What Is the Difference Between Undue Influence and Lack of Capacity in a BC Will Dispute?

This answer covers the law of British Columbia.

Lack of capacity asks whether the will-maker was mentally able to make a will, while undue influence asks whether someone else's pressure produced it. Under the Wills, Estates and Succession Act, a will-maker must be mentally capable, and the test for capacity comes from case law rather than the statute itself.

Proof works differently for each ground. For capacity, if suspicious circumstances relate to mental capacity, the person defending the will must establish capacity. For undue influence, the challenger first has to show the other person was in a position of potential dependence or domination and that the position was used to influence the will. Only then must the defender prove that no undue influence was exercised. Without that showing, the person alleging undue influence or fraud bears the burden.

Suspicious circumstances can raise a suspicion of fraud or undue influence, but they do no more than rebut the presumption of capacity and of knowledge and approval. Because the same facts can sometimes be argued under more than one ground, which one fits your situation depends on the evidence, and a lawyer can help you sort that out.