Introduction

Globally, charitable giving satisfies important needs for individuals in the society including the provision of resources such as blood, organs, time, and money. According to the CAF World Giving Index 2021, while some forms of giving are on the rise (e.g., helping a stranger, donating money), others (e.g., volunteering time) have reduced [1]. Charitable organisations are dependent on people giving to promote initiatives that reduce poverty, advance education, aid health services, and increase societal benefit. In addition to assisting the recipients and charitable organisations, giving has been shown to benefit the donor also (e.g., physical and mental health benefits from donating time) [2,3]. Research on donation behaviour has established demographic determinants of charitable giving; for example, donors tend to be married, well-educated, and of an older age [4]. However, to promote public donations, charities benefit from an understanding of the psychological determinants of charitable donation behaviours.

While some authors view charitable giving as relating to financial donations [5], other approaches are broader in their terminology and include helping others and volunteering in addition to donating money [1]. For the scope of the present study, the review will focus on a broad definition of charitable giving as the voluntary donation of one’s personal resources (time, services, or assets/goods) to charity that provides benefits to others [6].

Researchers have often drawn on multidisciplinary approaches to understand giving. For instance, in their seminal work of people’s charitable financial donations, Bekkers and Wiepking [5] encompassed perspectives including economics, psychology, sociology, and biology and proposed eight mechanisms driving monetary donation (e.g., costs and benefits, altruism, values, and efficacy). Measures have been developed reflecting major underlying motives (e.g., trust, altruism, tax benefits) for donating money to charities [7]. In relation to predicting people’s giving behaviours, there is a number of proposed constructs and models to encapsulate the underlying psychological determinants of people’s giving decisions. Some commonly identified psychological constructs proposed to impact on people’s giving decisions include empathy [8], altruism [5], warm glow [9], social norms [10], and self/role identity [11]. For more comprehensive models of psychological determinants, examples include the Volunteers Function Index (VFI) developed by Clary and Snyder [12] to represent the underlying motivations people have for volunteering time. Other models of volunteering consider also process and structure characteristics and different types of volunteering, including the Volunteer Process Model and Episodic Volunteer Engagement and Retention Model [13,14]. Across some charitable giving behaviours (e.g., volunteering time, blood donation), distinctions are made for the purpose of conceptualising psychological determinants in different stages of the donation ‘career’, such as the initiation stage of giving. In the context of behaviours where a person’s willingness may be a more appropriate outcome than behavioural enactment (e.g., organ donation upon death), the Prototype Willingness Model [15] has been used to gauge people’s openness to give rather than actual donation.

One theoretical framework that has been examined across a wide variety of donation behaviours and is one of the major models applied in the field of psychological determinants of charitable giving is the Theory of Planned Behaviour. The Theory of Planned Behaviour [16] offers a parsimonious explanation of behaviour with intention conceptualised as the proximal determinant of people’s behavioural decisions. The determinants of intention are favourable/unfavourable evaluations of the behaviour (attitude), perceptions of normative pressure to perform the behaviour (subjective norm), and perceived ability/efficacy of performing the behaviour (perceived behavioural control; PBC). When PBC represents actual control over a target behaviour if it is not completely volitional, PBC directly influences behaviour. Attitude, subjective norm, and PBC are belief based with behavioural beliefs (advantages and disadvantages of behavioural performance) underlying attitude, normative beliefs (approval of specific referents for behavioural performance) underpinning subjective norm, and control beliefs (barriers and facilitators of behavioural performance) as the determinants of PBC.

With its inclusion of PBC, the TPB is an extension of the Theory of Reasoned Action (TRA) [17] which focused on the role of attitude and subjective norm on intention, and intention on behaviour. Despite criticisms of the TPB in the health domain [18] focusing on a lack of predictive validity of the standard TPB constructs and the lack of effectiveness of the TPB to provide practitioners with useful behavioural interventions which Ajzen [19] subsequently refuted, the model has garnered much evidence over time, with meta-analytic support across general behavioural domains [20,21].

