There are several research topics which twist the thaumaturges' soul and perhaps mind, due to the contemplation of dire truths not available to common folk. (But is it truly insanity, when the "voices in your head" teach you actual knowledge?) In any case, such topics are known as "forbidden knowledge", and the effects of delving into them are summarized as warp.
In the Thauminomicon, forbidden-knowledge topics appear with a pulsing black aura around them. When you mouse-over the research, it will hint at the amount of warp it will give you under the research name. There are different levels ranging from Mostly harmless all the way up to Taboo. The higher the tier is, the more warp you'll gain.
In current versions, the warp gained by research is half permanent warp, and half "regular" (formerly, "sticky") warp. Even crafting the items from forbidden research (at least the first time) grants warp, but of a weaker kind: Crafting forbidden items gives half regular warp, and half temporary warp. Note that while crafting such items only adds warp the first time, it continues to count as "doing something warpy", so if the player's warp effects had faded from "good behavior", they will be reawakened.
While it is technically possible to play while avoiding all forbidden research, this is very difficult, as several of the topics are fundamental to further work: Research Mastery makes a big difference in playing the minigame, and the Brain in a Jar is required to learn Infusion Enchanting. Other topics are merely very useful: Infernal Furnace, Flesh golems, and so on. Furthermore, many of the advanced topics such as voidmetal and the Staff Core of the Primal also require warp even to see the topics -- they are so twisted that "sane" folks simply cannot comprehend them. Such are the temptations of power!