Google "mental illness" and "affected parties" or "affected family". You might get some helpful hits. This is a crucially important and , in my view, under researched field. Good luck. Best wishes Paul
Many thanks for your answer! I´m still curious though of your knowledge of studies where next of kin to mentally ill persons are interviewed. Likewise I also would appreciate your thoughts of the possible benefits of that kind of studies.
There is a semi-structured interview called "Camberwell Family Interview" to assess the environmental factors of family members (carers) attitude on the prognosis and predict the relapse rate of patients with schizophrenia. But not directly related to decision making competence. Check the key word "Expressed Emotion" and Julian Leff & Christian Vaughn. These information may help you.
If you let go of the interview method, you could focus on the history of the issue of mental competence, which mostly used in forensic psychiatry, but would shed light on your question. See for example the book by Daniel Robinson Wild Beasts and Idle Humours The Insanity Defense from Antiquity to the Present (Harvard University Press, 1998). Here is the summary:
How does the law regard and define mental incompetence, when faced with the problem of meting out justice? To what extent has the law relied on extra-legal authorities—be they religious or scientific—to frame its own categories of mental incompetence and madness? Wild Beasts and Idle Humours takes us on an illuminating journey through the changing historical landscape of human nature and offers an unprecedented look at the legal conceptions of insanity from the pre-classical Greek world to the present. Although actual trial records are either totally lacking or incomplete until the eighteenth century, there are other sources from which the insanity defenses can be constructed.
In this book Daniel N. Robinson, a distinguished historian of psychology, pores over centuries of written law, statements by legal commentators, summaries of crimes, and punishments, to glean from these sources an understanding of epochal views of responsibility and competence. From the Greek phrenesis to the Roman notions of furiosus and non compos mentis, from the seventeenth-century witch trials to today’s interpretation of mens rea, Robinson takes us through history and provides the intricate story of how the insanity defense has been construed as a meeting point of the law and those professions that chart human behavior and conduct: namely religion, medicine, and psychology. The result is a rare historical account of “insanity” within Western civilization.
Wild Beasts and Idle Humours will be essential reading for anyone interested in the evolution of thinking not merely about legal insanity but about such core concepts as responsibility, fitness for the rule of law, competence to enter into contracts and covenants, the role of punishments, and the place of experts within the overall juridical context.