This paper investigates the mechanism of the Journal Impact Factor (JIF). Although created as a journal selection tool the indicator is probably the central quantitative indicator for measuring journal quality. The focus is journal self-citations as the treatment of these in analyses and evaluations is highly disputed. The role of self-citations (both self-citing rate and self-cited rate) is investigated on a larger scale in this analysis in order to achieve statistical reliable material that can further qualify that discussion. Some of the hypotheses concerning journal self-citations are supported by the results and some are not.