Online Appendix to “The Spread of Secrecy: Covert Military Alliances and Portfolio Consistency” Raymond Kuo This online appendix for “The Spread of Secrecy: Covert Military Alliances and Portfolio Consistency” contains: 1. An explanation for the paper’s choice of unit of analysis; 2. Evidence for the claim that partial and full secrecy should be treated synony- mously in the statistical analysis; 3. A description of Klier and McMillen (2005)’s spatial logit model; 4. The calculations underlying Altonji et al’s robustness check for the strength of omitted variables; 5. Additional statistical test: A two-stage test for selection into alliances; 6. Additional statistical test: The more public alliances you have, the less you will create a secret one; 7. Additional statistical test: Joining a subsequent public alliance increases secret alliance failure; and 8. Additional statistical test: A hegemon’s secret pact induces secrecy in other states’ military alliances. 1