David Petraeus is probably the last person you might have expected to wreck his career with an email scandal.
Petraeus is smart: He graduated in the top five percent of his class at West Point and went on to earn a Ph.D.
Petraeus has self-control: His self-discipline was " legendary," according to Time Magazine.
And Petraeus knows what he's doing: During his time as a four-star general and as director of the CIA, he acquired an intimate knowledge of how easily email can be hacked.
And that's why it's so incredible that even Petraeus did the dumbest thing imaginable when it came to his email: He trusted it with his secrets.
Two limitations of the human mind
The Petraeus fiasco involves a sordid and tawdry " love pentagon" with five very high-level professionals flirting, harassing, investigating, gossiping, sexting and more.
It also involves issues of national security, inter-agency rivalry, legal boundaries around surveillance and more.
But I'm not concerned about any of that. What I care about is Gmail.
This whole mess came to light - and careers and, probably, marriages were ruined, and the government was shamed - because of evidence harvested from Google's Gmail.
Everybody should know -- and CIA directors should definitely know -- that cloud-based email services are not safe places to put things you don't want anyone to see.
In fact, Petraeus used an old trick he probably learned from Al Qaeda militants.
The trick was this: Instead of sending email to his biographer/girlfriend, Petraeus simply kept messages in a draft folder without sending them. Both parties had the email address, and simply logged in and read the drafts.
Well, probably. But the reason Petraeus did such a dumb thing is the same reason why half the celebrities in Hollywood end up with naked cellphone pictures on the Internet, the same reason kids entering the workforce increasingly don't get hired because of some online photo or comment, the same reason countless families argue over things posted or emailed, and the same reason people get fired every day over things they say online.
That reason is a fundamental flaw in human nature, which is that we think our potential audience is the same as our intended audience.
When teenagers post ill-advised pictures or comments on Facebook, they're thinking about a small number of friends reading it. While they're thinking of those people, their brains prevent them from thinking about grandma, or the teacher they friended, or the friend who copies the content and broadcasts it publicly, or the future HR manager vetting job candidates.
The writer H.P. Lovecraft talked about "the inability of the human mind to correlate all its contents." He described the experience of this limitation poetically: "We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far."
It's not that humans are dumb, but that we're single-minded. We're often unable to use the knowledge we have for our own good because when we think about one thing, we forget about other things.
The second flaw in human nature is something economists call "present bias." When the reward is now, but the risk later, we can't help but to embrace the reward and ignore the risk.
"Present bias" is why people get into crippling debt, take dangerous drugs, overeat and voluntarily do other things that cause regret. We do it because the benefit is now and the regret is later.
Email feels now, but email is forever. And so are social media and other online activities.
But they don't have to be
The benefits of self-destruction
A good rule of thumb is to only post or send something online if you would be happy to show it to your mother, children, partner and boss.
But there's a loophole. A category of free services lets you communicate everything else with very low risk.
The handiest solution is "email" that self-destructs, like the taped messages on Mission: Impossible.
The way these work is that you type your message on a website, rather than sending email. The site will send email, not with the message, but with a link.
In some cases, the services will allow the recipients to read the message once, after which time it's deleted. In others, you can set an expiration date.
Note that the "Destructing Message" service has an interesting twist: It doesn't identify the sender. Of course, you can identify yourself in the message, but you don't have to. It's both temporary and anonymous. Some email services, including Gmail, may block incoming mail from Destructing Message.
There's a related type of service that's useful when you want to keep a link private. You paste your link into the service, and set the "expiration date." Then, the service creates a temporary link that leads to the real link.
Note that Dying Links is highly configurable, enabling you to specify a delayed activation, an expiration date and time, and even a maximum number of clicks before it expires. It also shortens URLs.
Sometimes you just want to show someone a picture, but you want to do it securely and privately. In that case, you might try SnapChat.
SnapChat is an iOS and Android app that lets you send pictures from your phone to a list of recipients, who can view the picture only from within the SnapChat application, and only for a maximum of 10 seconds. The sender can even choose to set the time limit to less than 10 seconds.
While the picture is displayed, the screen capture feature on the recipient's phone is disabled.
Are these services unethical?
Some of you might think that these services are unethical and are only for people with shameful secrets to hide.
But I'm not here to lecture you about your ethics. That's your business, not mine.
I'm here to lecture you about being smart when using email, social networks and other online communications media. Whether you use these services or not, you should know they exist, just in case.
Learn from Petraeus's bad example: Start exercising common sense and avoiding the traps of myopic thinking and "present bias." And start taking advantage of some of the free and easy apps that can keep your secrets secret. With these tools, you can enjoy the benefits of online communication without the risks.
You're a smart person (if you're reading my column, you MUST be smart). So don't do something dumb like Petraeus did. Be smart -- and communicate smart.
Mike Elgan writes about technology and tech culture. You can contact Mike and learn more about him at Elgan.com, or subscribe to his free email newsletter, Mike's List. You can also see more articles by Mike Elgan on Computerworld.com.