This study presents an improvement of the King-Fullerton framework for calculating the marginal effective tax rate (METR) for active owners of closely held corporations in a dual income tax system with income splitting rules. The original King and Fullerton model was not modeled to incorporate this type of rule, making it difficult to fully calculate the METR in countries with a dual income tax. The model developed in this paper offers a more general method with less restrictive assumptions than earlier analyses of a dual income tax system. To illustrate the results, the model is applied to the Swedish dual income tax system and is contrasted with earlier works, revealing that the METR for new share issues may have been overestimated in earlier calculations.
Our model provides a more comprehensive and flexible toolbox for calculating the METR in a dual income tax system with income splitting rules and improves the possibilities to evaluate how changes in the regulatory framework may affect the METR and the neutrality between investment opportunities. As such, the results are relevant not only for Sweden but also for other countries that have implemented a dual income tax system or are considering doing so.
