What is the "general rule" on contribution between parties jointly charged for negligence?

It is singularly unfortunate, and has led to misunderstanding, that Merryweather v. Nixan should have been continually treated as stating the "general rule." As a matter of fact, that case states not the rule, but the exception. The general rule is that among persons jointly liable the law implies an assumpsit either for indemnity or contribution, and the exception is that no assumpsit, either express or implied, will be enforced among wilful tort-feasors or wrongdoers.

Reath, Contribution Between Persons Jointly Charged for Negligence--Merryweather v. Nixan, 12 Harv.L.Rev. 176, 177 (1898).