It is not altogether uncommon among the Indo-European languages to have what is called vowel reduction in unstressed syllables, by which is meant the substitution (typically, in a polysyllabic word, but not only) of a shorter and partially displaced vowel for what would otherwise, under stress, be a so-called full vowel. This happens, for instance, in standard Russian and Bulgarian, as it does in English.
The difference between the two Slavic languages and English is that whereas vowel reduction is completely regular in the native vocabulary of the former, it is subject to much variation in the latter. Thus a word like candidate can be pronounced either with secondary stress and retention of the full vowel in the final syllable, or with no secondary stress and the pronunciation of a schwa (ə) in the ultima. The character of the stress and the character of the vowel are linked: primary stress goes with full vowels, secondary stress or unstressedness with schwa. Any secondarily stressed or unstressed syllable can contain a schwa: the a in adept, the e in synthesis, the i in decimal, the o in harmony, the u in medium, the y in syringe are all schwas.
The appearance of schwas differs somewhat as between British and American English. In standard British English, schwa regularly appears in some words where American pronunciation retains a full vowel; cf. the differential phonetics of pentagon, Amazon, businessman, and even legislature. The case of the toponym Birmingham (Alabama vs. England) is a perfect illustration. Of course, in large part the two versions of English have the same vowel in unstressed syllables, but there is a marked propensity in British speech for schwas to occur where they do not in American. This means that where American English has a reduced vowel, so will British––but not vice versa.
One of the sign functions of vowel reduction is to underscore the unity of the word, that its constituents are subordinate to the unified integrity of the word as a whole, as a gestalt. Thus when, in British but not American English, the element man in businessman loses its full vowel character in this compound, it is IN THAT MEASURE also subordinated in value to the semantic unity of the compound. This is a matter of semantic hierarchy: constituents of words are always subordinate in meaning to that of the whole word of which they are parts. Vowel reduction and concomitant absence of secondary stress are a sign, therefore, of the word’s unity, of the parts being subordinate to the whole.
The difference between British and American speech in the distribution of reduced vowels in polysyllables is not au fond a matter of mere phonetics but of mentality. By consistently reducing vowels that are unreduced in American English, British English emphasizes the value of the whole at the expense of the parts, whereas American speech tends to give equal value to the vowels of secondarily stressed syllables. This phenomenon can be interpreted as a sign––an ICON, in fact, in that word’s proper semiotic sense––of what is fundamentally an ideological difference, always necessarily historical, between the two Englishes: British English here emphasizes the superordinate historical value (unconsciously) placed by its speakers on the unity of the nation manifested linguistically, whereas American English, by giving nearly equal value to part and whole, aligns itself ideologically with its enduring history as a revolutionary product, to this day less a nation than an uneasy federation of disparate states.