Proper execution of a legal instrument requires that the person signing have sufficient mental “capacity” to understand the implications of the document. While most people speak of legal “capacity” or “competence” as a rigid black line–either the person has it or doesn’t–in fact it can be quite variable depending on the person’s abilities and the function for which capacity is required.

One side of the capacity equation involves the client’s abilities, which may change from day to day (or even during the day), depending on the course of the illness, fatigue and the effects of medication. On the other side, greater understanding is required for some legal activities than for others. For instance, the capacity required for entering into a contract is higher than that required to execute a will.

As stated by the New York Court of Appeals (In re Estate of Kumstar, 66 N.Y.2d 691, 692 (N.Y. 1985)) in a dispute regarding testamentary capacity, the New York courts “must look to the following factors: (1) whether she understood the nature and consequences of executing a will; (2) whether she knew the nature and extent of the property she was disposing of; and (3) whether she knew those who would be considered the natural objects of her bounty and her relations with them” (Matter of Slade, 106 AD2d 914, 915; see also, Matter of Delmar, 243 NY 7).”

This is a relatively “low threshold,” meaning that signing a will does not require a great deal of capacity. The fact that the next day the testator does not remember the will signing and is not sufficiently “with it” to execute a will then does not invalidate the will if he understood it when he signed it. In contrast, the threshold for entering into contracts is fairly high.

The standards for entering into a contract are different because the individual must know not only the nature of her property and the person with whom she is dealing, but also the broader context of the market in which she is agreeing to buy or sell services or property. Competency to enter into a contract presupposes something more than a transient surge of lucidity. It requires the ability to comprehend the nature and quality of the transaction, together with an understanding of what is “going on,” but an ability to comprehend the nature and quality of the transaction, together with an understanding of its significance and consequences.

As a practical matter, in assessing a client’s capacity to execute a legal document, attorneys generally ask the question, “Is anyone going to challenge this transaction?” If a client of questionable capacity executes a will giving her estate to her husband, and then to her children if her husband does not survive her, it’s unlikely to be challenged. If, on the other hand, she executes a will giving her estate entirely to one daughter with nothing passing to her other children, the attorney must be more certain of being able to prove the client’s capacity.

While the standards may seem clear, applying them to particular clients may be difficult. The fact that a client does not know the year or the name of the President may mean she does not have capacity to enter into a contract, but not necessarily that she can’t execute a will or durable power of attorney. The determination mixes medical, psychological and legal judgments. It must be made by the attorney (or a judge, in the case of guardianship or probate determinations) based on information gleaned by the attorney in interactions with the client, from other sources such as family members and social workers, and, if necessary, from medical personnel. Doctors and psychiatrists cannot themselves make a determination as to whether an individual has capacity to undertake a legal commitment. But they can provide a professional evaluation of the person that will help an attorney or a court make this decision.

Because you need a third-party to assess capacity and because you need to be certain that the formal legal requirements are followed, it can be risky to prepare and execute legal documents on your own without representation by an attorney.