The Mariposa super mutants are a variety of super mutants common on the West Coast and in the Midwest that originated from the Mariposa Military Base in California. While some of them can be rather stupid, in general they are more intelligent and civilized than their distant Vault 87 super mutant cousins.
Super mutants were first created on the West Coast by Richard Grey, also known as "The Master". The great majority of super mutants produced by the Master and later his Lieutenant in Mariposa's Vats were big, dumb brutes. Physically, they were vastly superior to humans, but they had the intelligence of children. Which factor exactly determines the results of a "dipping" is still uncertain; prevalent theories are radiation damage and inoculation of the subjects. As a rule of thumb, the cleaner the subject, the better.