From CryptoDox, The Online Encyclopedia on Cryptography and Information Security
The increasing prominence of information technology and the Internet in employment, finance, recreation and other aspects of people's lives, and the ensuing introduction of secure transaction technology, has led to people accumulating a proliferation of accounts and passwords. According to British online-security consultant NTA Monitor the typical intensive computer user has 21 accounts that require a password.
Aside from contributing to stress password fatigue may encourage people to adopt habits that reduce the security of their protected information. For example, an account holder might use the same password for several different accounts, deliberately choose easy to remember passwords that are vulnerable to cracking, or rely on written records of their passwords.