In the article the author presents the principles of plain language and behavioural economy in official communication. She discusses a study whose aim was to verify whether the effectiveness of behavioural economy used in official letters created for the purpose of an experiment conducted by the World Bank and the National Revenue Administration in Poland in 2016 (Hernandez et al. 2017) was linked to the principles of plain language. In addition, she compares the results of the World Bank’s experiment with the results of a (qualitative and quantitative) linguistic analysis carried out in the presented article. The author seeks to verify whether letters regarded by the World Bank as the most effective in terms of tax collection will also be those best conforming to the rules of plain language. The hypothesis formulated in the article assumes that the letters written for the purpose of the experiment have varying levels of difficulty, which means — contrary to the experimenters’ assumptions — that they differ in terms of not only their persuasion technique (the so-called behavioural insight), but also style. If the hypothesis were to be confirmed, this would obviously undermine the conclusions of the experiment.