Some Web sites try to thwart the attackers by freezing an account for a certain period of time if too many incorrect passwords are typed. But experts say that the hackers simply learn to trick the system, by making guesses at an acceptable rate, for instance.

To improve security, some Web sites are forcing users to mix letters, numbers and even symbols in their passwords. Others, like Twitter, prevent people from picking common passwords.

Still, researchers say, social networking and entertainment Web sites often try to make life simpler for their users and are reluctant to put too many controls in place.

Even commercial sites like eBay must weigh the consequences of freezing accounts, since a hacker could, say, try to win an auction by freezing the accounts of other bidders.

Overusing simple passwords is not a new phenomenon. A similar survey examined computer passwords used in the mid-1990s and found that the most popular ones at that time were “12345,” “abc123” and “password.”

Why do so many people continue to choose easy-to-guess passwords, despite so many warnings about the risks?

Security experts suggest that we are simply overwhelmed by the sheer number of things we have to remember in this digital age.

“Nowadays, we have to keep probably 10 times as many passwords in our head as we did 10 years ago,” said Jeff Moss, who founded a popular hacking conference and is now on the Homeland Security Advisory Council. “Voice mail passwords, A.T.M. PINs and Internet passwords — it’s so hard to keep track of.”

In the idealized world championed by security specialists, people would have different passwords for every Web site they visit and store them in their head or, if absolutely necessary, on a piece of paper.

But bowing to the reality of our overcrowded brains, the experts suggest that everyone choose at least two different passwords — a complex one for Web sites were security is vital, such as banks and e-mail, and a simpler one for places where the stakes are lower, such as social networking and entertainment sites.

Mr. Moss relies on passwords at least 12 characters long, figuring that those make him a more difficult target than the millions of people who choose five- and six-character passwords.

“It’s like the joke where the hikers run into a bear in the forest, and the hiker that survives is the one who outruns his buddy,” Mr. Moss said. “You just want to run that bit faster.”

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