The variables investment, benefit, and yield were defined to study the influence of journal self-citations on the impact factor. Investment represents the share of journal self-citations that contribute to the impact factor. Benefit is defined as the ratio of journal impact factor including self-citations to journal impact factor without self-citations. Yield is the relationship between benefit and investment. I selected all journals included in 2008 in the Science Citation Index version of Journal Citation Reports. After deleting 482 records for reasons to be explained, I used a final set of 6,138 journals to study the distribution of the variables defined above. The distribution of benefit differed from the distribution of investment and yield. The top 20-ranked journals were not the same for all three variables. The yield of self-citations on the journal impact factor was, in general, very modest.