How common is self-citation in scholarly publication and does the practice vary by gender? Using novel methods and a dataset of 1.5 million research papers in the scholarly database JSTOR published between 1779-2011, we find that nearly 10% of references are self-citations by a paper's authors. We further find that over the years between 1779-2011, men cite their own papers 56% more than women do. In the last two decades of our data, men self-cite 70% more than women. Women are also more than ten percentage points more likely than men to not cite their own previous work at all. Despite increased representation of women in academia, this gender gap in self-citation rates has remained stable over the last 50 years. We break down self-citation patterns by academic field and number of authors, and comment on potential mechanisms behind these observations. These findings have important implications for scholarly visibility and likely consequences for academic careers.