The main goal of the dissertation study is the theoretical substantiation of the essence, evolution, and factors of transformational differentiation of the tax systems of the European Union member states, the development of methodological tools for quantitative assessment of their structural dynamics, taking into account integration processes, as well as the identification of patterns in the formation of effective models of tax regulation for the adaptation of the Ukrainian tax system in the context of its candidacy for the EU membership.
The dissertation explores the evolution as well as functional structure of tax systems, identifying their multidimensional institutional and legal mechanism of state regulation, which combines fiscal, economic, and social functions. It is determined that the development of tax policy concepts from classical economic approaches to supply-side economic models forms the basis for considering taxes not only as a means of accumulating revenue, but also as an instrument for stimulating economic activity, optimizing aggregate supply and stabilizing employment.
It has been revealed that in the context of globalization, digitalization, and integration transformations, the tax regulation system is acquiring new dimensions that determine its adaptability, transparency, administrative efficiency, and fairness in the distribution of the tax burden. At the same time, the structure, hierarchy, and interrelationships of the system components form the context for assessing fiscal efficiency, coordinating political priorities and optimizing public management strategies.
The mission of the EU member states’ the tax systems in ensuring economic growth and social justice has been studied. Stimulating innovation and green transformations is implemented as a strategic lever, transforming from a purely fiscal instrument into a comprehensive mechanism for macroeconomic regulation and ensuring sustainable development.
The asynchronous disposition of the tax systems of the EU member states is analyzed as a persistent form of fiscal asymmetry, manifested in significant variability in the tax burden (27–46% of GDP in 2023, average 40%), revenue structure, administrative efficiency, and progressive taxation. This unevenness determines unequal fiscal behavior and forms a dichotomy of economic development between the Union’s «centre» and periphery.
The Transitional Tax System Differentiation Index (TTSDI/TTSDI-UA) has been developed as an independent analytical tool for quantitative measurement of institutional and structural asynchrony of tax reforms in the EU member states. The index combines indicators of the digitization of public services, tax administration dynamics, changes in the tax structure, and the intensity of reforms with a proprietary weighting system adapted to OECD methodological approaches, which made it possible to identify clusters of transformational maturity, quantitatively record the unevenness of reforms, empirically identify groups of countries with high, medium, and low levels of transformational activity, as well as form an analytical basis for assessing fiscal convergence and the adaptation of candidate countries’ tax systems to the EU standards.
An assessment of the current state of the Ukrainian tax system was carried out based on the adapted TTSDI-UA transformation differentiation index, which revealed a moderate and volatile level of transformation changes with a predominance of indirect taxes, a high level of shadow economy, the presence of tax gaps, and fragmented reforms in the context of a war economy.
There have been developed the forecast scenarios for the transformation of the Ukrainian tax system for 2025–2027 by using regression models of income with 95% confidence intervals and taking into account military uncertainty.
The directions for structural reform of the Ukrainian tax system based on the EU practices have been substantiated. These include improving the quality of the tax base by inventorying tax breaks and reducing leakage, scaling up digital administration tools, institutionalizing short reform cycles with transparent KPIs, and taking into account environmental taxation (CBAM).
Keywords: tax system, the EU, the EU member states, international economic integration, fiscal policy, tax burden, competition, competitiveness, tax regulation, digitalization.
