First published October 1999. Last modified 03-Dec-2011.
Other signs to look for:
Deviant musical tastes.
Marilyn Manson and "industrial" electronic music are the current chief suspects, but Judas Priest, Deep Purple, AC/DC, The Beatles, Elvis Presley and Chuck Berry have not yet been exonerated. Anything sung in German to a heavy beat sounds like a Hitler Youth recruiting song, which means it probably is.
Excludes alcohol and tobacco, unless the suspect is over the age of 18 (or 21 in some jurisdictions). In some communities, use of alcohol, tobacco or caffeine is always evil. Check your local regulations.
Internet access permits disturbed individuals to find recipes for bombs and poisons. Restricting Net access only to persons over the age of 90 and kept under constant armed guard will drive other persons seeking this subversive information to actually visit public libraries, in which they can be trapped and burned.
Erotic books, videos and computer files are obviously associated with murderous rampages. Name one mass murderer that didn't masturbate.
For safety's sake, whenever you see anybody of whom it could be said "he was a quiet man", perform a Citizen's Arrest at once.
Strong interest in war and/or firearms.
90% of teenage boys should, for this reason, be incarcerated immediately.
Mood swings and depression.
We'd better make that 100% of all teenagers, regardless of gender.
Sympathy for offenders.
Any student who admits to understanding how people who have been bullied might rather like to shoot the bullies is probably about to run amok.
Possession of weapons.
Any weapons. Remember, pocketknives don't kill people - people who own pocketknives kill people.
Belief in evolution.
It is well known that evolution requires atheism, and that all atheists are amoral creatures, many of whom eat human flesh.
A teenager with an interest in fastidious dressing is clearly deeply aberrant. Note, however, that some particularly flamboyant apparent Goths may in fact be science fiction geeks dressed up as Centauri noblemen. Since no science fiction geek has yet killed a load of people, the plastic battery-powered phaser is not yet considered to be dangerous.
Disrespect for authority.
Today's youth should be as morally upright as are the leaders of the free world, and should never consider violence as a way of settling disputes, regardless of provocation, any more than the USA or NATO do.
("How to Spot a Psychopath" is also the name of my blog.)
Other ha-ha-only-serious pages on Dan's Data: