They settled on seven deadly sins to parallel the seven virtues.
I believe it was the other way around, the seven virtues were created to parallel the seven deadly sins. In any case seven was a number with religious significance, e.g., seven days of the week.
There's a couple different lists of seven virtues floating around in Christian tradition. One was created to parallel the seven deadly sins, and works well in that role but less well as a typology of virtue. The other's a slightly clunky melding of the New Testament virtues of faith, hope, and love (or charity) and the much older cardinal virtues (originally Platonic) of prudence, justice, temperance, and courage (or fortitude). That works fairly well as a typology of Christian virtue despite some overlap, but does a much worse job of paralleling the deadly sins.
So I was reading the list of http://en.wikipedia.org/wiki/Seven_deadly_sins and I was impressed with the list (seeing how many of these sins are what ultimately bring down many major historical figures). I also recognize how many of these sins were responsible for some of my major setbacks in life, and am thinking of creative ways to reduce their effects (by putting value on things that don't involve any of those sins).
I'm curious: to what extent do the "seven deadly sins" cover the most common reasons why people engage in self-defeating behavior? Are there any major omissions in the list of "seven deadly sins"? If you were to make a list of "X deadly sins", which sins would you include?
As examples: should excessive guilt be counted as a sin? Should stupidity be counted as a sin? What about being excessively "autistic"?
Which of the "Seven deadly sins" do you think are most applicable to LessWrong posters? To what extent are they responsible for akrasia?