Major depressive disorder (MDD) is one of the most common mental disorders in older people. There are several biological, psychological, and social factors associated with this disorder.
Mental health disorders are associated with reduced functional capacity and quality of life in the elderly. Major depressive disorder (MDD) is one of the most common mental health disorders in older people 
. The estimated number of people with MDD worldwide in 2015 was 300 million 
. In Portugal, the prevalence of MDD in the population over 65 years old is 7.5–12.6% 
MDD is a clinical condition characterised by a feeling of sadness and loss of interest in activities that were once perceived as enjoyable. To be considered a disorder, rather than just a normal reaction to life events, these symptoms must persist for at least 2 weeks and must be generally accompanied by changes in appetite and sleep patterns, fatigue, difficulty concentrating, indecision, suicidal thoughts, or feelings of worthlessness, helplessness, and despair 
MDD in older people involves the psychological, biological and social domains. There are several associated factors within these three domains that may have started in an earlier stage of life or that may be directly related to the ageing process. These factors may pertain to pathologies or health-related changes such as reduced physical strength, sensory acuity and information-processing speed and social changes such as retirement, widowhood, and loneliness. These changes may lead to the physical and social conditions present in depressive disorders.
MDD in older people is often undervalued and not treated due to its non-specific symptoms, or because it is confused with other comorbidities, such as cardiac pathologies, diabetes mellitus, malignant neoplasms, infections, and major neurocognitive disorders 
. All these conditions may contribute to the onset of MDD in older people, and this disorder also acts in a bidirectional manner on the aforementioned diseases.
In many cases, MDD coexists with other comorbidities (major neurocognitive disorder (MND), etc.) in the same individual. Therefore, differential diagnosis is essential. It also needs to be clarified if MDD is a risk factor for MND, or if it is a phase that may trigger this disease. Conversely, MND may be a risk factor for MDD due to the patient’s behavioural and emotional reactions to MND 
The presence of depressive symptoms that do not yet meet the diagnostic criteria for MDD is nonetheless an indicator of the risk of developing MDD. As such, and to prevent MDD, it is essential to recognise its predictors, give them individual attention, and not regard them as merely consequences of other comorbidities 
. In addition, when determining the presence of these factors in the older population, it is essential to integrate a biopsychosocial perspective to avoid the negative stereotyping of ageing and elderly people as sad and normally depressed.
Within the scope of the literature review in this study, the predictors of MDD can be grouped into sociodemographic, behavioural, health, and life event predictors. The sociodemographic predictors include the following: (i) sex (MDD is more common in women than in men, and this difference persists into old age for many reasons; feelings of loneliness and a low self-perception of health are common among depressed women 
); (ii) age (in most of the previous studies, the increased presence of MDD and depressive symptoms was found in older age groups 
); (iii) education level (individuals with low education levels have a higher risk of developing depressive symptoms; a higher education level is a resource for individuals when faced with stressful situations); (iv) marital status (there are significantly more cases of MDD among divorced and widowed individuals than among married individuals 
; stressful events such as divorce, or the loss of family members and friends, may predispose one to a depressive state 
); and (v) social support, both formal and informal (a multidimensional concept referring to the material and psychological resources to which people have access through their social networks 
; the maintenance of social networks in the elderly provides them with psychological and social well-being 
; low social support can be considered a risk factor for an individual’s health 
As for the behavioural predictors of MDD, they include (i) nutritional status (we must consider the bidirectionality of nutritional status and MDD, as MDD can interfere with nutritional status, debilitating the elderly or making them malnourished, and the debilitated/malnourished state of the elderly can lead to MDD—in many studies, MDD has been correlated with weight loss 
) and (ii) physical exercise (many studies have provided empirical evidence of the relationship between physical exercise and MDD in the elderly; in one study 
, the MDD levels of older people before and after their participation in the Portuguese National Walking and Running Programme, were examined with a 6-month interval and the depressive symptoms showed significant improvements from the pre- to the post-test. In another study 
, the MDD levels of older people who did various types of physical exercise were assessed, and it was found that the exercises reduced their MDD levels).
For the health predictors of MDD, they include (i) chronic diseases such as cardiovascular diseases, diabetes, and hypertension (several studies have shown that chronic disease is the factor most associated with MDD in older people 
, and that quality of life is compromised by the number of chronic diseases that an elderly person has, contributing to MDD; conversely, MDD also often worsens the clinical condition) and (ii) cognition (several studies have shown that functional capacity is positively correlated with an individual’s cognitive impairment 
; older people who show greater cognitive decline also show greater functional impairment 
, and consequently, isolation and MDD).
Finally, with regard to the life event predictors of MDD, the literature has suggested isolation and the consequent emotional state of loneliness as common denominators, generating a bidirectional effect to MDD which may complicate research and make it difficult to determine their cause–effect relationships of MDD 
2. Major Depressive Disorder in Older People
MDD is one of the most frequently observed mental disorders in the elderly 
and has a major impact on both the patient and the caregiver, compromising their quality of life. It may even be a risk factor for many other health conditions, including MND 
. The elderly experience frequent losses of family, social and economic support (retirement, greater physical decline, more physical illnesses, disability, etc.), often leading to isolation and loneliness, which may trigger MDD. However, MDD in older people is seldom recognised 
, and depressive symptoms are often attributed to physiological causes, or regarded as being associated with other comorbidities.