Although people once thought that the levelling influence of radio and television would decrease the regional accents, broadcasting actually has very little impact on one's accent. In fact, except for the smallest, isolated dialects, dialects have become much stronger than they were even 50 or 100 years ago. The Northern cities vowel shift, the California vowel, and the Canadian vowel shift, which affects part of the Western United States are all recent innovations in the pronunciation of vowels. The merger of the vowels in words such as "cot" and "caught", is also a fairly recent innovation, which affects almost half of the United States, causes speakers with the merger to be unable to produce a distinction in those vowels, or even hear a difference when they hear speakers from non-mergin