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.