Usually if it is common, both are true. The population is about 51% women and 49% men. For one man to have 4 wives that usually means 2 or more men have no wives. So, guess who gets the ladies. The ruling class. Not coincidentally, such areas are where you can often get suicide bombers. Especially if your religion promises the 40 virgins in the next life. The drive to have sex is generally the most powerful motivator there is.