Survivorship is not an inheritance, and the distinction has practical consequences. When a joint tenant dies, the surviving owners do not receive anything from the estate; the deceased owner's interest simply ends and the title is already in the survivors. Because nothing passes through the estate, the asset is outside probate, outside the will, and generally outside the reach of the process that gives creditors a window to make claims against the estate. That is the entire attraction of the arrangement, and it is also why it is so often set up casually and then discovered to have overridden a carefully written plan.
It has to be declared, and the statutes are explicit about the direction of the default. California Civil Code section 686 provides that "every interest created in favor of several persons in their own right is an interest in common, unless acquired by them in partnership, for partnership purposes, or unless declared in its creation to be a joint interest ... or unless acquired as community property." New York's Estates, Powers and Trusts Law section 6-2.2(a) reaches the same place from the opposite legal tradition: a disposition to two or more persons creates in them a tenancy in common "unless expressly declared to be a joint tenancy." Two states, one a community property state and one a common law state, both starting from tenancy in common. A deed that says only "to A and B" is generally not a joint tenancy, and the words on the instrument are what decide it.
California adds a requirement other states do not, which is a warning against generalizing. Civil Code section 683(a) defines a joint interest as one "owned by two or more persons in equal shares, by a title created by a single will or transfer, when expressly declared in the will or transfer to be a joint tenancy." Equal shares is part of California's statutory definition; New York's provision contains no such requirement. The older common law doctrine that a joint tenancy requires four matching characteristics, usually called the four unities, has been modified by statute in many states. Anything specific about how a joint tenancy is formed is a question for the law of the state where the property sits.
A joint tenant can usually destroy the survivorship alone, and often silently. California Civil Code section 683.2(a) provides that a joint tenant may sever a joint tenancy in real property as to their own interest "without the joinder or consent of the other joint tenants", either by conveying legal title to a third person or by executing a written instrument evidencing the intent to sever, including a deed naming the severing joint tenant as the transferee. What survives severance is a tenancy in common: the co-owners still own the property together, and the survivorship is gone. There is a recording safeguard at section 683.2(c), under which a severance by deed or declaration does not defeat the other owners' survivorship unless it was recorded before the severing tenant's death, or was notarized no earlier than three days before that death and recorded within seven days after it. That safeguard exists precisely because a secret severance would otherwise let one owner keep the survivorship if they outlived the others and cancel it if they did not.
Adding a co-owner to a deed is a gift, and adding one to a bank account is not. The two are governed by different paragraphs of the same Treasury regulation. Under Treasury Regulation section 25.2511-1(h)(4), depositing money into a joint bank account is not a completed gift when the account is opened, because the depositor can still withdraw the whole balance; the gift happens only when the other owner draws on it for their own benefit. Under section 25.2511-1(h)(5), buying property and taking title jointly is a completed gift of half the value immediately, because the transfer cannot be undone unilaterally. Consumer writing routinely groups both under one rule about adding a name, and that rule is wrong for one half of its own question. The other consequences, that a co-owner's creditors and a co-owner's divorce can now reach the property, and that the giver has handed over control of half of it, apply to both.
The gift usually does not remove the property from the giver's taxable estate, which is the reverse of what most people adding a name expect. Internal Revenue Code section 2040(a) includes in a decedent's gross estate the value of property held as joint tenants with right of survivorship "except such part thereof as may be shown to have originally belonged to such other person and never to have been received or acquired by the latter from the decedent for less than an adequate and full consideration in money or money's worth." A child who was simply added to a deed contributed nothing and received the interest from the parent, so neither branch of that exception is available and the whole property is generally counted rather than half. Section 2040(b) is the carve-out and it reaches spouses only: where the decedent and their spouse are the only joint tenants, exactly half is included regardless of who paid. Full inclusion is not purely bad news, because property included in a decedent's gross estate is generally revalued to its date-of-death value for income tax purposes under section 1014, so the survivor's basis follows the inclusion.
The arrangement is efficient and inflexible, and the inflexibility is the risk. A joint tenancy set up between two people works exactly as intended so long as circumstances do not change. It does not adjust for a beneficiary who later needs protecting, it does not accommodate unequal contributions, it cannot be revised by the will that revises everything else, and where several children are added as joint tenants the whole property lands with whichever of them lives longest, regardless of what the parent intended for the others. Where a house is the main asset, the tools designed for the job, which include transfer-on-death deeds in states that allow them and trusts, do the same probate avoidance without handing over ownership during life.
