The last theory advanced during the last century is social one, advocated by the American linguist William Labov. What he found was that at the beginning a small part of a population pronounces certain words that have, for example, the same vowel, differently than the rest of the population. This occurs naturally since humans don't all reproduce exactly the same sounds. However, at some later point in time, for some reason this difference in pronunciation starts to become a signal for social and cultural identity. Others of the population who wish to be identified with the group either consciously or unconsciously adopt this difference, exaggerate it, and apply it to change the pronunciation of other words. If given enough time, the change ends up affecting all words that posses the same vowel so that this become a regular linguistic sound change.