Generally speaking, if a decedent owned property as joint tenants, the entire value of the property is included in his estate. There are a couple exceptions to this rule.
Contribution. The amount included is limited by the contribution made by the deceased joint tenant. Thus, if it can be shown that a joint tenant made a contribution to the principal of the property, then only that percentage of the property’s value will be included in the decedent’s estate. For example, if Abel and Cain each contributed $1,000 to buy some property to buy $2,000 worth of stock. The stock appreciates and is worth $5,000 at Abel’s untimely death. Only 50% of the value of that property ($2,500) will be included in Abel’s estate. This concept may be better understood with this equation:
Contribution by decedent/Total contribution by all tenants x value of joint property = Amount included in decedent’s estate
Any consideration by the surviving co-tenant does not include amounts that were originally gifted to the surviving co-tenant by the decedent. For example, Abel gave Cain $5,000 and Cain deposited it into an account and added Abel as a joint tenant. If Abel dies before Cain, the entire $5,000 would be included in Abel’s estate.
Tenancy by the Entirety (husband and wife as joint tenants): When two people are married and hold property either as joint tenants or as tenants by the entirety, exactly one-half of the value of the property is included in the decedent’s estate. The amount contributed by either spouse does not matter.