The laws referred to in this chapter are those affecting adults that are in operation in England and Wales at the time of writing. Similar or equivalent laws exist in most liberal democracies, albeit not necessarily as statutes.

Liaison psychiatrists are frequently referred patients who refuse medical or surgical treatment and who appear to be mentally disturbed. Referrers in the past would usually ask that these patients be detained under the Mental Health Act 1983. Occasionally it was a correct request, usually it was not; thus, liaison psychiatrists need to fully understand the concepts of mental incapacity and best interests and the applicability of the Mental Health Act if the assessment or treatment is not for mental disorder (Feldman, 1998).

In more recent years, after the passage of new statutes, the importance of mental incapacity and the Mental Capacity Act 2005 has filtered through to front-line general hospital staff, who may now ask a more sophisticated question: ‘Will you come and assess the patient's capacity … please?’ Less often is the referrer aware that the liaison psychiatrist needs to know what specific decision it is that the patient appears unable to make, or that it is primarily the referrer's responsibility to make this assessment if they are the treating doctor: but yes, of course, the liaison psychiatrist will be happy to provide a second opinion on the patient's capacity to decide whatever needs to be decided and talk the treating doctor through the legal niceties. It is sometimes necessary to see the patient with the general hospital consultant, as the psychiatrist may not understand the full complexities of the treatment that is being discussed, so will need to observe the treating doctor explain it to their patient.

A sound understanding of mental health law is an essential element of the liaison psychiatrist's knowledge, an ability to assess capacity an essential skill, and a confident approach to these tasks will give psychiatrists the right attitudes of patience, calm and helpfulness with general hospital colleagues, who are often confused and unsure of their legal responsibilities.