In South African law, the determination of mental capacity is governed by the Mental Health Care Act, 2002 (Act No. 17 of 2002) and the Promotion of Equality and Prevention of Unfair Discrimination Act, 2000 (Act No. 4 of 2000). The assessment of mental capacity is crucial in various legal contexts, such as making decisions about the person's ability to manage their own affairs, consent to medical treatment, or participate in legal proceedings.

The Mental Health Care Act provides the legal framework for the admission and treatment of individuals with mental disorders, and it outlines the process for determining a person's mental capacity. According to the Act, a person is deemed to lack capacity if they are unable to appreciate the nature and consequences (effect) of their decision. The general rule is that a person must have the mental capability to understand the nature and consequences of the document they are signing. This means that they should be able to comprehend what the document is, what it means for them, and what the potential outcomes of signing it could be.  Regarding the legal capacity to sign documents in South Africa, If someone has a severe mental illness, dementia, or other conditions affecting their cognitive abilities, they may be found to lack the mental capacity to sign legal documents. The key question is whether they can understand the implications of the document at the time they sign it, not necessarily whether they can remember it later. When questions arise about a person's mental capacity, a medical or psychiatric assessment is typically conducted by a qualified medical practitioner or a psychiatrist. This assessment evaluates the individual's mental state, cognitive abilities, and understanding of the relevant decision they need to make. In legal proceedings, such as those involving contracts, property matters, or testamentary capacity (the capacity to make a valid will), the court may rely on expert testimony and medical reports to determine whether the individual had the necessary mental capacity at the time the decision was made.

It is important to note that the determination of mental capacity is context-specific and decision-specific. In other words, an individual may be considered to have capacity for some decisions but lack capacity for others, depending on the complexity of the decision and their mental state at the time. Moreover, South African law recognizes that individuals with mental disabilities have the right to be supported and accommodated in making decisions. There is a move away from a paternalistic approach that solely focuses on determining incapacity and rather towards a supported decision-making model that respects an individual's autonomy and seeks to enable their participation in decision-making to the greatest extent possible.

It's essential to consult legal professionals familiar with South African law for precise advice on specific cases involving mental capacity. Laws and regulations may change, so staying up-to-date with the latest legal developments is crucial.