A sanity system

[dataweaver]

During GURPS Horror playtest, this system was suggested as a sort of psychological parallel to the regular Injury, Illness, and Fatigue rules, with Stress being roughly equivalent to Fatigue Points and Derangement being analogous to Hit Points. It was proposed due to feedback from a playtester who also wasn't terribly fond of GURPS Cthulupunk's Sanity-Blasting Fright Checks. Also of note are the subsequent Madness Check rules: in my games, I use Madness Checks instead of Sanity-Blasting Fright Checks, with the psychological Conditions replacing the usual Fright Check results. That is, I have regular Fright Checks (which generate Stress) and Madness Checks (which generate Derangement and inflict psychological Conditions).

I tend to be a bit more lenient than GURPS Horror in terms of what happens when you accumulate too much Derangement: instead of extra Derangement being converted into Disadvantages, I simply inflict an appropriate Incapacitating Affliction (such as catatonia) on the victim until she has a chance to recover. Likewise, too much Stress incapacitates the character just as too much Fatigue renders her unconscious. Since Incapacitating Conditions take the character out of the scene, they also tend to limit further psychological harm: if you're insensate, you're not making any more Fright or Madness Checks. It also means that the only way permanent psychological damage can occur is by rolling high on a Fright or Madness Check.

To really push the analogy all the way, I add a Composure stat, which is to mental health as HT is to physical health: completely independent of IQ, this stat is used as the basis for Madness checks, replaces IQ as the default basis for Influence skills and Will, and determines how much of a Derangement penalty you can have. It starts at 10, and buying it up costs 10 points per +1.

Note that I do this even in games without Madness Checks: I like having Influence skills be independent of IQ; and I only forego Composure in games which are light on both Horror and Social Engineering elements.