal Question & Aggregation

🎯 Essential Doctrinal Takeaways

• Complete diversity requires that no plaintiff share citizenship with any defendant at the time of filing.
• A single plaintiff may aggregate all claims against a single defendant to exceed $75,000, regardless of whether they are transactionally related.
• Under Louisville & Nashville Railroad v. Mottley, anticipating a federal defense does NOT confer federal question jurisdiction.
• Under 28 U.S.C. § 1367(b), in diversity-only cases, plaintiffs cannot assert supplemental claims against third-party defendants joined under Rule 14.

Federal subject matter jurisdiction is non-waivable and may be challenged by any party or raised sua sponte by the court at any point in the litigation, including on appeal. Under 28 U.S.C. § 1332, diversity jurisdiction demands two distinct requirements: complete diversity of citizenship (Strawbridge v. Curtiss) and an amount in controversy exceeding $75,000 exclusive of interest and costs.

For amount in controversy purposes, a single plaintiff suing a single defendant is permitted to aggregate any and all claims, whether factually related or totally independent. However, multiple plaintiffs cannot aggregate separate and distinct claims against a single defendant unless they share a common, undivided legal interest (such as co-owners of a single piece of property). Furthermore, a plaintiff cannot aggregate separate claims against multiple defendants unless the defendants are jointly liable.

Under 28 U.S.C. § 1331, federal question jurisdiction exists only when a federal question is presented on the face of the plaintiff's properly pleaded complaint. Under Louisville & Nashville Railroad Co. v. Mottley, a plaintiff cannot create federal question jurisdiction by anticipating that the defendant will raise a federal defense or by arguing that a federal statute invalidates an anticipated state defense.

⚠️ NextGen Bar Examination Traps & Pitfalls

• Assuming a single plaintiff can only aggregate claims that arise from the same transaction or occurrence.
• Believing federal preemption raised in an answer or anticipated in the complaint creates § 1331 jurisdiction.
• Overlooking the § 1367(b) statutory bar on diversity plaintiffs asserting claims against impleaded third parties.