The TPB is open to the inclusion of additional constructs provided they can be conceived as causal factors, add independent variance over and above the standard predictors, and can be applied to a range of behaviours in the interest of parsimony [16]. Arguably the most consistent addition in TPB examinations of charitable giving behaviour assesses a person’s sense of moral obligation to assist. Based on these and other findings in areas comprising a moral component (e.g., road safety and other decisions with legal and moral ramifications), researchers have argued for the inclusion of a moral/personal norm in the TPB [22]. Moral/personal norm reflects internalised rules or values with moral overtones [23] and engaging (or not engaging) in the target behaviour will, thus, result in self-approval or disapproval. Moral norm predicts additional variance in intention over and above the standard TPB predictors and is relevant across a range of behaviours involving an ethical/moral component [23,24]. The intention to donate blood is moderately associated with moral norm according to a previous meta-analysis [25]. As charitable giving results in significant consequences for the welfare and benefit of other people and groups of people, a sense of moral obligation is likely influential in decisions to engage in the behaviour. As it has been recognised independently as a useful addition to the TPB in previous meta-analytic studies, the construct of moral norm will be included also in this systematic synthesis.

Although a large body of literature has applied the TRA/TPB to understand charitable giving, there is a lack of a systematic synthesis and meta-analysis which enables evaluation of the consistency (or reasons for inconsistency) of results and provides more accurate estimates of TPB associations in this context. The only exception is a systematic review and meta-analysis of blood donation intention and behaviour; this study highlighted the role of attitude and PBC in blood donation intention and behaviour [25]. A review with a broader scope for donation behaviours is of interest to establish whether the strong associations identified in the review of blood donation are equally strong across other charitable giving domains (e.g., organs, money) and whether there is an overarching set of strong TPB associations shared across charitable behaviour types or stronger associations unique to one behavioural domain. Previous research has found high correlations between people giving both time and money [26,27] without as many examinations of whether the determinants of people’s giving across domains are of equal strength. Therefore, this review will examine one of the predominant social psychological models commonly employed to understand and predict people’s giving behaviours in the TPB. Of note, the model has been used by researchers to predict multiple charitable giving behaviours simultaneously [28].

Evaluating the contribution of more general decision-making models is this context is timely given critiques of the utility and validity of the TPB to represent adequately the variability in people’s behavioural decisions [18] and researchers’ efforts to propose models reflecting people’s decisions targeting specific donation behaviours [14]. Consequently, it is of benefit to researchers and practitioners alike to establish the extent to which parsimonious, general models of decision making such as the TPB are efficacious to represent the main determinants of people’s giving behaviours across a range of donation contexts. In this vein, clarity may be provided to identify whether there is sufficient evidence to support a general model of people’s charitable giving across distinct giving domains or if efforts are needed to continue to develop tailored predictive models unique to separate giving behaviours. Recognition of the key constructs of people’s giving behaviours allows practitioners from charitable organisations to focus recruitment and retention strategies on the most relevant underlying cognitions to promote giving intentions and behaviours.

The overall aim of this study, then, is to systematically review studies that have adopted a TPB approach to examine people’s giving intentions and behaviours and test the utility of the model as a parsimonious representation of the determinants of people’s giving decisions. The objectives of this systematic review of the charitable donation literature are to: (1) synthesise through meta-analysis TPB relationships, incorporating measures of moral/personal norm; (2) test the predictive utility of the TPB, with the addition of moral norm, within the context of charitable donations; and (3) explore relevant sample and methodological moderators of these associations. Specifically, seven moderators were considered. Sample characteristics of age, participant gender, and sample type (i.e., student status) were examined consistent with previous TPB meta-analyses examining sample-based differences [25,29]. These analyses remained exploratory in nature due to previously reported differences across types of donation behaviours. A potentially important methodological moderator to consider was the degree of specificity in the operationalisation of the target behaviour [30]. In the context of the TPB, the principles of Target, Action, Context, and Time (TACT) [31] were formulated to ensure alignment between the theoretical concept and measured outcome and to guide questionnaire construction. The principles promote specifying who performs the behaviour (Target), what the target behaviour is (Action), where or how the behaviour is performed (Context), and when the behaviour is performed (Time). In the current review, studies that adhered to the TACT principles were expected to render stronger TPB associations. Additional methodological considerations of attrition rates and follow-up lengths were investigated also, with higher attrition rates and shorter follow-up periods for measuring prospective behaviour expected to result in stronger associations. Finally, given the range of charitable donation behaviours covered, the type of donation behaviour was examined in an explorative manner.