In developing countries, tax revenues are essential for financing public goods but they are afflicted by an extensive shadow economy and extensive tax evasion. Among taxes, personal income taxation is one of the most commonly used and oldest instruments of fiscal policy. In developing countries, a substantial part of their tax gap is due to non-filers and potential taxpayers in the informal sector who not only evade taxes but also fail to even register as taxpayers. The problem of tax return non-filing in Pakistan results in low tax revenues, which fuels substantial budget deficits, increased foreign and local debts, ever increasing debt servicing, inflation and, ultimately, economic growth is affected. Efforts to enhance tax compliance can be a way to broaden the tax base so that burgeoning concerns regarding tax evasion can be addressed. Therefore, the main objective of this study is to propose and test a conceptual model of mediating mechanism between tax compliance determinants and puzzle of tax compliance behaviour of non-filers in Pakistan. Despite the quality of extensive studies on tax compliance determinants and compliance behaviour, little attention has been paid to understanding the mediating mechanism which is termed the ‘black box’. In order to unlock this black box, I have utilized the theoretical lenses of theory of moral suasion, deterrence theory and the theory of planned behaviour (TPB). According to the theory of moral suasion, an appeal to influence or change certain behaviour is made by institutions or individuals to others. In other words, it is an appeal to others for altruistic behaviour. Based on this notion, it is proposed in the conceptual model that moral suasion enhances attitude and tax morale, which results in improvement in tax compliance behaviour leading to more tax revenues collection to finance public goods. Deterrence is a theory from behavioural psychology for preventing or controlling actions or behaviour through fear of punishment or retribution. Deterrence theory asserts enforcement of land laws through the use of hard measures (penalty and punishment). Based on the goal of deterrence theory, it is proposed that there might be non-filers who do not respond to moral suasion; for such persons, the use of deterrence becomes inevitable. The intention behind the use of moral suasion and deterrence theories is the utilization of the carrot and stick approach by considering the dynamics of human behaviour. The optimal use of both policy instruments can change noncompliant behaviour of non-filers into compliant behaviour. According to TPB, human behaviour is influenced by beliefs. The TPB suggests that attitude, subjective norms and perceived behavioural control change an individual’s behavioural intentions and behaviour. Building on the argument of TPB, it is proposed that the behaviour of individual non-filers should be examined under the lens of TPB and that the impact of belief would change human compliance behaviour leading to a broadening of the tax base. The target population consisted of novel data with the unique idea of using first-hand data mining of individual non-filer persons. Data were collected using the survey method. A total of 1400 questionnaires were circulated, out of which 1359 were returned. After screening, 720 questionnaires were found eligible for data analysis. On the basis of conceptual model and literature, 33 hypotheses were developed and tested by employing multiple regression analysis, check of mediation, reliability analysis and principal component analysis. The results indicate that subjective norms, deterrence, moral persuasion, tax awareness, fairness and tax morale influence attitude towards behaviour (mediating variable) positively and significantly. In addition, self-corruption negatively and significantly influences attitude towards behaviour. On the other hand, no significant impact of tax complexity on attitude towards compliance is found. Furthermore, attitude towards behaviour has positive and significant impact on intention to comply (dependent variable and predictor of tax compliance behaviour). Likewise, subjective norms, deterrence, moral persuasion, tax awareness and fairness influence tax morale (mediating variable) positively and significantly. In addition, a negative and significant impact of tax complexity and self-corruption on attitude towards behaviour is found. Moreover, tax morale has positive and significant impact on tax compliance behaviour. Multiple regression analysis revealed that attitude towards behaviour partially mediates the relationship between subjective norms, moral persuasion, tax awareness, fairness and tax morale with tax compliance behaviour. In addition, attitude towards behaviour fully mediates the relationship between deterrence and self-corruption with tax compliance behaviour, whereas no mediation has been found between tax complexity and tax compliance behaviour. Tax morale partially mediates the relationship between subjective norms, deterrence, moral persuasion, tax awareness and fairness with tax compliance behaviour. In addition, tax morale fully mediates the relationship between tax complexity and self-corruption with tax compliance behaviour. Moreover, isolated positive and significant effects of perceived behavioural control on tax compliance behaviour were found. The findings of this study are largely supported by the literature with a few interesting exceptions. Along with practical policy implications, this study provides a sound theoretical contribution to tax literature with respect to relationship between tax determinants and tax compliance behaviour in the context of individual non-filers in Pakistan. This study uses the lenses of TPB, moral suasion and deterrence theories with attitudinal and tax morale mediating model of determinants and tax compliance behaviour relationships along with the carrot and stick approach. By using the said approach, the study attempts not only to unlock the ‘black box’ of determinants and compliance behaviour, but could also serve as one way for optimum tax compliance level among individual non-filers in Pakistan. The study also provides insights to tax authorities, educational policy makers and tax professionals for understanding tax (non)compliance as a behavioural phenomenon in Pakistan, particularly admitting attitudinal engagement in tax compliance determinants and tax compliance behaviour of individual non-filers, which might help them to devise a supportive tax compliance system.