Torts - Joint Liability. Rutman v. Rabinowitz
In Rutman v. Rabinowitz (Ont CA, 2018) the Court of Appeal considered joint liability in defamation on a 'concerted action' theory:
 Concerted action may occur in a variety of ways. Generally, it involves a common design or conspiracy. In Botiuk v. Toronto Free Press Publications Ltd., 1995 CanLII 60 (SCC),  3 S.C.R. 3, the Supreme Court of Canada adopted the following formulation of the law regarding concerted action liability as set out by John G. Fleming in The Law of Torts, 8th ed. (Sydney: Law Book Co., 1992), at p. 255:
The critical element of [concerted action liability] is that those participating in the commission of the tort must have acted in furtherance of a common design. … Broadly speaking, this means a conspiracy with all participants acting in furtherance of the wrong, though it is probably not necessary that they should realize they are committing a tort. The difficulty, of course, is determining the degree of involvement or connection necessary to meet the requirements of concerted action liability. Canadian authorities suggest that concerted action liability arises when a tort is committed in furtherance of a common design or plan, by one party on behalf of or in concert with another party: see Lewis N. Klar & Cameron S.G. Jefferies, Tort Law, 6th ed. (Toronto: Thomson Reuters, 2017), at p. 657; G.H.L. Fridman, The Law of Torts in Canada, 3rd ed. (Toronto: Carswell, 2010), at p. 856. In The Law of Torts, 10th ed. (Sydney: Thomson Reuters, 2011), at p. 302, Fleming puts it this way: “[k]nowingly assisting, encouraging or merely being present as a conspirator at the commission of the wrong would suffice, so too would any form of ‘inducement, incitement or persuasion’ which procures the commission of the wrong.” And, W. Page Keeton, in Prosser and Keeton on the Law of Torts, 5th ed. (Minnesota: West Publishing Co., 1984), at p. 323, states:
All those who, in pursuance of a common plan or design to commit a tortious act, actively take part in it, or further it by cooperation or request, or who lend aid or encouragement to the wrongdoer, or ratify and adopt the wrongdoer’s acts done for their benefit, are equally liable. The key point is that concerted action liability is a fact-sensitive concept. Lord Neuberger emphasized as much in Sea Shepherd, at para. 56, reiterating Bankes LJ’s admonition from The Koursk,  P 140, at p. 151 that “[i]t would be unwise to attempt to define the necessary amount of connection”, and that each case “must depend on its own circumstances”. We agree.