The world’s population is ageing, and the percentage of older people will continue to rise in the coming years. Whereas the percentage of people aged 65 or higher was 9.1% among the world’s population in 2019, it is estimated that in 2050 older people will account for 15.9%. In Europe and Northern America, by 2050 one in four persons could be aged 65 years or over. Moreover, within that same timeframe, the number of people aged 80 years or over is expected to triple from 143 million in 2019 to 426 million worldwide . Because of the ageing population, the demand for care is also rising. It has been established that, as people age, they suffer from more (chronic) illnesses, needing long-term care. Among people aged 65 years or older, 70% suffer from a chronic disease. In the population aged 75 years or older suffering from a chronic disease, 63% have two or more chronic illnesses . This results in the care volume increasing by 4% every year in the time period 2000–2010, leading to higher overall healthcare costs .
In order to face these rising care demands and healthcare costs, Western governments are steering away from formal care and towards informal care. Consequently, older people are encouraged to stay at home longer instead of moving into nursing homes. In order to make this feasible, such people will have to rely on their direct network of family and friends to ensure informal care . Given the trend of an ageing society and policy reforms, the need for informal care will increase in the coming years.
But how can informal care be defined? Informal care consists of all aid to a person in need of care from someone in his or her direct environment. This also entails less intensive help, help towards members of the household, and help towards institutional residents. Informal care goes further than so-called regular help. Regular help – the aid that may reasonably be expected towards members of the household – means, for instance, the care for children. Examples of informal care activities are emotional support, administrative help, guidance in arranging appointments, transport, domestic and personal care . It should be stressed that informal care is unpaid, results from social rather than professional relations, and entails long-term care for sick family members or friends .
In 2014, more than a third of the European population (34.3%) gave some type of informal care. Most informal carers are aged between 50 and 75 and take care of partners or parents (in law) . The largest group of people in need of informal care are those aged 75 and over .
Even though the need for informal carers is rising, current research reports that the potential support ratio – defined as the number of people of working age (25 to 64 years) per person aged 65 years or over – is rapidly decreasing. By 2050, it is estimated that, for 48 Western countries, the potential support ratio will have more than halved to below two; this means fewer than two potential informal carers per elderly person .
Besides the pressing future shortage of informal carers, Dutch research has shown that almost one in 10 informal carers feels overloaded with the care demand. Even though research also stresses the dual nature of informal caregiving, thus having both negative and positive aspects (e.g. [9, 10]), one in four providing long-term and intensive informal care feels stressed out. Not only do they feel as if the care demand is never lifted from their shoulders, but also they struggle with the upkeep of their own household and thus become overstrained . These figures are alarming, especially as the Dutch healthcare system and its formal care services are advanced relative to other countries.
Caregiver burden can be defined  as ‘a multidimensional response to the negative appraisal and perceived stress resulting from taking care of an ill individual’ (p. 846). Overburdening threatens both the physical and the psychological health of caregivers. It has been shown that overstrained caregivers use more healthcare services and prescribed medication than non-caregivers, indicating a decline in physical health . Moreover, stressed caregivers report more feelings of depression, perceived lack of coping mechanisms, and concerns about their poor quality of life . If this burden is combined with the decreasing number of potential informal carers, it becomes apparent that the caregiver burden needs to be alleviated in order to cope with the future rising demand for informal care.
Research has already been undertaken regarding the determinants of informal caregiver burden. However, most literature reviews have focused on homogeneous diagnostic groups. For instance, a systematic review that focused on caregiver burden in dementia  led to several categories of both patient and caregiver determinants, such as the patient’s need for support, (cognitive) function disorders, and the caregiver’s social functioning, self-efficacy, and coping traits. Other reviews have focused on caring for elderly patients with cancer , amyotrophic lateral sclerosis , Parkinson’s disease , and mental disorders including bipolar disorder  and eating disorders . Caregiver burden may indeed vary across diseases; however, exclusively focusing on specific diseases limits the identification of common determinants of caregiver burden across diseases .
The lack of knowledge on common determinants may hamper the development of effective policies and interventions that aim to relieve caregiver burden. Current interventions generally focus on providing caregivers with both direct and indirect support, such as emotional support, advice on coping, and respite services to reduce the amount of care burden . However, systematic reviews on such interventions show that the effects on caregiver burden are very small or even insignificant [23, 24].
To inform future policy and intervention development, the aim of the present review is to synthesise the literature on the common determinants of caregiver burden in Western countries, to ultimately ensure future continuation of informal care in the home context, and to improve or sustain the quality of life of caregivers and patients alike. The literature review aims to answer the following research question: What are the determinants of caregiver burden among informal carers?
Given its multidimensionality, the caregiver burden construct has many different definitions. According to George and Gwyther , it entails physical, psychological, emotional, financial, and social stressors that individuals experience because of providing care. Depending on the scope of the research and included facets, the caregiver burden definition differs per study .
