Is a private email merely a leak that hasn’t happened yet? It is starting to seem that way after the number of hacking scandals in recent years. We still don’t know the culprit behind the recent hack of the Democratic National Committee’s computers, although President Barack Obama has tied Russia to the operation. The theft seems too sophisticated to blame on a pimply teenager in a bedroom.

But the fact that the reason remains a mystery (was it to help Donald Trump, embarrass the U.S. or settle some private score?) highlights a longstanding difficulty in plugging leaks: People divulge secrets for all sorts of reasons—from the vindictive to the virtuous, and everything in between.

The most popular category is sheer ineptitude. The Romans discovered that it was all too easy for a secret papyrus to end up in the wrong hands. In 63 B.C., Roman Senator Lucius Sergius Catilina, known as Catiline, almost succeeded in overthrowing the political establishment in a violent coup. But his co-conspirators allowed unvetted messengers to carry highly incriminating letters that were intercepted by Senate agents. The coup fizzled out as soon as the letters were made public.

Other famous victims of self-incrimination include Mary, Queen of Scots (1542-87), whose letters approving a plot to assassinate Elizabeth I were easily deciphered by Elizabeth’s spies, and Thomas Hutchinson (1711-80), the royal governor of Massachusetts whose anticolonist musings passed from Benjamin Franklin to Samuel Adams to a furious American public. The former lost her head; the latter helped Britain lose 13 colonies.

Leaks don’t only come from careless plotters, of course. The tradition of whistleblowing—the so-called virtuous leak—may have started in the fifth century B.C. in Sparta, when Gen. Pausanias started a traitorous correspondence with the enemy Persians. Hoping to prevent his plans from leaking, Pausanias’s letters always included the instruction to kill the messenger upon receipt. The scheme failed after a suspicious messenger opened his letter and, on reading the instruction, promptly gave up the general to the authorities.

The idea of holding the great and powerful to account received a boost in the 18th century when a German professor named August Ludwig von Schlözer decided that secrecy generated incompetence (or worse). Until the Prussian authorities shut him down in 1795, von Schlözer sought to publish every piece of information and official record that came his way in his journal, the State Reports.

But for every public-minded von Schlözer—or, for that matter, Daniel Ellsberg, the former Rand Corp. military analyst who in 1971 leaked the Pentagon Papers—there’s a self-important Charles Marvin. A disgruntled clerk in the British Foreign Office, Marvin sold a copy of the secret 1878 Anglo-Russian Treaty to the press within hours of its signing. The ensuing brouhaha helped no one except Germany, which was trying to isolate Britain ahead of the Congress of Berlin, where the great powers of Europe met to decide the fate of the Balkans. But at Marvin’s subsequent trial, his lawyer offered a novel defense: The Foreign Office regularly leaked its own documents and his client was merely being entrepreneurial. The magistrate threw out the case, suggesting a wide skepticism about governments and leaks even back then.

Yet the self-serving Marvin leak—like so many others in its wake—wasn’t a victimless crime. However full of unpalatable compromises, the treaty was built on the truth that an imperfect peace is often better than all-out war. No matter what the Julian Assanges of the world may say, the adage “Finders keepers, losers weepers” works poorly when lives are at stake.