Excerpt:

A new study published on Thursday in The American Journal of Psychiatry suggests that dosage may play a role. It found that among people who took high doses of prescription amphetamines such as Vyvanse and Adderall, there was a fivefold increased risk of developing psychosis or mania for the first time compared with those who weren’t taking stimulants.

Perhaps this explains some of what goes on at LessWrong and in other rationalist circles.