Joint tenancy with right of survivorship is a form of co-ownership. Co-owners in a joint tenancy must have equal ownership shares and equal authority over the property, whether it's a bank account, brokerage account or real estate. If one joint tenant dies, right of survivorship means her co-owner or owners divide up her share equally, regardless of her will or her heirs' wishes. It's much harder to contest survivorship than to challenge a will.
One point on which the right of survivorship might be contested is whether the co-ownership documents were drawn up correctly. Courts assume that joint bank accounts, for instance, do not have right of survivorship unless it's specifically stated. If the joint tenants didn't fill out their paperwork according to state law and bank requirements, a court might decide that there's no proof the right of survivorship exists.
If the documentation holds up, the burden of proof is on the person challenging the right of survivorship. The Smarter Dollars website states that right of survivorship trumps wills, domestic-partner agreements, written contracts and probate laws for people who die without a will. Another factor is that the joint tenant can drain the bank account or dispose of jointly-owned property quickly, making the issue moot by placing the property out of the heirs' reach.
Someone planning to challenge the right of survivorship to a jointly-owned bank account can ask the bank or the estate executor to put a freeze on it until any questions are resolved. Even if both names are on the account and the paperwork is in order, if it can be shown that the surviving tenant didn't put any money into the account, a court might consider that it wasn't true joint tenancy and distribute the money according to the deceased's will.
In certain circumstances, an established right of survivorship won't come into play. For instance, if the joint tenants die together in a fire or a car accident, it might be impossible to determine who died first, so each tenant's share will go to their respective heirs. If one tenant is convicted of murdering his co-owner, he cannot benefit from the crime, so the deceased's share will go to his other co-owners or, if there are none, to his heirs.