1. Definitions.

Adults	Patients who were ≥ 18 years old. If some studies had an age range from, for example, 16 years upwards, and a majority of participants were over 18 years of age, we will include these study participants as adults
Children and adolescents	Children (from birth to 18 years) were considered as a separate group of participants, as they have: • different patterns of psychopathology/mental disorders; and • different help‐seeking behaviours that would, therefore, require different interventions, in different settings (e.g. schools) and a different approach to care (e.g. worker interventions such as teacher‐led interventions)
Promotion	Promotion is an approach aimed at strengthening positive aspects of mental health and psychosocial well‐being; it includes, for example, components to foster pro‐social behavior, self‐esteem, positive coping with stress, and decision‐making capacity (National Academies of Sciences 2019; WHO 2014). Prevention is an approach aimed at reducing the likelihood of future disorder within the general population or among people who are identified as being at risk for developing a full‐blown disorder (Eaton 2012; Tol 2015)
Universal prevention	Universal prevention includes strategies that can be offered to the whole population, based on evidence that prevention strategies are likely to provide some benefit to all (i.e. reduce the probability of a disorder), which clearly outweighs the costs and risks of negative consequences. Examples of common universal prevention interventions include: community‐wide provision of information on the negative effects of alcohol misuse; protection against human rights violations in the whole population (e.g. community mobilisation to reduce gender‐based violence); and community‐wide efforts to improve livelihood as a key protective factor for mental health (e.g. working on lifting restrictions of movement and employment for everyone in a refugee camp)
Selective prevention	Selective prevention refers to strategies that are targeted to subpopulations identified as being at elevated risk for a disorder; it includes: support for children whose parents have a mental illness; strengthening of community networks for vulnerable individuals by activating social networks and supportive communication; and stress management training in communities affected by chronic poverty.
Indicated prevention	Indicated prevention includes strategies that are targeted to individuals who are identified (or individually screened) as having increased vulnerability for a disorder based on some individual assessment. These interventions include: mentoring programmes aimed at teachers and caregivers of children with behavioural problems; and prevention of postnatal depression in women with heightened levels of prenatal symptoms (Institute of Medicine 2009). These interventions may be delivered at an individual or group level. They include antenatal and postnatal classes, parenthood classes, and continuity of care (home visits, follow‐ups)
First‐level care, primary care, and community care	First‐level contact with formal health services consists of community‐based interventions or primary care interventions (or both), on their own or attached to hospital settings, provided they had no specialist input apart from supervision (modified from Wiley‐Exley 2007). This would include promotion or prevention programmes in outpatient clinics or primary care practices. This would not include programmes in hospitals unless these programmes were providing prevention interventions to outpatients. Community programmes involve detection of mental disorders in all age groups, often done outside the health facility, for example, through school, training, and other community settings
Low‐ and middle‐income country (LMIC)	Any country that has ever been an LMIC, as defined by the World Bank lists of LMICs
Primary care health workers (PHWs)	Health workers who are not specialising in mental disorders or have not received in‐depth professional specialist training in this clinical area. They work in primary care centres or in the community. These individuals include doctors, nurses, auxiliary nurses, lay health workers, and allied health personnel such as social workers and occupational therapists. This category does not include professional specialist health workers such as psychiatrists, psychiatric nurses, or mental health social workers. For inclusion, PHWs received some training in mental conditions (in the control group or in the intervention group), but this would not constitute a professional category. Study authors made a judgement of what constitutes ’some training’. Examples of ’some training’ may include an undergraduate module or a short course in mental health
Community workers (CWs)	People involved as community‐level workers but who are not within the health sector, as many people, particularly adolescents and young adults, have limited contact with health workers. This category includes teachers/trainers/support workers from schools and colleges, along with other volunteers or workers within community‐based networks or non‐governmental organisations. These CWs have an important role, particularly in promotion of mental health and detection of mental disorders (Patel 2007a; Patel 2008). We excluded from this review studies that looked at informal care provided by family members or that extended care only to members of their own family (i.e. who were unavailable to other members of the community). As was previously highlighted in Lewin’s Cochrane Review, “these interventions are qualitatively different from other LHW [lay health worker] interventions included in this review given that parents or spouses have an established close relationship with those receiving care, which could affect the process and effects of the intervention” (Lewin 2010)
Primary‐level workers (PWs)	Broad term to encompass both CWs and PHWs