[2] If parties have joint liability, each of them is liable up to the full amount of the relevant obligation.
The Law Commission of New Zealand summarises the issue nicely: Joint and several liability is premised on the theory that the defendants are in the best position to apportion damages amongst themselves.
[2] For example, where an uninsured drunk driver causes an accident that results in injury, the plaintiff may sue an additional defendant, along with the drunk driver, such as suing the state highway department alleging that a highway defect contributed to the accident, hoping that the additional defendant will be found partly responsible.
About two dozen have reformed the rule, with several (Alaska, Arizona, Kansas, Utah, Vermont, Oklahoma, and Wyoming) abolishing it.
[4] Some US jurisdictions have imposed limits on joint and several liability, but without completely abolishing that doctrine.
For example, If Ann is struck by a car driven by Bob, who was served alcohol in Charlotte's bar (and the state has dramshop laws), then both Bob and Charlotte's bar may be held jointly liable for Ann's injuries.