Moreover, the distinction between objective and subjective burden adds to the complexity of the caregiver burden construct. Montgomery, Gonyea, and Hooyman  state that: ‘objective burden is defined as the extent of disruptions or changes in various aspects of the caregivers’ life and household. Subjective burden is defined as the caregivers’ attitudes toward or emotional reactions to the caregiving experience’ (p. 21). Thus, objective burden entails overt aspects, such as time spent on caregiving and the nature and number of tasks, whereas subjective burden encompasses caregiver perceptions of the care demands and their consequences, such as negative mood states and anxiety.
The multidimensionality of, and the objective–subjective distinction within, caregiver burden are both reflected in the Adapted Stress Model (ASM). The ASM describes the relationships between various categories of determinants that ultimately cause perceived caregiver burden (Fig. 1). Causality between the different constructs is implied from the left side of the ASM to the right, i.e. stressors positively affect caregiver burden, meaning that caregivers experience more stress than non-caregivers.
The ASM is based on stress theories, notably the transactional model of stress and coping  and the stress process model , as well as role theory .
Going from the left side of the model to the right, the ASM allows for the formulation of specific hypotheses, including their rationale.
H1: direct effects of stressors on perceived caregiver burden
Whereas H2, H3, and H4 describe indirect (i.e. mediated) effects of general stressors through role and intrapsychic stressors, it is hypothesised that each class of stressor in the ASM may also directly result in perceived caregiver burden.
H2: general stressors result in an estimated number of hours spent on caregiving
The general stressors are the patient’s cognitive status, problematic behaviour, dependency level, specific illness, and care demands. Together, these five general stressors lead to the caregiver’s primary appraisal of the severity of the stressor , which translates into the estimated number of hours that need to be spent on caregiving.
H3: the estimated number of caregiving hours leads to role conflict and role overload
As a result of the estimated number of caregiving hours, caregivers may experience role conflict and role overload as role stressors. Role conflicts occur when the expectations of the various roles fulfilled by the caregiver become incompatible . Theoretically, this is in line with secondary appraisal in the transactional model of stress and coping, which is the appraisal of one’s resources (e.g. time) to manage the stressor . In the case of informal care, the informal carer role may lead to conflict with the roles of being a parent or being an employee.
Role overload happens when people lack sufficient resources and time to fulfil all obligations linked to their roles . For instance, an increase in the number of caregiving hours may result in role overload when the caregiver feels he or she lacks sufficient hours in the day to complete all tasks associated with every role he or she occupies.
H4: role stressors may lead to intrapsychic stressors
Role conflict and overload may result in role captivity, the feeling of entrapment within the caregiving role and the related experience of self-loss . For instance, caregivers may give up activities related to other roles, such as quality time with their partner, in order to take care of their ill parent. It has been established that role captivity and loss of self can lead to higher feelings of depression and burden [32, 33]. In addition to this situational burden, role and intrapsychic stressors may negatively affect self-esteem and feelings of competence.
H5: coping strategies and social support moderate the relationships between stressors and perceived caregiver burden
Finally, the relationships between stressors and perceived caregiver burden may be moderated by coping strategies and perceived social support. Coping can be defined as the cognitive, emotional, and behavioural efforts to manage the internal and the environmental demands that challenge or exceed someone’s resources . According to Carver, Scheier, and Weintraub , coping efforts can be divided into three categories: problem-focused coping, emotional-focused coping, and dysfunctional coping. Problem-focused coping entails strategies used to solve certain problems, such as thinking about the steps that need to be taken in order to solve the problem. When the caregiver tries to reduce or eliminate negative feelings connected to the role, such as through seeking distraction, this can be considered a type of emotional-focused coping. Lastly, dysfunctional coping happens when people do not accept the problem at hand and try to ignore or even deny reality, and this is positively associated with feelings of anxiety, depression, and perceived burden [35, 36]. In contrast, problem-focused coping does not appear to be related to caregiver burden [36, 37]. Whether emotional-focused coping and burden are related is not consistent in the current literature. However, cases of informal care among palliative cancer patients or people suffering from dementia show that emotional-focused coping leads to a decrease in perceived burden [37, 38].
Perceived social support is the second moderator in the ASM. In general, informal caregivers receiving social support from their own contacts or professionals experience less burden than others . According to Thompson, Futterman, Gallagher-Thompson, Rose, and Lovett , social support that allows caregivers to engage in social interaction for fun and recreation is the most important in decreasing perceived burden. Other forms of social support are informational and emotional support and tangible assistance. Respite care is a special type of social support where social interaction and tangible assistance are combined. Here, the care tasks are temporarily lifted from the informal caregiver’s shoulders in order to alleviate burden. Moreover, respite care should free up time, so caregivers can perform other roles. However, results on the effectiveness of respite care on caregiver burden are heterogeneous .